path: root/org-drill.el

;;; org-drill.el --- Self-testing using spaced repetition -*- lexical-binding: t -*-

;;; Header:

;; Maintainer: Phillip Lord <phillip.lord@russet.org.uk>
;; Author: Paul Sexton <eeeickythump@gmail.com>
;; Version: 2.7.0
;; Package-Requires: ((emacs "25.3") (seq "2.14") (org "9.6") (persist "0.3"))
;; Keywords: games, outlines, multimedia

;; URL: https://github.com/cjennings/org-drill
;;
;; This file is not part of GNU Emacs.
;;
;; Copyright (C) 2018-2019 Phillip Lord
;; Copyright (C) 2010-2015 Paul Sexton
;;
;;
;; This program is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;;
;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
;;
;;

;;; Commentary:
;;
;; Within an Org mode outline or outlines, headings and associated content are
;; treated as "flashcards".  Spaced repetition algorithms are used to conduct
;; interactive "drill sessions", where a selection of these flashcards is
;; presented to the student in random order.  The student rates his or her
;; recall of each item, and this information is used to schedule the item for
;; later revision.
;;
;; Each drill session can be restricted to topics in the current buffer
;; (default), one or several files, all agenda files, or a subtree.  A single
;; topic can also be tested.
;;
;; Different "card types" can be defined, which present their information to
;; the student in different ways.
;;
;; See the file README.org for more detailed documentation.

;;; Code:

(require 'calendar)
(require 'cl-lib)
(require 'eieio)
(require 'org)
(require 'org-agenda)
(require 'org-id)
(require 'persist)
(require 'seq)

(defgroup org-drill nil
  "Options concerning interactive drill sessions in Org mode (org-drill)."
  :tag "Org-Drill"
  :group 'org-link)

(defgroup org-drill-display nil
  "How cards, clozes, and the session are displayed."
  :tag "Org-Drill Display"
  :group 'org-drill)

(defgroup org-drill-algorithm nil
  "Spaced-repetition scheduling algorithm and its parameters."
  :tag "Org-Drill Algorithm"
  :group 'org-drill)

(defgroup org-drill-session nil
  "What gets drilled and how a session is bounded."
  :tag "Org-Drill Session"
  :group 'org-drill)

(defgroup org-drill-leech nil
  "Handling of leeches: items that are failed repeatedly."
  :tag "Org-Drill Leech"
  :group 'org-drill)

(defgroup org-drill-statistics nil
  "Persistent session log and the statistics dashboard.
The dashboard is opt-in via \\='M-x org-drill-statistics\\='; the session log
itself is updated automatically at the end of every completed (non-suspended)
drill session.  See docs/design/stats-dashboard.org for the design rationale."
  :tag "Org-Drill Statistics"
  :group 'org-drill)

(defconst org-drill-version "2.7.0"
  "Version of the org-drill package.
Keep this in sync with the Version header at the top of this file.")

;;;###autoload
(defun org-drill-version ()
  "Report the installed org-drill version in the echo area.
Returns the version string so it is useful in non-interactive code too."
  (interactive)
  (message "org-drill %s" org-drill-version)
  org-drill-version)

(defcustom org-drill-question-tag
  "drill"
  "Tag for topics which are review topics."
  :group 'org-drill-session
  :type 'string)

(defvar org-drill-leitner-tag "leitner"
  "Tag marking entries reviewed via the Leitner box system.")

(defcustom org-drill-maximum-items-per-session
  30
  "Each drill session will present at most this many topics for review.
Nil means unlimited."
  :group 'org-drill-session
  :type '(choice integer (const nil)))

(defcustom org-drill-maximum-duration
  20
  "Maximum duration of a drill session, in minutes.
Nil means unlimited."
  :group 'org-drill-session
  :type '(choice integer (const nil)))

(defcustom org-drill-on-timeout-action
  'finish-current
  "What to do with unfinished cards when `org-drill-maximum-duration' is reached.

- `finish-current' (default): keep the historical behavior.  The
  card in progress and any cards queued for re-drilling this session
  (the \\='again\\=' queue) are still presented before the session ends.
- `discard-current': end the session as soon as the time limit is
  reached, dropping the in-progress card and the again-queue.  The
  dropped cards are left untouched — they keep whatever scheduling
  they already had and simply turn up again next session."
  :group 'org-drill-session
  :type '(choice (const finish-current) (const discard-current)))

(defcustom org-drill-item-count-includes-failed-items-p
  nil
  "If non-nil, count failed items in overall count.

If nil (default), only successful items count towards this
total."
  :group 'org-drill-session
  :type 'boolean)

(defcustom org-drill-failure-quality
  2
  "Lower bound for an recall to be marked as failure.

If the quality of recall for an item is this number or lower,
it is regarded as an unambiguous failure, and the repetition
interval for the card is reset to 0 days.  If the quality is higher
than this number, it is regarded as successfully recalled, but the
time interval to the next repetition will be lowered if the quality
was near to a fail.

By default this is 2, for SuperMemo-like behaviour.  For
Mnemosyne-like behaviour, set it to 1.  Other values are not
really sensible."
  :group 'org-drill-algorithm
  :type '(choice (const 2) (const 1)))

(defcustom org-drill-forgetting-index
  10
  "The maximum percentage of items that can be forgotten before a warning.

What percentage of items do you consider it is \\='acceptable\\=' to
forget each drill session? The default is 10%.  A warning message
is displayed at the end of the session if the percentage forgotten
climbs above this number."
  :group 'org-drill-algorithm
  :type 'integer)

(defcustom org-drill-leech-failure-threshold
  15
  "Threshold before a item is defined as a leech.

If an item is forgotten more than this many times, it is tagged
as a \\='leech\\=' item."
  :group 'org-drill-leech
  :type '(choice integer (const nil)))

(defcustom org-drill-leech-method
  'skip
  "How should \\='leech items\\=' be handled during drill sessions?
Possible values:
- nil :: Leech items are treated the same as normal items.
- skip :: Leech items are not included in drill sessions.
- warn :: Leech items are still included in drill sessions,
  but a warning message is printed when each leech item is
  presented."
  :group 'org-drill-leech
  :type '(choice (const warn) (const skip) (const nil)))

(defface org-drill-visible-cloze-face
  '((t (:foreground "darkseagreen")))
  "The face used to hide the contents of cloze phrases."
  :group 'org-drill-display)

(defface org-drill-visible-cloze-hint-face
  '((t (:foreground "dark slate blue")))
  "The face used to hide the contents of cloze phrases."
  :group 'org-drill-display)

(defface org-drill-hidden-cloze-face
  '((t (:foreground "deep sky blue" :background "blue")))
  "The face used to hide the contents of cloze phrases."
  :group 'org-drill-display)

(defcustom org-drill-use-visible-cloze-face-p
  nil
  "Highlight cloze-deleted text."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-auto-enable-mode t
  "When non-nil, enable `org-drill-mode' automatically in Org buffers that
contain drill cards — headings tagged with `org-drill-question-tag' or
`org-drill-leitner-tag'.  This scopes cloze fontification to buffers that
actually hold cards instead of installing it in every Org buffer."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-hide-item-headings-p
  nil
  "If non-nil, conceal headings during a drill session.

You may want to enable this behaviour if item headings or tags
contain information that could \\='give away\\=' the answer."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-new-count-color
  "royal blue"
  "Foreground colour for remaining new items."
  :group 'org-drill-display
  :type 'color)

(defcustom org-drill-mature-count-color
  "green"
  "Foreground colour for remaining mature items.

Mature items are due for review, but are not new."
  :group 'org-drill-display
  :type 'color)

(defcustom org-drill-failed-count-color
  "red"
  "Foreground colour for remaining failed items."
  :group 'org-drill-display
  :type 'color)

(defcustom org-drill-done-count-color
  "sienna"
  "Foreground colour for reviewed items."
  :group 'org-drill-display
  :type 'color)

(defcustom org-drill-left-cloze-delimiter
  "["
  "String used within org buffers to delimit cloze deletions."
  :group 'org-drill-display
  :type 'string)

(defcustom org-drill-right-cloze-delimiter
  "]"
  "String used within org buffers to delimit cloze deletions."
  :group 'org-drill-display
  :type 'string)

(setplist 'org-drill-cloze-overlay-defaults
          `(display ,(format "%s...%s"
                             org-drill-left-cloze-delimiter
                             org-drill-right-cloze-delimiter)
                    face org-drill-hidden-cloze-face
                    window t))

(setplist 'org-drill-hidden-text-overlay
          '(invisible t))

(setplist 'org-drill-replaced-text-overlay
          '(display "Replaced text"
                    face default
                    window t))

(defvar org-drill-hint-separator "||"
  "Delimiter in cloze expression for hints.")

(defun org-drill--compute-cloze-regexp ()
  "Return a regexp that detects clozes.

The inner match is constrained to non-newline characters so a cloze
stays within one line.  An older version used
`[[:cntrl:][:graph:][:space:]]' which silently included newline,
letting a stray `[' match all the way to a `]' several lines later
and bleeding the cloze face onto intervening org headings (upstream
issue #38)."
  (concat "\\("
          (regexp-quote org-drill-left-cloze-delimiter)
          "[^\n]+?\\)\\(\\|"
          (regexp-quote org-drill-hint-separator)
          ".+?\\)\\("
          (regexp-quote org-drill-right-cloze-delimiter)
          "\\)"))

(defun org-drill--compute-cloze-keywords ()
  "Return a fontification spec that detects cloze keywords."
  (list (list (org-drill--compute-cloze-regexp)
              (cl-copy-list '(1 'org-drill-visible-cloze-face nil))
              (cl-copy-list '(2 'org-drill-visible-cloze-hint-face t))
              (cl-copy-list '(3 'org-drill-visible-cloze-face nil)))))

(defvar-local org-drill-cloze-regexp
  (org-drill--compute-cloze-regexp)
  "Regexp that detects cloze.

This is buffer-local variable.")

(defvar-local org-drill-cloze-keywords
  (org-drill--compute-cloze-keywords)
  "Fontification form for cloze.

This is a buffer-local variable.")

;; Keys pressed during a drill session to quit, edit the item, etc.
;; These are defcustoms so they can be rebound from customize-group.
(defcustom org-drill--quit-key ?q
  "Character to quit the session."
  :group 'org-drill-session
  :type 'character)
(defcustom org-drill--edit-key ?e
  "Character to suspend the session."
  :group 'org-drill-session
  :type 'character)
(defcustom org-drill--help-key ??
  "Character to show help."
  :group 'org-drill-session
  :type 'character)
(defcustom org-drill--skip-key ?s
  "Character to skip to the next item."
  :group 'org-drill-session
  :type 'character)
(defcustom org-drill--tags-key ?t
  "Character to edit the tags."
  :group 'org-drill-session
  :type 'character)
(defcustom org-drill--undo-key ?u
  "Character to undo the most recent rating during a session.
Pressing it at the rating prompt restores the previous card's
scheduling data and re-queues that card (see `org-drill-undo-last-rating')."
  :group 'org-drill-session
  :type 'character)

(defcustom org-drill-undo-limit
  3
  "How many recent ratings can be undone with `org-drill--undo-key'.
Each rating snapshots the card's scheduling state; only this many of the
most recent snapshots are kept."
  :group 'org-drill-session
  :type 'integer)

(defcustom org-drill-card-type-alist
  '((nil org-drill-present-simple-card)
    ("simple" org-drill-present-simple-card)
    ("simpletyped" org-drill-present-simple-card-with-typed-answer)
    ("twosided" org-drill-present-two-sided-card nil t)
    ("multisided" org-drill-present-multi-sided-card nil t)
    ("hide1cloze" org-drill-present-multicloze-hide1)
    ("hide2cloze" org-drill-present-multicloze-hide2)
    ("show1cloze" org-drill-present-multicloze-show1)
    ("show2cloze" org-drill-present-multicloze-show2)
    ("multicloze" org-drill-present-multicloze-hide1)
    ("hidefirst" org-drill-present-multicloze-hide-first)
    ("hidelast" org-drill-present-multicloze-hide-last)
    ("hide1_firstmore" org-drill-present-multicloze-hide1-firstmore)
    ("show1_lastmore" org-drill-present-multicloze-show1-lastmore)
    ("show1_firstless" org-drill-present-multicloze-show1-firstless)
    ("conjugate"
     org-drill-present-verb-conjugation
     org-drill-show-answer-verb-conjugation)
    ("decline_noun"
     org-drill-present-noun-declension
     org-drill-show-answer-noun-declension)
    ("spanish_verb" org-drill-present-spanish-verb))
  "Alist associating card types with presentation functions.

Each entry in the alist takes the form:

;;; (CARDTYPE QUESTION-FN [ANSWER-FN DRILL-EMPTY-P])

Where CARDTYPE is a string or nil (for default), and QUESTION-FN
is a function which takes no arguments and returns a boolean
value.

When supplied, ANSWER-FN is a function that takes one argument --
that argument is a function of no arguments, which when called,
prompts the user to rate their recall and performs rescheduling
of the drill item.  ANSWER-FN is called with the point on the
active item's heading, just prior to displaying the item's
\\='answer\\='.  It can therefore be used to modify the appearance of
the answer.  ANSWER-FN must call its argument before returning.

When supplied, DRILL-EMPTY-P is a boolean value, default nil.
When non-nil, cards of this type will be presented during tests
even if their bodies are empty."
  :group 'org-drill-session
  :type '(alist :key-type (choice string (const nil))
                :value-type function))

(defcustom org-drill-treat-headline-as-card-p nil
  "When non-nil, treat a drill entry with an empty body as a valid card.

By default a drill entry whose body is empty is skipped during a
session unless its card type opts in via the DRILL-EMPTY-P slot of
`org-drill-card-type-alist'.  When this is non-nil, an empty-bodied
entry is presented as a card with the heading itself as the question,
regardless of card type.  This covers hierarchical-notes decks where
the heading is the prompt and the answer lives in child entries, and
decks where the heading alone is the card (upstream issues #30 and #41)."
  :group 'org-drill-session
  :type 'boolean)

(defcustom org-drill-card-tags-alist
  '(("explain" nil org-drill-explain-answer-presenter
     org-drill-explain-cleaner))
  "Alist associating tags with presentation functions.

The alist is of the form (TAG QUESTION-PRESENTER ANSWER-PRESENTER CLEANER).

When a card with the relevant TAG is tested, QUESTION-PRESENTER
will be called when the card is displayed to the user,
ANSWER-PRESENTER will be called with point in the entry when the
answer is displayed to the user and CLEANER will be called when
the answer is accepted.  In all cases, point will be in the card
in question when the function is called.  All values may be nil in
which case no function will be called."
  :group 'org-drill-session
  :type '(alist :key-type (choice string (const nil))
                :value-type function))

(defcustom org-drill-scope
  'file
  "The scope to search for drill items in a session.

This can be any of:

file                 The current buffer, respecting the restriction if any.
                     This is the default.
tree                 The subtree started with the entry at point
file-no-restriction  The current buffer, without restriction
file-with-archives   The current buffer, and any archives associated with it.
agenda               All agenda files
agenda-with-archives All agenda files with any archive files associated
                     with them.
directory            All files with the extension '.org' in the same
                     directory as the current file (includes the current
                     file if it is an .org file.)
 (FILE1 FILE2 ...)   If this is a list, all files in the list will be scanned."
  ;; Note -- meanings differ slightly from the argument to org-map-entries:
  ;; 'file' means current file/buffer, respecting any restriction
  ;; 'file-no-restriction' means current file/buffer, ignoring restrictions
  ;; 'directory' means all *.org files in current directory
  :group 'org-drill-session
  :type '(choice (const :tag "The current buffer, respecting the restriction if any." file)
                 (const :tag "The subtree started with the entry at point" tree)
                 (const :tag "The current buffer, without restriction" file-no-restriction)
                 (const :tag "The current buffer, and any archives associated with it." file-with-archives)
                 (const :tag "All agenda files" agenda)
                 (const :tag "All agenda files with any archive files associated with them." agenda-with-archives)
                 (const :tag "All files with the extension '.org' in the same directory as the current file (includes the current file if it is an .org file.)"  directory)
                 (repeat :tag "List of files to scan for drill items." file)))

(defcustom org-drill-match
  nil
  "If non-nil, a string specifying a tags/property/TODO query.

During drill sessions, only items that match this query will be
considered."
  :group 'org-drill-session
  :type '(choice (const nil) string))

(defcustom org-drill-save-buffers-after-drill-sessions-p
  t
  "If non-nil, prompt to save all modified buffers when a session ends."
  :group 'org-drill-session
  :type 'boolean)

;; ADR 2026-05-27 — default scheduling algorithm (upstream issue #46).
;;
;; Context.  The default for the past several years has been SM5.  Issue
;; #46 (2022-07) asked whether SM2 or Simple8 would be better defaults
;; out of the box.  The decision was deferred until #147 (the card-state
;; refactor) landed, which it now has.
;;
;; The three candidates as implemented here:
;;
;; - SM2: classic SuperMemo 2.  Carries no algorithm-level state beyond
;;   the per-card item-data.  Simple, well-validated, no persistence-file
;;   surface.  The conservative default in many SR implementations.
;;
;; - SM5: SuperMemo 5 with a per-user optimal-factor matrix persisted
;;   via `persist-defvar' (`org-drill-sm5-optimal-factor-matrix').  The
;;   matrix really does adapt across sessions, so SM5's "learns over
;;   time" claim materializes for each user.  The downside is the
;;   persistence-file dependency — upstream issue #45 surfaced "End of
;;   file during parsing" raised from inside `persist', and we now wrap
;;   the load in a `condition-case' that falls back to a fresh nil
;;   matrix.  A SM5 user whose persist file rots silently regresses to
;;   learning-from-scratch every session.
;;
;; - Simple8: modernized SM-family scheduler.  Learns per-card difficulty
;;   from quality grades and failure count, no global matrix.  Adjusts
;;   intervals for early or late reviews (delta-days), a real-world
;;   concern that SM2 and SM5 don't address.  No persistence-file
;;   dependency.  The "boring modern" choice.
;;
;; Decision.  Simple8.  Per-card difficulty learning gives most of SM5's
;; adaptation value without the persist-file fragility, and delta-days
;; handling matches how people actually use spaced repetition (cards do
;; get reviewed late).  SM5 stays available for users who want the
;; persisted optimal-factor matrix; SM2 stays available for users who
;; want the simplest possible scheduler.
;;
;; Consequences.  New installs: Simple8 by default.  Existing users with
;; `org-drill-spaced-repetition-algorithm' set in their config: no
;; change.  Existing users on the previous default with no setting in
;; their config: their next session schedules under Simple8, which uses
;; a different per-card state shape than SM5.  No state migration is
;; performed — Simple8 reads what it can from the existing item-data
;; (last-interval, repetitions, failures, total-repeats, meanq) and
;; carries on.  SM5's optimal-factor matrix is preserved on disk and
;; available if the user switches back.

(defcustom org-drill-spaced-repetition-algorithm
  'simple8
  "Which SuperMemo spaced repetition algorithm to use for scheduling items.
Available choices are:
- SM2 :: the SM2 algorithm, used in SuperMemo 2.0
- SM5 :: the SM5 algorithm, used in SuperMemo 5.0
- Simple8 :: a modified version of the SM8 algorithm.  SM8 is used in
  SuperMemo 98. The version implemented here is simplified in that while it
  \\='learns\\=' the difficulty of each item using quality grades and number of
  failures, it does not modify the matrix of values that
  governs how fast the inter-repetition intervals increase.  A method for
  adjusting intervals when items are reviewed early or late has been taken
  from SM11, a later version of the algorithm, and included in Simple8.

The default is Simple8; see the ADR comment above the defcustom in the
source for the rationale."
  :group 'org-drill-algorithm
  :type '(choice (const sm2) (const sm5) (const simple8)))

;; Wrap `persist-defvar' in `condition-case' so a corrupted persist
;; file (upstream issue #45 — "End of file during parsing" raised from
;; deep inside the persist package) doesn't prevent org-drill from
;; loading.  Fall back to a fresh nil matrix when persist-load fails.
(condition-case err
    (persist-defvar org-drill-sm5-optimal-factor-matrix
      nil
      "DO NOT CHANGE THE VALUE OF THIS VARIABLE.

Persistent matrix of optimal factors, used by the SuperMemo SM5
algorithm. The matrix is saved at the end of each drill session.

Over time, values in the matrix will adapt to the individual user's
pace of learning.")
  (error
   (message
    "org-drill: failed to load persisted SM5 matrix (%s); using fresh state"
    err)
   (defvar org-drill-sm5-optimal-factor-matrix nil
     "Persistent matrix of optimal factors (fallback after load failure).")))

;; Statistics session log.  Records one entry per completed drill session
;; so the stats dashboard (docs/design/stats-dashboard.org) has
;; a temporal axis to render trends against.  Same persist-defvar +
;; condition-case pattern as the SM5 matrix above.  See
;; `org-drill--session-log-quarantine' for the corrupt-file recovery
;; behavior — the dashboard's spec adds a rename-and-quarantine step on
;; top of the SM5 path's fresh-start fallback.
(cl-defstruct org-drill-session-record
  "One completed drill session, persisted to `org-drill-session-log'.
Slots match the v0 stats-dashboard spec; see
docs/design/stats-dashboard.org for field semantics."
  start-time      ; float — `float-time' at session start
  end-time        ; float — `float-time' at session end
  scope           ; symbol or list — `org-drill-scope' at start
  algorithm       ; symbol — `org-drill-spaced-repetition-algorithm' at start
  qualities       ; vector of int — every 0-5 quality entered, in order
  pass-percent    ; int — (count qualities > failure-quality) / total * 100
  new-count       ; int — new entries at session end
  mature-count    ; int — young-mature + old-mature entries at session end
  failed-count    ; int — failed entries at session end
  cram-mode)      ; bool — cram-mode value at session start

(defun org-drill--session-log-quarantine ()
  "Rename a corrupt `org-drill-session-log' persist file out of the way.
The file is renamed to a `.corrupt-YYYY-MM-DDTHHMMSS' sibling so the
next save doesn't overwrite a potentially recoverable file.  The
timestamp includes seconds so a repeat corruption on the same day does
not silently overwrite the earlier quarantine.  No-op if the file
can't be located or doesn't exist.

Locates the on-disk file via `persist--file-location', which is an
internal symbol in the `persist' package.  The `fboundp' gate keeps a
future persist release that renames it from breaking
package load — at the cost of becoming a silent no-op on that release,
which would let the next save overwrite the corrupt file.  Worth a
follow-up if persist changes its API."
  (when (fboundp 'persist--file-location)
    (let ((file (ignore-errors
                  (persist--file-location 'org-drill-session-log))))
      (when (and file (file-exists-p file))
        (let ((quarantine (format "%s.corrupt-%s"
                                  file
                                  (format-time-string "%Y-%m-%dT%H%M%S"))))
          (ignore-errors (rename-file file quarantine t))
          (lwarn 'org-drill :warning
                 "Corrupt session-log file moved to %s; starting fresh."
                 quarantine))))))

(condition-case err
    (persist-defvar org-drill-session-log
      nil
      "List of `org-drill-session-record' values, newest first.
Updated at the end of every completed (non-suspended) drill session by
`org-drill-record-session'.  Read by the stats dashboard
(`org-drill-statistics', when implemented) to render trend panels.")
  (error
   (message
    "org-drill: failed to load persisted session log (%s); using fresh state"
    err)
   (ignore-errors (org-drill--session-log-quarantine))
   (defvar org-drill-session-log nil
     "Persistent session log (fallback after load failure).")))

(defcustom org-drill-sm5-initial-interval
  4.0
  "In the SM5 algorithm, the initial interval after the first
successful presentation of an item is always 4 days. If you wish to change
this, you can do so here."
  :group 'org-drill-algorithm
  :type 'float)

(defcustom org-drill-add-random-noise-to-intervals-p
  nil
  "If true, the number of days until an item's next repetition
will vary slightly from the interval calculated by the SM2
algorithm. The variation is very small when the interval is
small, but scales up with the interval."
  :group 'org-drill-algorithm
  :type 'boolean)

(defcustom org-drill-adjust-intervals-for-early-and-late-repetitions-p
  nil
  "If true, when the student successfully reviews an item 1 or more days
before or after the scheduled review date, this will affect that date of
the item's next scheduled review, according to the algorithm presented at
 [[http://www.supermemo.com/english/algsm11.htm#Advanced%20repetitions]].

Items that were reviewed early will have their next review date brought
forward. Those that were reviewed late will have their next review
date postponed further.

Note that this option currently has no effect if the SM2 algorithm
is used."
  :group 'org-drill-algorithm
  :type 'boolean)

(defcustom org-drill-cloze-text-weight
  4
  "For card types \\='hide1_firstmore\\=', \\='show1_lastmore\\=' and \\='show1_firstless\\=',
this number determines how often the \\='less favoured\\=' situation
should arise. It will occur 1 in every N trials, where N is the
value of the variable.

For example, with the hide1_firstmore card type, the first piece
of clozed text should be hidden more often than the other
pieces. If this variable is set to 4 (default), the first item
will only be shown 25% of the time (1 in 4 trials). Similarly for
show1_lastmore, the last item will be shown 75% of the time, and
for show1_firstless, the first item would only be shown 25% of the
time.

If the value of this variable is NIL, then weighting is disabled, and
all weighted card types are treated as their unweighted equivalents."
  :group 'org-drill-display
  :type '(choice integer (const nil)))

(defcustom org-drill-cram-hours
  12
  "When in cram mode, items are considered due for review if
they were reviewed at least this many hours ago."
  :group 'org-drill-session
  :type 'integer)

;;; NEW items have never been presented in a drill session before.
;;; MATURE items HAVE been presented at least once before.
;;; - YOUNG mature items were scheduled no more than
;;;   ORG-DRILL-DAYS-BEFORE-OLD days after their last
;;;   repetition. These items will have been learned 'recently' and will have a
;;;   low repetition count.
;;; - OLD mature items have intervals greater than
;;;   ORG-DRILL-DAYS-BEFORE-OLD.
;;; - OVERDUE items are past their scheduled review date by more than
;;;   LAST-INTERVAL * (ORG-DRILL-OVERDUE-INTERVAL-FACTOR - 1) days,
;;;   regardless of young/old status.

(defcustom org-drill-days-before-old
  10
  "When an item's inter-repetition interval rises above this value in days,
it is no longer considered a \\='young\\=' (recently learned) item."
  :group 'org-drill-algorithm
  :type 'integer)

(defcustom org-drill-overdue-interval-factor
  1.2
  "An item is considered overdue if its scheduled review date is
more than (ORG-DRILL-OVERDUE-INTERVAL-FACTOR - 1) * LAST-INTERVAL
days in the past. For example, a value of 1.2 means an additional
20% of the last scheduled interval is allowed to elapse before
the item is overdue. A value of 1.0 means no extra time is
allowed at all - items are immediately considered overdue if
there is even one day's delay in reviewing them. This variable
should never be less than 1.0."
  :group 'org-drill-algorithm
  :type 'float)

(defcustom org-drill-learn-fraction
  0.5
  "Fraction between 0 and 1 that governs how quickly the spaces
between successive repetitions increase, for all items. The
default value is 0.5. Higher values make spaces increase more
quickly with each successful repetition. You should only change
this in small increments (for example 0.05-0.1) as it has an
exponential effect on inter-repetition spacing."
  :group 'org-drill-algorithm
  :type 'float)

(defcustom org-drill-presentation-prompt-with-typing nil
  "Non-nil indicates that answers should be given in a buffer."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-cloze-length-matches-hidden-text-p
  nil
  "If non-nil, when concealing cloze deletions, force the length of
the ellipsis to match the length of the missing text. This may be useful
to preserve the formatting in a displayed table, for example."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-hide-modeline-during-session
  t
  "If non-nil, hide the modeline during drill sessions.
This provides a cleaner, more focused display for reading drill cards.
The modeline is automatically restored when the session ends."
  :group 'org-drill-display
  :type 'boolean)

(defcustom org-drill-text-size-during-session
  20
  "Font size (in points) to use during drill sessions.
Set to nil to use the default font size without scaling.
Typical values are 16-24 for comfortable reading."
  :group 'org-drill-display
  :type '(choice (const :tag "Use default size" nil)
                 (integer :tag "Font size in points")))

(defcustom org-drill-use-variable-pitch
  nil
  "If non-nil, use variable-pitch font during drill sessions.
This can make text more readable for long-form content."
  :group 'org-drill-display
  :type 'boolean)

(defvar org-drill--saved-modeline-format nil
  "Saved modeline format before drill session started.")

(defvar org-drill--saved-text-scale nil
  "Saved text scale level before drill session started.")

(defvar org-drill--saved-variable-pitch-mode nil
  "Saved variable-pitch-mode state before drill session started.")

(defvar org-drill--saved-display-buffer nil
  "Buffer in which the active drill session set up its display.
Captured at setup so restore can target the same buffer even if the
user has switched away by the time `org-drill--restore-display' runs.")

(defvar org-drill-display-answer-hook nil
  "Hook called when `org-drill' answers are displayed.")

(defvar org-drill-before-session-hook nil
  "Hook run before starting a drill session.
Called in the drill buffer after opening the file but before
presenting the first card. Useful for setting up display preferences
like fonts, text scale, or window layout.")

(defvar org-drill-after-session-hook nil
  "Hook run after ending a drill session.
Called when the session ends normally, is quit by the user, or
encounters an error. Useful for restoring display preferences
or performing cleanup.")

(defclass org-drill-session ()
  ((qualities :initform nil)
   (start-time
    :initform 0.0
    :documentation "Time at which the session started"
    :type float)
   (scope-at-start
    :initform nil
    :documentation
    "Value of `org-drill-scope' captured at session start.  Stored on the
stats-dashboard session record verbatim so a mid-session change to the
defcustom doesn't misrepresent what was actually drilled.")
   (algorithm-at-start
    :initform nil
    :documentation
    "Value of `org-drill-spaced-repetition-algorithm' captured at session
start.  Same rationale as `scope-at-start'.")
   (new-entries :initform nil)
   (dormant-entry-count :initform 0)
   (due-entry-count :initform 0)
   (overdue-entry-count :initform 0)
   (due-tomorrow-count :initform 0)
   (overdue-entries
    :initform nil
    :documentation
    "List of markers for items that are
considered 'overdue', based on the value of
ORG-DRILL-OVERDUE-INTERVAL-FACTOR.")
   (young-mature-entries
    :initform nil
    :documentation "List of markers for mature entries whose last inter-repetition
interval was <= ORG-DRILL-DAYS-BEFORE-OLD days.")
   (old-mature-entries
    :initform nil
    :documentation "List of markers for mature entries whose last inter-repetition
interval was greater than ORG-DRILL-DAYS-BEFORE-OLD days.")
   (failed-entries :initform nil)
   (again-entries :initform nil)
   (done-entries :initform nil)
   (undo-stack
    :initform nil
    :documentation "Stack of pre-rating scheduling snapshots, most recent
first, used by `org-drill-undo-last-rating'.  Capped at `org-drill-undo-limit'.")
   (current-item
    :initform nil
    :documentation "Set to the marker for the item currently being tested.")
   (cram-mode
    :initform nil
    :documentation "Are we in 'cram mode', where all items are considered due
for review unless they were already reviewed in the recent past?")
   (warned-about-id-creation
    :initform nil
    :documentation "Have we warned the user about ID creation this session?")
   (overdue-data :initform nil)
   (cnt :initform 0)
   (exit-kind
    :initform nil
    :documentation "Return value from typed answers which use recursive edit.")
   (typed-answer
    :initform nil
    :documentation "The last answer typed by the user.")
   (drill-answer
    :initform nil
    :documentation "The correct answer when an item is being
presented. If this variable is non-nil, the default presentation
function will show its value instead of the default behaviour of
revealing the contents of the drilled item.

This variable is useful for card types that compute their answers
-- for example, a card type that asks the student to translate a
random number to another language.")
   (end-pos :initform nil))
  :documentation "An org-drill session object carries data about
  the current state of a particular org-drill session." )

(defvar org-drill-current-session nil
  "If non-nil, the current session.

The current session is an `org-drill-session' object.")

(defvar org-drill-last-session nil
  "If non-nil, the last session.

This can be used to resume the last session.")

(defvar org-drill-cards-in-this-emacs 0
  "The total number of cards displayed in this Emacs invocation.

This variable is not functionally important, but is used for
  debugging.")

(defvar org-drill-scheduling-properties
  '("LEARN_DATA" "DRILL_LAST_INTERVAL" "DRILL_REPEATS_SINCE_FAIL"
    "DRILL_TOTAL_REPEATS" "DRILL_FAILURE_COUNT" "DRILL_AVERAGE_QUALITY"
    "DRILL_EASE" "DRILL_LAST_QUALITY" "DRILL_LAST_REVIEWED"))

(defcustom org-drill-lapse-threshold-days
  90
  "Number of days overdue before an entry is considered lapsed.

When an entry is more than this many days overdue and
`org-drill--lapse-very-overdue-entries-p' is non-nil, the entry
is treated as lapsed and will be scheduled as a failure (quality 2)
even if answered correctly."
  :group 'org-drill-algorithm
  :type 'integer)

(defvar org-drill--lapse-very-overdue-entries-p nil
  "If non-nil, entries more than `org-drill-lapse-threshold-days' overdue are lapsed.
This means that when the item is eventually re-tested it will be
treated as \\='failed\\=' (quality 2) for rescheduling purposes,
regardless of whether the test was successful.")


;;; Make the above settings safe as file-local variables.
(put 'org-drill-question-tag 'safe-local-variable 'stringp)
(put 'org-drill-maximum-items-per-session 'safe-local-variable
     '(lambda (val) (or (integerp val) (null val))))
(put 'org-drill-maximum-duration 'safe-local-variable
     '(lambda (val) (or (integerp val) (null val))))
(put 'org-drill-failure-quality 'safe-local-variable 'integerp)
(put 'org-drill-forgetting-index 'safe-local-variable 'integerp)
(put 'org-drill-leech-failure-threshold 'safe-local-variable 'integerp)
(put 'org-drill-lapse-threshold-days 'safe-local-variable 'integerp)
(put 'org-drill-leech-method 'safe-local-variable
     '(lambda (val) (memq val '(nil skip warn))))
(put 'org-drill-use-visible-cloze-face-p 'safe-local-variable 'booleanp)
(put 'org-drill-hide-item-headings-p 'safe-local-variable 'booleanp)
(put 'org-drill-treat-headline-as-card-p 'safe-local-variable 'booleanp)
(put 'org-drill-spaced-repetition-algorithm 'safe-local-variable
     '(lambda (val) (memq val '(simple8 sm5 sm2))))
(put 'org-drill-sm5-initial-interval 'safe-local-variable 'floatp)
(put 'org-drill-add-random-noise-to-intervals-p 'safe-local-variable 'booleanp)
(put 'org-drill-adjust-intervals-for-early-and-late-repetitions-p
     'safe-local-variable 'booleanp)
(put 'org-drill-cram-hours 'safe-local-variable 'integerp)
(put 'org-drill-learn-fraction 'safe-local-variable 'floatp)
(put 'org-drill-days-before-old 'safe-local-variable 'integerp)
(put 'org-drill-overdue-interval-factor 'safe-local-variable 'floatp)
(put 'org-drill-scope 'safe-local-variable
     '(lambda (val) (or (symbolp val) (listp val))))
(put 'org-drill-match 'safe-local-variable
     '(lambda (val) (or (stringp val) (null val))))
(put 'org-drill-save-buffers-after-drill-sessions-p 'safe-local-variable 'booleanp)
(put 'org-drill-cloze-text-weight 'safe-local-variable
     '(lambda (val) (or (null val) (integerp val))))
(put 'org-drill-left-cloze-delimiter 'safe-local-variable 'stringp)
(put 'org-drill-right-cloze-delimiter 'safe-local-variable 'stringp)

;;;; Utilities ================================================================
(defmacro org-drill-pop-random (place)
  "Remove an item randomly from PLACE."
  (let ((index-var (cl-gensym)))
    `(if (null ,place)
         nil
       (let ((,index-var (cl-random (length ,place))))
         (prog1 (nth ,index-var ,place)
           (setf ,place (append (cl-subseq ,place 0 ,index-var)
                                (cl-subseq ,place (1+ ,index-var)))))))))

(defmacro org-drill-push-end (val place)
  "Add VAL to the end of the sequence stored in PLACE. Return the new
value."
  `(setf ,place (append ,place (list ,val))))

(defun org-drill-round-float (floatnum fix)
  "Round the floating point number FLOATNUM to FIX decimal places.
Example: (round-float 3.56755765 3) -> 3.568"
  (let ((n (expt 10 fix)))
    (/ (float (round (* floatnum n))) n)))

(defun org-drill-command-keybinding-to-string (cmd)
  "Return a human-readable description of the key/keys to which the command
CMD is bound, or nil if it is not bound to a key."
  (let ((key (where-is-internal cmd overriding-local-map t)))
    (if key (key-description key))))

(defun org-drill-time-to-inactive-org-timestamp (time)
  "Convert TIME into org-mode timestamp."
  (format-time-string (org-time-stamp-format t 'no-bracket) time))

(defun org-drill-map-entries (func &optional scope drill-match &rest skip)
  "Like `org-map-entries', but only drill entries are processed."
  (let ((org-drill-match (or drill-match org-drill-match)))
    (apply 'org-map-entries func
           (concat "+" org-drill-question-tag
                   (if (and (stringp org-drill-match)
                            (not (member (elt org-drill-match 0) '(?+ ?- ?|))))
                       "+" "")
                   (or org-drill-match ""))
           (org-drill-current-scope scope)
           skip)))

(defun org-drill-current-scope (scope)
  "Translate SCOPE into an scope suitable for `org-map-entries'.

If scope is NIL, then use `org-drill-scope'.

Returns scope as defined by `org-map-entries'"
  (let ((scope (or scope org-drill-scope)))
    (cl-case scope
      (file nil)
      (file-no-restriction 'file)
      (directory
       (directory-files
        (file-name-directory (buffer-file-name))
        t "^[^.].*\\.org$"))
      (t scope))))

(defmacro org-drill-with-hidden-cloze-text (&rest body)
  "Eval BODY with clozed text hidden."
  (declare (debug t))
  `(progn
     (org-drill-hide-clozed-text)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unhide-clozed-text))))

(defmacro org-drill-with-hidden-cloze-hints (&rest body)
  "Eval BODY with cloze hints hidden."
  (declare (debug t))
  `(progn
     (org-drill-hide-cloze-hints)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unhide-text))))

(defmacro org-drill-with-hidden-comments (&rest body)
  "Eval BODY with comments hidden."
  (declare (debug t))
  `(progn
     (if org-drill-hide-item-headings-p
         (org-drill-hide-heading-at-point))
     (org-drill-hide-comments)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unhide-text))))

(defmacro org-drill-with-card-display (&rest body)
  "Eval BODY in the standard card-display envelope.
Combines `with-hidden-comments', `with-hidden-cloze-hints', and
`with-hidden-cloze-text' — the wrap most card presenters open with."
  (declare (debug t) (indent 0))
  `(org-drill-with-hidden-comments
    (org-drill-with-hidden-cloze-hints
     (org-drill-with-hidden-cloze-text
      ,@body))))

(defun org-drill-days-since-last-review ()
  "Nil means a last review date has not yet been stored for
the item.
Zero means it was reviewed today.
A positive number means it was reviewed that many days ago.
A negative number means the date of last review is in the future --
this should never happen."
  (let ((datestr (org-entry-get (point) "DRILL_LAST_REVIEWED")))
    (when datestr
      (- (time-to-days (current-time))
         (time-to-days (apply 'encode-time
                              (org-parse-time-string datestr)))))))

(defun org-drill-hours-since-last-review ()
  "Like `org-drill-days-since-last-review', but return value is
in hours rather than days."
  (let ((datestr (org-entry-get (point) "DRILL_LAST_REVIEWED")))
    (when datestr
      (floor
       (/ (- (time-to-seconds (current-time))
             (time-to-seconds (apply 'encode-time
                                     (org-parse-time-string datestr))))
          (* 60 60))))))

(defun org-drill-entry-p (&optional marker)
  "Is MARKER, or the point, in a \\='drill item\\='? This will return nil if
the point is inside a subheading of a drill item -- to handle that
situation use `org-part-of-drill-entry-p'."
  (save-excursion
    (when marker
      (org-drill-goto-entry marker))
    (member org-drill-question-tag (org-get-tags nil t))))

(defun org-drill-goto-entry (marker)
  "Switch to the buffer and position of MARKER."
  (switch-to-buffer (marker-buffer marker))
  (goto-char marker))

(defun org-drill-part-of-drill-entry-p ()
  "Is the current entry either the main heading of a \\='drill item\\=',
or a subheading within a drill item?"
  (or (org-drill-entry-p)
      ;; Does this heading INHERIT the drill tag
      (member org-drill-question-tag (org-get-tags))))

(defun org-drill-goto-drill-entry-heading ()
  "Move the point to the heading which holds the :drill: tag for this
drill entry."
  (unless (org-at-heading-p)
    (org-back-to-heading))
  (unless (org-drill-part-of-drill-entry-p)
    (error "Point is not inside a drill entry"))
  (while (not (org-drill-entry-p))
    (unless (org-up-heading-safe)
      (error "Cannot find a parent heading that is marked as a drill entry"))))

(defun org-drill-entry-leech-p ()
  "Is the current entry a \\='leech item\\='?"
  (and (org-drill-entry-p)
       (member "leech" (org-get-tags nil t))))

(defun org-drill-entry-days-overdue (session)
  "Returns:
- NIL if the item is not to be regarded as scheduled for review at all.
  This is the case if it is not a drill item, or if it is a leech item
  that we wish to skip, or if we are in cram mode and have already reviewed
  the item within the last few hours.
- 0 if the item is new, or if it scheduled for review today.
- A negative integer - item is scheduled that many days in the future.
- A positive integer - item is scheduled that many days in the past."
  (cond
   ((oref session cram-mode)
    (let ((hours (org-drill-hours-since-last-review)))
      (and (org-drill-entry-p)
           (or (null hours)
               (>= hours org-drill-cram-hours))
           0)))
   (t
    (let ((item-time (org-get-scheduled-time (point))))
      (cond
       ((or (not (org-drill-entry-p))
            (and (eql 'skip org-drill-leech-method)
                 (org-drill-entry-leech-p)))
        nil)
       ((null item-time)                ; not scheduled -> due now
        0)
       (t
        (- (time-to-days (current-time))
           (time-to-days item-time))))))))

(defun org-drill-entry-overdue-p (session &optional days-overdue last-interval)
  "Return non-nil if entry that is scheduled DAYS-OVERDUE days in the past,
and whose last inter-repetition interval was LAST-INTERVAL, should be
considered \\='overdue\\='. If the arguments are not given they are extracted
from the entry at point."
  (unless days-overdue
    (setq days-overdue (org-drill-entry-days-overdue session)))
  (unless last-interval
    (setq last-interval (org-drill-entry-last-interval 1)))
  (and (numberp days-overdue)
       (> days-overdue 1)               ; enforce a sane minimum 'overdue' gap
       (> last-interval 0)              ; prevent division by zero
       ;;(> due org-drill-days-before-overdue)
       (> (/ (+ days-overdue last-interval 1.0) last-interval)
          org-drill-overdue-interval-factor)))

(defun org-drill-entry-due-p (session)
  "Return non-nil if the entry at point is overdue.

The SESSION can affect the definition of overdue."
  (let ((due (org-drill-entry-days-overdue session)))
    (and (not (null due))
         (not (cl-minusp due)))))

(defun org-drill-entry-new-p ()
  "Return non-nil if the entry at point is new."
  (and (org-drill-entry-p)
       (let ((item-time (org-get-scheduled-time (point))))
         (null item-time))))

(defun org-drill-entry-last-quality (&optional default)
  "Return the SM quality score for entry at point, or DEFAULT."
  (let ((quality (org-entry-get (point) "DRILL_LAST_QUALITY")))
    (if quality
        (string-to-number quality)
      default)))

(defun org-drill-entry-failure-count ()
  "Return the SM failure count for entry at point."
  (let ((quality (org-entry-get (point) "DRILL_FAILURE_COUNT")))
    (if quality
        (string-to-number quality)
      0)))

(defun org-drill-entry-average-quality (&optional default)
  "Return the SM average quality for entry at point."
  (let ((raw-value (org-entry-get (point) "DRILL_AVERAGE_QUALITY")))
    (if raw-value
        (string-to-number raw-value)
      (or default nil))))

(defun org-drill-entry-last-interval (&optional default)
  "Return the SM last interval for entry at point."
  (let ((raw-value (org-entry-get (point) "DRILL_LAST_INTERVAL")))
    (if raw-value
        (string-to-number raw-value)
      (or default 0))))

(defun org-drill-entry-repeats-since-fail (&optional default)
  "Return the SM repeats since fail for entry at point."
  (let ((raw-value (org-entry-get (point) "DRILL_REPEATS_SINCE_FAIL")))
    (if raw-value
        (string-to-number raw-value)
      (or default 0))))

(defun org-drill-entry-total-repeats (&optional default)
  "Return the SM total number of repeats for the entry at point."
  (let ((raw-value (org-entry-get (point) "DRILL_TOTAL_REPEATS")))
    (if raw-value
        (string-to-number raw-value)
      (or default 0))))

(defun org-drill-entry-ease (&optional default)
  "Return the SM ease for the entry at point."
  (let ((raw-value (org-entry-get (point) "DRILL_EASE")))
    (if raw-value
        (string-to-number raw-value)
      default)))

(defun org-drill-random-dispersal-factor ()
  "Return arandom number between 0.5 and 1.5.

This returns a strange random number distribution. See
http://www.supermemo.com/english/ol/sm5.htm for details."
  (let ((a 0.047)
        (b 0.092)
        (p (- (cl-random 1.0) 0.5)))
    (cl-flet ((sign (n)
                    (cond ((zerop n) 0)
                          ((cl-plusp n) 1)
                          (t -1))))
      (/ (+ 100 (* (* (/ -1 b) (log (- 1 (* (/ b a ) (abs p)))))
                   (sign p)))
         100.0))))

(defun org-drill-early-interval-factor (optimal-factor
                                                optimal-interval
                                                days-ahead)
  "Arguments:
- OPTIMAL-FACTOR: interval-factor if the item had been tested
exactly when it was supposed to be.
- OPTIMAL-INTERVAL: interval for next repetition (days) if the item had been
tested exactly when it was supposed to be.
- DAYS-AHEAD: how many days ahead of time the item was reviewed.

Returns an adjusted optimal factor which should be used to
calculate the next interval, instead of the optimal factor found
in the matrix."
  (let ((delta-ofmax (* (1- optimal-factor)
                    (/ (+ optimal-interval
                          (* 0.6 optimal-interval) -1) (1- optimal-interval)))))
    (- optimal-factor
       (* delta-ofmax (/ days-ahead (+ days-ahead (* 0.6 optimal-interval)))))))

(defun org-drill--safe-read-learn-data (learn-str)
  "Safely parse LEARN_DATA string, validating it contains only numbers.
Returns the parsed list or nil if invalid or unsafe."
  (when (and learn-str (stringp learn-str))
    (condition-case nil
        (let ((data (car (read-from-string learn-str))))
          ;; Validate it's a list of at least 3 elements
          (when (and (listp data)
                     (>= (length data) 3)
                     ;; Validate first 3 elements are numbers
                     (numberp (nth 0 data))
                     (numberp (nth 1 data))
                     (numberp (nth 2 data)))
            data))
      (error nil))))

;;; Card-state struct ---------------------------------------------------------
;;
;; ADR (2026-05-27): the schedulers and the item-data round-trip used to pass
;; the same recall fields around as long positional lists, in three different
;; orderings — `get-item-data'/`store-item-data' as
;; (last-interval repetitions failures total-repeats meanq ease), the schedulers
;; taking (last-interval n ef quality failures meanq total-repeats ...), and the
;; schedulers returning (next-interval repetitions ef failures meanq
;; total-repeats ...).  Three call sites re-ordered between the shapes by hand,
;; each re-order a latent positional bug, and a new scheduler had to match all
;; three layouts.
;;
;; `org-drill-card-state' bundles the shared stored fields under named slots so
;; the shape is explicit and the manual re-ordering goes away.  `next-interval'
;; and `quality' stay outside the struct: the former is scheduling output, the
;; latter is the rating just entered — neither is persisted recall state.
;;
;; Migration is staged: this struct lands first (inert), then the item-data
;; round-trip adopts it, then each scheduler in turn.
(cl-defstruct org-drill-card-state
  last-interval repetitions ease failures meanq total-repeats)

(defun org-drill-get-item-data ()
  "Return an `org-drill-card-state' with the stored recall data for the item at point.

Slots:
- LAST-INTERVAL is the interval in days that was used to schedule the item's
  current review date.
- REPETITIONS is the number of times the item has been successfully recalled
  without any failures. It is reset to 0 upon failure to recall the item.
- FAILURES is the total number of times the user has failed to recall the item.
- TOTAL-REPEATS includes both successful and unsuccessful repetitions.
- MEANQ is the mean quality of recall of the item over all its repetitions,
  successful and unsuccessful.
- EASE is a number reflecting how easy the item is to learn. Higher is easier."
  (let ((learn-str (org-entry-get (point) "LEARN_DATA"))
        (repeats (org-drill-entry-total-repeats :missing)))
    (cond
     (learn-str
      (let ((learn-data (org-drill--safe-read-learn-data learn-str)))
        (if learn-data
            (make-org-drill-card-state
             :last-interval (nth 0 learn-data)
             :repetitions (nth 1 learn-data)
             :failures (org-drill-entry-failure-count)
             :total-repeats (nth 1 learn-data)
             :meanq (org-drill-entry-last-quality)
             :ease (nth 2 learn-data))   ; EF
          ;; If LEARN_DATA is invalid/unsafe, fall through to next case
          (if (not (eql :missing repeats))
              (make-org-drill-card-state
               :last-interval (org-drill-entry-last-interval)
               :repetitions (org-drill-entry-repeats-since-fail)
               :failures (org-drill-entry-failure-count)
               :total-repeats (org-drill-entry-total-repeats)
               :meanq (org-drill-entry-average-quality)
               :ease (org-drill-entry-ease))
            ;; Virgin item
            (make-org-drill-card-state
             :last-interval 0 :repetitions 0 :failures 0
             :total-repeats 0 :meanq nil :ease nil)))))
     ((not (eql :missing repeats))
      (make-org-drill-card-state
       :last-interval (org-drill-entry-last-interval)
       :repetitions (org-drill-entry-repeats-since-fail)
       :failures (org-drill-entry-failure-count)
       :total-repeats (org-drill-entry-total-repeats)
       :meanq (org-drill-entry-average-quality)
       :ease (org-drill-entry-ease)))
     (t  ; virgin item
      (make-org-drill-card-state
       :last-interval 0 :repetitions 0 :failures 0
       :total-repeats 0 :meanq nil :ease nil)))))

(defun org-drill-store-item-data (state)
  "Store the recall data in STATE, an `org-drill-card-state', into the item at point.
STATE's LAST-INTERVAL slot holds the interval to schedule going forward (the
caller passes the scheduler's next-interval there)."
  (org-entry-delete (point) "LEARN_DATA")
  (org-set-property "DRILL_LAST_INTERVAL"
                    (number-to-string
                     (org-drill-round-float (org-drill-card-state-last-interval state) 4)))
  (org-set-property "DRILL_REPEATS_SINCE_FAIL"
                    (number-to-string (org-drill-card-state-repetitions state)))
  (org-set-property "DRILL_TOTAL_REPEATS"
                    (number-to-string (org-drill-card-state-total-repeats state)))
  (org-set-property "DRILL_FAILURE_COUNT"
                    (number-to-string (org-drill-card-state-failures state)))
  (org-set-property "DRILL_AVERAGE_QUALITY"
                    (number-to-string
                     (org-drill-round-float (org-drill-card-state-meanq state) 3)))
  (org-set-property "DRILL_EASE"
                    (number-to-string
                     (org-drill-round-float (org-drill-card-state-ease state) 3))))

;;; SM2 Algorithm =============================================================
(defun org-drill-determine-next-interval-sm2 (state quality)
  "Return the SM2 schedule for STATE after a review of recall QUALITY (0-5).

STATE is an `org-drill-card-state' carrying LAST-INTERVAL, REPETITIONS,
EASE (the \\='easiness factor\\='), FAILURES, MEANQ and TOTAL-REPEATS.
QUALITY is the rating just entered.

Returns a list:
  (INTERVAL REPEATS EF FAILURES MEAN TOTAL-REPEATS OFMATRIX), where:
- INTERVAL is the number of days until the item should next be reviewed
- REPEATS is incremented by 1.
- EF is modified based on the recall quality for the item.
- OF-MATRIX is not modified."
  (let ((last-interval (org-drill-card-state-last-interval state))
        (n (org-drill-card-state-repetitions state))
        (ef (org-drill-card-state-ease state))
        (failures (org-drill-card-state-failures state))
        (meanq (org-drill-card-state-meanq state))
        (total-repeats (org-drill-card-state-total-repeats state)))
  (if (zerop n) (setq n 1))
  (if (null ef) (setq ef 2.5))
  (setq meanq (if meanq
                  (/ (+ quality (* meanq total-repeats 1.0))
                     (1+ total-repeats))
                quality))
  (cl-assert (> n 0))
  (cl-assert (and (>= quality 0) (<= quality 5)))
  (if (org-drill--quality-failed-p quality)
      ;; When an item is failed, its interval is reset to 0,
      ;; but its EF is unchanged
      (list -1 1 ef (1+ failures) meanq (1+ total-repeats)
            org-drill-sm5-optimal-factor-matrix)
    ;; else:
    (let* ((next-ef (org-drill-modify-e-factor ef quality))
           (interval
            (cond
             ((<= n 1) 1)
             ((= n 2)
              (cond
               (org-drill-add-random-noise-to-intervals-p
                (cl-case quality
                  (5 6)
                  (4 4)
                  (3 3)
                  (2 1)
                  (t -1)))
               (t 6)))
             (t (* last-interval next-ef)))))
      (list (if org-drill-add-random-noise-to-intervals-p
                (+ last-interval (* (- interval last-interval)
                                    (org-drill-random-dispersal-factor)))
              interval)
            (1+ n)
            next-ef
            failures meanq (1+ total-repeats)
            org-drill-sm5-optimal-factor-matrix)))))

;;; SM5 Algorithm =============================================================
(defun org-drill-modify-e-factor (ef quality)
  "Return new e-factor given existing EF and QUALITY."
  (if (< ef 1.3)
      1.3
    (+ ef (- 0.1 (* (- 5 quality) (+ 0.08 (* (- 5 quality) 0.02)))))))

(defun org-drill-modify-of (of q fraction)
  "Return modify of."
  (let ((temp (* of (+ 0.72 (* q 0.07)))))
    (+ (* (- 1 fraction) of) (* fraction temp))))

(defun org-drill-set-optimal-factor (n ef of-matrix of)
  "Set the optimal factor."
  (let ((factors (assoc n of-matrix)))
    (if factors
	(let ((ef-of (assoc ef (cdr factors))))
	  (if ef-of
	      (setcdr ef-of of)
	    (push (cons ef of) (cdr factors))))
      (push (cons n (list (cons ef of))) of-matrix)))
  of-matrix)

(defun org-drill-initial-optimal-factor-sm5 (n ef)
  "Return initial optimal factor."
  (if (= 1 n)
      org-drill-sm5-initial-interval
    ef))

(defun org-drill-get-optimal-factor-sm5 (n ef of-matrix)
  "Return optimal factor."
  (let ((factors (assoc n of-matrix)))
    (or (and factors
             (let ((ef-of (assoc ef (cdr factors))))
               (and ef-of (cdr ef-of))))
        (org-drill-initial-optimal-factor-sm5 n ef))))

(defun org-drill-inter-repetition-interval-sm5 (last-interval n ef &optional of-matrix)
  "Return repetition interval."
  (let ((of (org-drill-get-optimal-factor-sm5 n ef (or of-matrix
                                             org-drill-sm5-optimal-factor-matrix))))
    (if (= 1 n)
        of
      (* of last-interval))))

(defun org-drill-determine-next-interval-sm5 (state quality of-matrix &optional delta-days)
  "Return the SM5 schedule for STATE after a review of recall QUALITY (0-5).

STATE is an `org-drill-card-state'.  OF-MATRIX is the optimal-factor
matrix.  DELTA-DAYS, when set, adjusts the interval for an early or late
repetition."
  (let ((last-interval (org-drill-card-state-last-interval state))
        (n (org-drill-card-state-repetitions state))
        (ef (org-drill-card-state-ease state))
        (failures (org-drill-card-state-failures state))
        (meanq (org-drill-card-state-meanq state))
        (total-repeats (org-drill-card-state-total-repeats state)))
  (if (zerop n) (setq n 1))
  (if (null ef) (setq ef 2.5))
  (cl-assert (> n 0))
  (cl-assert (and (>= quality 0) (<= quality 5)))
  (unless of-matrix
    (setq of-matrix org-drill-sm5-optimal-factor-matrix))
  (setq of-matrix (copy-tree of-matrix))

  (setq meanq (if meanq
                  (/ (+ quality (* meanq total-repeats 1.0))
                     (1+ total-repeats))
                quality))

  (let ((next-ef (org-drill-modify-e-factor ef quality))
        (old-ef ef)
        (new-of (org-drill-modify-of (org-drill-get-optimal-factor-sm5 n ef of-matrix)
                           quality org-drill-learn-fraction))
        (interval nil))
    (when (and org-drill-adjust-intervals-for-early-and-late-repetitions-p
               delta-days (cl-minusp delta-days))
      (setq new-of (org-drill-early-interval-factor
                    (org-drill-get-optimal-factor-sm5 n ef of-matrix)
                    (org-drill-inter-repetition-interval-sm5
                     last-interval n ef of-matrix)
                    delta-days)))
    (setq of-matrix
          (org-drill-set-optimal-factor n next-ef of-matrix
                              (org-drill-round-float new-of 3))) ; round OF to 3 d.p.
    (setq ef next-ef)
    (cond
     ;; "Failed" -- reset repetitions to 0,
     ((org-drill--quality-failed-p quality)
      (list -1 1 old-ef (1+ failures) meanq (1+ total-repeats)
            of-matrix))     ; Not clear if OF matrix is supposed to be
                                        ; preserved
     (t
      (setq interval (org-drill-inter-repetition-interval-sm5
                      last-interval n ef of-matrix))
      (if org-drill-add-random-noise-to-intervals-p
          (setq interval (* interval (org-drill-random-dispersal-factor))))
      (list interval
            (1+ n)
            ef
            failures
            meanq
            (1+ total-repeats)
            of-matrix))))))

;;; Simple8 Algorithm =========================================================
(defun org-drill-simple8-first-interval (failures)
  "Arguments:
- FAILURES: integer >= 0. The total number of times the item has
  been forgotten, ever.

Returns the optimal FIRST interval for an item which has previously been
forgotten on FAILURES occasions."
  ;; SM8 first-interval model.  An item never forgotten gets ~2.4849 days;
  ;; each prior failure shrinks that by a factor of e^-0.057 (~5.5% per
  ;; failure).  Both constants are the empirical fit from the SM8 algorithm
  ;; and are not meant to be tuned individually.
  (* 2.4849 (exp (* -0.057 failures))))

(defun org-drill-simple8-interval-factor (ease repetition)
  "Arguments:
- EASE: floating point number >= 1.2. Corresponds to `AF' in SM8 algorithm.
- REPETITION: the number of times the item has been tested.
1 is the first repetition (ie the second trial).
Returns:
The factor by which the last interval should be
multiplied to give the next interval. Corresponds to `RF' or `OF'."
  ;; 1.2 is the SM8 floor for the interval factor: it never drops below 1.2,
  ;; so intervals always grow.  The amount above the floor decays as
  ;; learn-fraction raised to log2(repetition), pulling later repetitions
  ;; toward the floor.
  (+ 1.2 (* (- ease 1.2) (expt org-drill-learn-fraction (log repetition 2)))))

(defun org-drill-simple8-quality->ease (quality)
  "Return theease (`AF' in the SM8 algorithm) which corresponds
to a mean item quality of QUALITY."
  ;; Quality (0-5 mean recall score) maps to ease/AF through this 4th-degree
  ;; polynomial, a least-squares fit carried over from the SM8 algorithm.
  ;; The five coefficients are empirical and only meaningful together, so
  ;; treat them as one fitted curve rather than independent knobs.
  (+ (* 0.0542 (expt quality 4))
     (* -0.4848 (expt quality 3))
     (* 1.4916 (expt quality 2))
     (* -1.2403 quality)
     1.4515))

(defun org-drill-determine-next-interval-simple8 (state quality &optional delta-days)
  "Return the Simple8 schedule for STATE after a review of recall QUALITY (0-5).

STATE is an `org-drill-card-state'.  DELTA-DAYS is how many days overdue
the item was when reviewed: 0 = on the scheduled day, +N = N days late,
-N = N days early.

Returns the new item data, as a list of 6 values:
- NEXT-INTERVAL
- REPEATS
- EASE
- FAILURES
- AVERAGE-QUALITY
- TOTAL-REPEATS.
See the documentation for `org-drill-get-item-data' for a description of these."
  (let ((last-interval (org-drill-card-state-last-interval state))
        (repeats (org-drill-card-state-repetitions state))
        (failures (org-drill-card-state-failures state))
        (meanq (org-drill-card-state-meanq state))
        (totaln (org-drill-card-state-total-repeats state)))
  (cl-assert (>= repeats 0))
  (cl-assert (and (>= quality 0) (<= quality 5)))
  (cl-assert (or (null meanq) (and (>= meanq 0) (<= meanq 5))))
  (let ((next-interval nil))
    (setf meanq (if meanq
                    (/ (+ quality (* meanq totaln 1.0)) (1+ totaln))
                  quality))
    (cond
     ((org-drill--quality-failed-p quality)
      (cl-incf failures)
      (cl-incf totaln)
      (setf repeats 0
            next-interval -1))
     ((or (zerop repeats)
          (zerop last-interval))
      (setf next-interval (org-drill-simple8-first-interval failures))
      (cl-incf repeats)
      (cl-incf totaln))
     (t
      (let* ((use-n
              (if (and
                   org-drill-adjust-intervals-for-early-and-late-repetitions-p
                   (numberp delta-days) (cl-plusp delta-days)
                   (cl-plusp last-interval))
                  (+ repeats (min 1 (/ delta-days last-interval 1.0)))
                repeats))
             (factor (org-drill-simple8-interval-factor
                      (org-drill-simple8-quality->ease meanq) use-n))
             (next-int (* last-interval factor)))
        (when (and org-drill-adjust-intervals-for-early-and-late-repetitions-p
                   (numberp delta-days) (cl-minusp delta-days))
          ;; The item was reviewed earlier than scheduled.
          (setf factor (org-drill-early-interval-factor
                        factor next-int (abs delta-days))
                next-int (* last-interval factor)))
        (setf next-interval next-int)
        (cl-incf repeats)
        (cl-incf totaln))))
    (list
     (if (and org-drill-add-random-noise-to-intervals-p
              (cl-plusp next-interval))
         (* next-interval (org-drill-random-dispersal-factor))
       next-interval)
     repeats
     (org-drill-simple8-quality->ease meanq)
     failures
     meanq
     totaln
     ))))

;;; Essentially copied from `org-learn.el', but modified to
;;; optionally call the SM2 or simple8 functions.
(defun org-drill-smart-reschedule (quality &optional days-ahead)
  "If DAYS-AHEAD is supplied it must be a positive integer. The
item will be scheduled exactly this many days into the future."
  (let ((delta-days (- (time-to-days (current-time))
                       (time-to-days (or (org-get-scheduled-time (point))
                                         (current-time)))))
        (ofmatrix org-drill-sm5-optimal-factor-matrix)
        ;; Entries can have weights, 1 by default. Intervals are divided by the
        ;; item's weight, so an item with a weight of 2 will have all intervals
        ;; halved, meaning you will end up reviewing it twice as often.
        ;; Useful for entries which randomly present any of several facts.
        (weight (org-entry-get (point) "DRILL_CARD_WEIGHT")))
    (if (stringp weight)
        (setq weight (string-to-number weight)))
    (let ((state (org-drill-get-item-data)))
      (cl-destructuring-bind (next-interval repetitions ease
                                         failures meanq total-repeats
                                         &optional new-ofmatrix)
          (cl-case org-drill-spaced-repetition-algorithm
            (sm5 (org-drill-determine-next-interval-sm5 state quality ofmatrix))
            (sm2 (org-drill-determine-next-interval-sm2 state quality))
            (simple8 (org-drill-determine-next-interval-simple8
                      state quality delta-days)))
        (if (numberp days-ahead)
            (setq next-interval days-ahead))

        (if (and (null days-ahead)
                 (numberp weight) (cl-plusp weight)
                 (not (cl-minusp next-interval)))
            (setq next-interval
                  (max 1.0 (+ (org-drill-card-state-last-interval state)
                              (/ (- next-interval
                                    (org-drill-card-state-last-interval state))
                                 weight)))))

        (org-drill-store-item-data
         (make-org-drill-card-state
          :last-interval next-interval :repetitions repetitions :ease ease
          :failures failures :meanq meanq :total-repeats total-repeats))

        (if (eql 'sm5 org-drill-spaced-repetition-algorithm)
            (setq org-drill-sm5-optimal-factor-matrix new-ofmatrix))

        ;; Pick the schedule based on days-ahead.  The optional arg may be
        ;; nil (callers omit it to mean "use the algorithm-computed
        ;; next-interval").  The original cond compared `(= 0 days-ahead)'
        ;; before any type guard, which crashed on nil.
        (cond
         ((and (numberp days-ahead) (= 0 days-ahead))
          (org-schedule '(4)))
         ((and (numberp days-ahead) (cl-minusp days-ahead))
          (org-schedule nil (current-time)))
         (t
          (let ((interval (if (numberp days-ahead) days-ahead next-interval)))
            (org-schedule nil (time-add (current-time)
                                        (days-to-time
                                         (round interval)))))))))))

(defun org-drill-hypothetical-next-review-date (quality)
  "Return aninteger representing the number of days into the future
that the current item would be scheduled, based on a recall quality
of QUALITY."
  (let ((weight (org-entry-get (point) "DRILL_CARD_WEIGHT"))
        (state (org-drill-get-item-data)))
      (if (stringp weight)
          (setq weight (string-to-number weight)))
      (cl-destructuring-bind (next-interval _repetitions _ease
                                         _failures _meanq _total-repeats
                                         &optional _ofmatrix)
          (cl-case org-drill-spaced-repetition-algorithm
            (sm5 (org-drill-determine-next-interval-sm5
                  state quality org-drill-sm5-optimal-factor-matrix))
            (sm2 (org-drill-determine-next-interval-sm2 state quality))
            (simple8 (org-drill-determine-next-interval-simple8
                      state quality)))
        (cond
         ((not (cl-plusp next-interval))
          0)
         ((and (numberp weight) (cl-plusp weight))
          (+ (org-drill-card-state-last-interval state)
             (max 1.0 (/ (- next-interval
                            (org-drill-card-state-last-interval state))
                         weight))))
         (t
          next-interval)))))

(defun org-drill-hypothetical-next-review-dates ()
  "Return hypothetical next review dates."
  (let ((intervals nil))
    (dotimes (quality 6)
      (push (max (or (car intervals) 0)
                 (org-drill-hypothetical-next-review-date quality))
            intervals))
    (reverse intervals)))

(defun org-drill--read-key-sequence (prompt)
  "Just like `read-key-sequence' but with input method turned off.

Uses `deactivate-input-method' / `activate-input-method' rather than
`set-input-method', because `(set-input-method nil)' clears
`default-input-method' as a side effect when no input method is
currently active (upstream issues #52 and #58)."
  (let ((old-input-method current-input-method))
    (unwind-protect
        (progn
          (when current-input-method
            (deactivate-input-method))
          (read-key-sequence prompt))
      (when old-input-method
        (activate-input-method old-input-method)))))

(defun org-drill--read-rating-key (typed-answer rating-help-block)
  "Read a rating key (0-5) or control key for a drill prompt.

Loops on `read-key-sequence' until the user presses 0..5, the
configured quit key, the configured edit key, or C-g.  Arrow / scroll
/ wheel events navigate the visible buffer.  The configured tags key
runs `org-set-tags-command'.  The configured help key flips the
prompt to include RATING-HELP-BLOCK above the standard prompt line.

Returns the chosen character.

TYPED-ANSWER, when non-nil, is rendered as a `Your answer: ...' line
above the prompt — used by the typed-answer flow to show the user
what they just typed.

RATING-HELP-BLOCK is the multi-line ratings explanation shown on
the help key.  It's a plain string (no trailing newline), with the
caller supplying the wording appropriate to its scheduler (SM,
Leitner, etc.).

Shared by `org-drill-reschedule' and `org-drill-leitner-rebox'."
  (let ((ch nil)
        (input nil)
        (typed-answer-statement (if typed-answer
                                    (format "Your answer: %s\n" typed-answer)
                                  ""))
        (key-prompt (format "(0-5, %c=help, %c=edit, %c=tags, %c=undo, %c=quit)"
                            org-drill--help-key
                            org-drill--edit-key
                            org-drill--tags-key
                            org-drill--undo-key
                            org-drill--quit-key)))
    (save-excursion
      (while (not (memq ch (list org-drill--quit-key
                                 org-drill--edit-key
                                 org-drill--undo-key
                                 7          ; C-g
                                 ?0 ?1 ?2 ?3 ?4 ?5)))
        (run-hooks 'org-drill-display-answer-hook)
        (setq input (org-drill--read-key-sequence
                     (if (eq ch org-drill--help-key)
                         (format "%s\n\n%sHow well did you do? %s"
                                 rating-help-block
                                 typed-answer-statement
                                 key-prompt)
                       (format "%sHow well did you do? %s"
                               typed-answer-statement key-prompt))))
        (cond
         ((stringp input)
          (setq ch (elt input 0)))
         ((and (vectorp input) (symbolp (elt input 0)))
          (cl-case (elt input 0)
            (up (ignore-errors (forward-line -1)))
            (down (ignore-errors (forward-line 1)))
            (left (ignore-errors (backward-char)))
            (right (ignore-errors (forward-char)))
            (prior (ignore-errors (scroll-down))) ; pgup
            (next (ignore-errors (scroll-up)))))  ; pgdn
         ((and (vectorp input) (listp (elt input 0))
               (eventp (elt input 0)))
          (cl-case (car (elt input 0))
            (wheel-up (ignore-errors (mwheel-scroll (elt input 0))))
            (wheel-down (ignore-errors (mwheel-scroll (elt input 0)))))))
        (if (eql ch org-drill--tags-key)
            (org-set-tags-command))))
    ch))

(defun org-drill--snapshot-entry-data ()
  "Capture the scheduling state of the entry at point for undo.
Returns (MARKER . DATA), where MARKER points at the entry heading and
DATA is an alist mapping each scheduling property (and the special
`scheduled' key) to its current value, or nil when unset.  Restore it
with `org-drill--restore-entry-data'."
  (cons (save-excursion (org-back-to-heading t) (point-marker))
        (cons (cons 'scheduled (org-entry-get (point) "SCHEDULED"))
              (mapcar (lambda (prop)
                        (cons prop (org-entry-get (point) prop)))
                      org-drill-scheduling-properties))))

(defun org-drill--restore-entry-data (snapshot)
  "Restore the entry scheduling state captured in SNAPSHOT.
A property absent at snapshot time is deleted; the SCHEDULED line is put
back, or removed if there was none."
  (org-with-point-at (car snapshot)
    (dolist (cell (cdr snapshot))
      (cond
       ((eq (car cell) 'scheduled)
        (if (cdr cell)
            (org-schedule nil (cdr cell))
          (org-schedule '(4))))
       ((cdr cell)
        (org-set-property (car cell) (cdr cell)))
       (t
        (org-delete-property (car cell)))))))

(defun org-drill--push-undo-snapshot (session)
  "Snapshot the entry at point onto SESSION's undo stack, capped at
`org-drill-undo-limit'."
  (push (org-drill--snapshot-entry-data) (oref session undo-stack))
  (when (> (length (oref session undo-stack)) org-drill-undo-limit)
    (setf (oref session undo-stack)
          (cl-subseq (oref session undo-stack) 0 org-drill-undo-limit))))

(defun org-drill-undo-last-rating (session)
  "Undo the most recent rating in SESSION.
Restore the card's pre-rating scheduling data, drop the recorded quality,
and re-queue the card so it is presented again.  Does nothing when there
is nothing to undo."
  (let ((snapshot (pop (oref session undo-stack))))
    (if (null snapshot)
        (message "Nothing to undo")
      (org-drill--restore-entry-data snapshot)
      (pop (oref session qualities))
      (push (car snapshot) (oref session again-entries))
      (message "Undid the last rating; that card will come around again"))))

(defun org-drill-reschedule (session)
  "Return qualityrating (0-5), or nil if the user quit."
  (let* ((next-review-dates (org-drill-hypothetical-next-review-dates))
         (rating-help (format "0-2 Means you have forgotten the item.
3-5 Means you have remembered the item.

0 - Completely forgot.
1 - Even after seeing the answer, it still took a bit to sink in.
2 - After seeing the answer, you remembered it.
3 - It took you awhile, but you finally remembered. (+%s days)
4 - After a little bit of thought you remembered. (+%s days)
5 - You remembered the item really easily. (+%s days)"
                              (round (nth 3 next-review-dates))
                              (round (nth 4 next-review-dates))
                              (round (nth 5 next-review-dates)))))
    (cl-block org-drill-reschedule
      ;; Loop so the undo key can take back the previous rating and then
      ;; return us to the prompt for the current card.
      (while t
        (let ((ch (org-drill--read-rating-key (oref session typed-answer)
                                              rating-help)))
          (cond
           ((eql ch org-drill--undo-key)
            (org-drill-undo-last-rating session))
           ((and (>= ch ?0) (<= ch ?5))
            (let ((quality (- ch ?0))
                  (failures (org-drill-entry-failure-count)))
              (unless (oref session cram-mode)
                ;; Snapshot the pre-rating state so this rating can be undone.
                (org-drill--push-undo-snapshot session)
                (save-excursion
                  (let ((quality (if (org-drill--entry-lapsed-p session) 2 quality)))
                    (org-drill-smart-reschedule quality
                                                (nth quality next-review-dates))))
                (push quality (oref session qualities))
                (cond
                 ((org-drill--quality-failed-p quality)
                  (when org-drill-leech-failure-threshold
                    (if (> (1+ failures) org-drill-leech-failure-threshold)
                        (org-toggle-tag "leech" 'on))))
                 (t
                  (let ((scheduled-time (org-get-scheduled-time (point))))
                    (when scheduled-time
                      (message "Next review in %d days"
                               (- (time-to-days scheduled-time)
                                  (time-to-days (current-time))))
                      (sit-for 0.5)))))
                (org-set-property "DRILL_LAST_QUALITY" (format "%d" quality))
                (org-set-property "DRILL_LAST_REVIEWED"
                                  (org-drill-time-to-inactive-org-timestamp (current-time))))
              (cl-return-from org-drill-reschedule quality)))
           ((eql ch org-drill--edit-key)
            (cl-return-from org-drill-reschedule 'edit))
           (t
            (cl-return-from org-drill-reschedule nil))))))))

(defun org-drill-hide-subheadings-if (test)
  "TEST is a function taking no arguments. TEST will be called for each
of the immediate subheadings of the current drill item, with the point
on the relevant subheading. TEST should return nil if the subheading is
to be revealed, non-nil if it is to be hidden.
Returns a list containing the position of each immediate subheading of
the current topic."
  (let ((drill-entry-level (org-current-level))
        (drill-sections nil))
    (org-fold-show-subtree)
    (save-excursion
      (org-map-entries
       (lambda ()
         (when (and (not (outline-invisible-p))
                    (> (org-current-level) drill-entry-level))
           (when (or (/= (org-current-level) (1+ drill-entry-level))
                        (funcall test))
             (outline-hide-subtree))
           (push (point) drill-sections)))
       t 'tree))
    (reverse drill-sections)))

(defun org-drill-hide-all-subheadings-except (heading-list)
  "Hide all subheadings except HEADING-LIST."
  (org-drill-hide-subheadings-if
   (lambda () (let ((drill-heading (org-get-heading t)))
           (not (member drill-heading heading-list))))))

(defun org-drill--quality-failed-p (quality)
  "Non-nil when QUALITY counts as a failure for scheduling purposes.
A failure rating sends the card back to box 1 (Leitner) or resets
the interval (SM*).  The threshold is `org-drill-failure-quality'."
  (<= quality org-drill-failure-quality))

(defun org-drill--maybe-prepend-leech-warning (prompt)
  "Prepend the leech warning to PROMPT when applicable.
The warning appears when `org-drill-leech-method' is `warn' and the
entry at point is a leech.  Otherwise PROMPT is returned unchanged.

Used by the three prompt builders (mini-buffer prompt, in-buffer
prompt, prompt-for-string) to share the same preamble."
  (if (and (eql 'warn org-drill-leech-method)
           (org-drill-entry-leech-p))
      (concat (propertize "!!! LEECH ITEM !!!
You seem to be having a lot of trouble memorising this item.
Consider reformulating the item to make it easier to remember.\n"
                          'face '(:foreground "red"))
              prompt)
    prompt))

(defcustom org-drill-show-outline-path-during-drill nil
  "When non-nil, show the card's outline path in the drill prompt.
The ancestor headings are joined with \" > \" and bracketed, e.g.
\"[Spanish > Greetings]\", and prepended to the prompt so you can see
where the current card sits in the deck.  Off by default."
  :group 'org-drill-display
  :type 'boolean)

(put 'org-drill-show-outline-path-during-drill 'safe-local-variable 'booleanp)

(defun org-drill--outline-path-string ()
  "Return the outline path of the entry at point, bracketed for the prompt.
Joins the ancestor headings with \" > \" and wraps them in brackets with a
trailing space, e.g. \"[Spanish > Greetings] \".  Returns the empty string
when the entry has no ancestors or point is not on a heading."
  (let ((path (ignore-errors (org-get-outline-path))))
    (if path
        (concat "[" (mapconcat #'identity path " > ") "] ")
      "")))

(defun org-drill--make-minibuffer-prompt (session prompt)
  "Make a mini-buffer for the SESSION, with PROMPT."
  (let ((status (cl-first (org-drill-entry-status session)))
        (mature-entry-count (+ (length (oref session young-mature-entries))
                               (length (oref session old-mature-entries))
                               (length (oref session overdue-entries)))))
    (format "%s %s %s %s %s %s"
            (propertize
             (char-to-string
              (cond
               ((eql status :failed) ?F)
               ((oref session cram-mode) ?C)
               (t
                (cl-case status
                  (:new ?N) (:young ?Y) (:old ?o) (:overdue ?!)
                  (t ??)))))
             'face `(:foreground
                     ,(cl-case status
                        (:new org-drill-new-count-color)
                        ((:young :old) org-drill-mature-count-color)
                        ((:overdue :failed) org-drill-failed-count-color)
                        (t org-drill-done-count-color))))
            (propertize
             (number-to-string (length (oref session done-entries)))
             'face `(:foreground ,org-drill-done-count-color)
             'help-echo "The number of items you have reviewed this session.")
            (propertize
             (number-to-string (+ (length (oref session again-entries))
                                  (length (oref session failed-entries))))
             'face `(:foreground ,org-drill-failed-count-color)
             'help-echo (concat "The number of items that you failed, "
                                "and need to review again."))
            (propertize
             (number-to-string mature-entry-count)
             'face `(:foreground ,org-drill-mature-count-color)
             'help-echo "The number of old items due for review.")
            (propertize
             (number-to-string (length (oref session new-entries)))
             'face `(:foreground ,org-drill-new-count-color)
             'help-echo (concat "The number of new items that you "
                                "have never reviewed."))
            (if org-drill-show-outline-path-during-drill
                (concat (org-drill--outline-path-string) prompt)
              prompt))))

(defun org-drill-presentation-prompt  (session &optional prompt)
  "Create a card prompt with a timer and user-specified menu.

Arguments:

PROMPT: A string that overrides the standard prompt.

RETURNS: An alist of the form ((<char> . <symbol>)...) where
         <char> is the character pressed and <symbol> is the
         returned value, which will normally be either a symbol,
         `t' or `nil'.

START-TIME: The time the card started to be displayed.  This
            defaults to (current-time), however, if the function
            is called multiple times from one card then it might
            be convenient to override this default.
"
  (if org-drill-presentation-prompt-with-typing
    (org-drill-presentation-prompt-in-buffer session prompt)
    (org-drill-presentation-prompt-in-mini-buffer session prompt)))

(defun org-drill-presentation-prompt-in-mini-buffer (session &optional prompt)
  "Prompt in the echo area for the user's response to the current card.
Show a running session timer, wait for a key, and handle the tags key by
editing tags and re-prompting.  Return nil if the user quit, `edit' or
`skip' for those keys, and t for any other key (reveal the answer).

PROMPT overrides the standard \"Press key for answer\" prompt."
  (let* ((item-start-time (current-time))
         (input nil)
         (ch nil)
         (prompt
          (or prompt
              (format (concat "Press key for answer, "
                              "%c=edit, %c=tags, %c=skip, %c=quit.")
                      org-drill--edit-key
                      org-drill--tags-key
                      org-drill--skip-key
                      org-drill--quit-key)))
         (full-prompt
          (org-drill--make-minibuffer-prompt session prompt)))
    (setq full-prompt (org-drill--maybe-prepend-leech-warning full-prompt))
    (while (memq ch '(nil org-drill--tags-key))
      (setq ch nil)
      (while (not (input-pending-p))
        (let ((elapsed (time-subtract (current-time) item-start-time)))
          (message (concat (if (>= (time-to-seconds elapsed) (* 60 60))
                               "++:++ "
                             (format-time-string "%M:%S " elapsed))
                           full-prompt))
          (sit-for 1)))
      (setq input (org-drill--read-key-sequence nil))
      (if (stringp input) (setq ch (elt input 0)))
      (if (eql ch org-drill--tags-key)
          (org-set-tags-command)))
    (cond
     ((eql ch org-drill--quit-key) nil)
     ((eql ch org-drill--edit-key) 'edit)
     ((eql ch org-drill--skip-key) 'skip)
     (t t))))

(defvar org-drill-presentation-timer nil
  "Timer for buffer-entry of answers")

(defvar org-drill-presentation-timer-calls 0
  "How many times the presentation timer has been called")

(defun org-drill-presentation-timer-cancel ()
  "Cancel the presentation timer."
  (when org-drill-presentation-timer
    (cancel-timer org-drill-presentation-timer))
  (setq org-drill-presentation-timer nil)
  (setq org-drill-presentation-timer-calls 0))

(defun org-drill-presentation-minibuffer-timer-function
    (item-start-time full-prompt)
  "Return prompt for mini-buffer in `org-drill-response-mode'."
  (let ((elapsed (time-subtract (current-time) item-start-time)))
    (message (concat (if (>= (time-to-seconds elapsed) (* 60 60))
                         "++:++ "
                       (format-time-string "%M:%S " elapsed))
                     full-prompt)))
  ;; if we have done it this many times, we probably want to stop
  (when (< 10 (cl-incf org-drill-presentation-timer-calls))
    (org-drill-presentation-timer-cancel)))

(define-derived-mode org-drill-response-mode nil "Org-Drill")
(define-key org-drill-response-mode-map [return] 'org-drill-response-rtn)
(define-key org-drill-response-mode-map (kbd "C-c C-q") 'org-drill-response-quit)
(define-key org-drill-response-mode-map (kbd "C-c C-e") 'org-drill-response-edit)
(define-key org-drill-response-mode-map (kbd "C-c C-s") 'org-drill-response-skip)
(define-key org-drill-response-mode-map (kbd "C-c C-t") 'org-drill-response-tags)

(defun org-drill-response-complete ()
  "Complete org-drill response mode."
  (kill-buffer (current-buffer))
  (exit-recursive-edit))

(defun org-drill-response-rtn ()
  "Exit response mode with return value."
  (interactive)
  (let ((session org-drill-current-session))
    (setf (oref session typed-answer) (buffer-string))
    (oset session exit-kind t)
    (org-drill-response-complete)))

(defun org-drill-response-quit ()
  "Exit response mode with quit."
  (interactive)
  (oset org-drill-current-session exit-kind 'quit)
  (org-drill-response-complete))

(defun org-drill-response-edit ()
  "Exit response mode with edit."
  (interactive)
  (oset org-drill-current-session exit-kind 'edit)
  (org-drill-response-complete))

(defun org-drill-response-skip ()
  "Exit response mode with skip."
  (interactive)
  (oset org-drill-current-session exit-kind 'skip)
  (org-drill-response-complete))

(defun org-drill-response-tags ()
  "Exit response mode with tags."
  (interactive)
  (oset org-drill-current-session exit-kind 'tags)
  (org-drill-response-complete))

(defun org-drill-response-get-buffer-create ()
  "Create a response buffer.

Propagates the caller's input method into the new buffer, but only when
one is actually active — otherwise `(set-input-method nil)' would clear
`default-input-method' as a side effect (upstream issues #52, #58)."
  (let ((local-current-input-method
         current-input-method))
    (with-current-buffer
        (get-buffer-create "*Org-Drill*")
      (erase-buffer)
      (org-drill-response-mode)
      (when local-current-input-method
        (activate-input-method local-current-input-method))
      (current-buffer))))

(defun org-drill-presentation-prompt-in-buffer (session &optional prompt)
  "Display drill for SESSION with PROMPT."
  (let* ((item-start-time (current-time))
         (prompt
          (or prompt
              (format (concat "Type answer then return, "
                              "C-c C-e=edit, C-c C-t=tags, C-c C-s=skip, C-c C-q=quit."))))
         (full-prompt
          (org-drill--make-minibuffer-prompt session prompt)))
    (setf (oref session drill-answer) nil)
    (setq full-prompt (org-drill--maybe-prepend-leech-warning full-prompt))
    (org-drill-presentation-timer-cancel)
    (setq org-drill-presentation-timer
          (run-with-idle-timer 1 t
                               #'org-drill-presentation-minibuffer-timer-function
                               item-start-time full-prompt)
          org-drill-presentation-timer-calls 0)
    (unwind-protect
        (save-window-excursion
          (let ((buf
                 (org-drill-response-get-buffer-create)))
            (select-window
             (display-buffer buf '(display-buffer-below-selected)))
            ;; Store the current session in a variable, so that it can
            ;; be picked up by the when we leave the buffer
            (setq-local org-drill-current-session session)
            (recursive-edit)
            (oref session exit-kind)))
      ;; Always cancel timer, even if recursive-edit exits abnormally
      (org-drill-presentation-timer-cancel))))

(cl-defun org-drill-presentation-prompt-for-string (session prompt)
  "Create a card prompt with a timer and user-specified menu.

Arguments:

PROMPT: A string that overrides the standard prompt.

START-TIME: The time the card started to be displayed.  This
            defaults to (current-time), however, if the function
            is called multiple times from one card then it might
            be convenient to override this default.
"
  (let* ((prompt
          (or prompt
              "Type your answer and press <Enter>: "))
         (full-prompt
          (org-drill--make-minibuffer-prompt session prompt)))
    (setq full-prompt (org-drill--maybe-prepend-leech-warning full-prompt))
    (setf (oref session drill-answer)
          (read-string full-prompt nil nil nil t))))

(defun org-drill-pos-in-regexp (pos regexp &optional nlines)
  "Return non-nil if POS is within REGEXP.

Normally only the current line is checked but NLINES can be checked instead.

If non-nil, returns (BEG . END) where beginning and end of the match are."
  (save-excursion
    (goto-char pos)
    (org-in-regexp regexp nlines)))

(defun org-drill-hide-region (beg end &optional text)
  "Hide the buffer region between BEG and END with an \\='invisible text\\='
visual overlay, or with the string TEXT if it is supplied."
  (let ((ovl (make-overlay beg end)))
    (overlay-put ovl 'category
                 'org-drill-hidden-text-overlay)
    (overlay-put ovl 'priority 9999)
    (when (stringp text)
      (overlay-put ovl 'invisible nil)
      (overlay-put ovl 'face 'default)
      (overlay-put ovl 'display text))))

(defun org-drill-hide-heading-at-point (&optional text)
  "Hide the heading at point."
  (unless (org-at-heading-p)
    (error "Point is not on a heading"))
  (save-excursion
    (let ((beg (point)))
      (end-of-line)
      (org-drill-hide-region beg (point) text))))

(defun org-drill-hide-comments ()
  "Hide comments."
  (save-excursion
    (while (re-search-forward "^#.*$" nil t)
      (org-drill-hide-region (match-beginning 0) (match-end 0)))))

(defun org-drill-hide-drawers ()
  "Hide all drawers in the current entry, including PROPERTIES drawer.
This is more reliable than `org-cycle-hide-drawers' for drill display.

Drawer-end was previously captured as the value of `(point)' after
the search — always a number, so the subsequent `(when drawer-end)'
guard was dead.  Capture the search result itself and gate on that,
otherwise a malformed drawer (no `:END:') ends up with a junk
overlay covering whatever range point happened to be at."
  (save-excursion
    (org-back-to-heading t)
    (let ((end (save-excursion (org-end-of-subtree t t))))
      (while (re-search-forward org-drawer-regexp end t)
        (let* ((drawer-start (match-beginning 0))
               (drawer-end (save-excursion
                             (when (re-search-forward
                                    "^[ \t]*:END:[ \t]*$" end t)
                               (point)))))
          (when drawer-end
            (org-drill-hide-region drawer-start drawer-end)))))))

(defun org-drill--setup-display ()
  "Set up display settings for drill session.
Saves current settings and applies drill-specific display preferences.
Records the current buffer so that `org-drill--restore-display' can
target it even if the user switches buffers mid-session."
  (setq org-drill--saved-display-buffer (current-buffer))

  ;; Save current text scale and apply new size (face is global).
  (when org-drill-text-size-during-session
    (setq org-drill--saved-text-scale
          (face-attribute 'default :height nil 'default))
    (set-face-attribute 'default nil :height (* org-drill-text-size-during-session 10)))

  ;; Save and enable variable-pitch mode (buffer-local minor mode).
  (when org-drill-use-variable-pitch
    (setq org-drill--saved-variable-pitch-mode
          (if (boundp 'variable-pitch-mode) variable-pitch-mode nil))
    (variable-pitch-mode 1))

  ;; Save and hide modeline (buffer-local).
  (when org-drill-hide-modeline-during-session
    (setq org-drill--saved-modeline-format mode-line-format)
    (setq-local mode-line-format nil)))

(defun org-drill--restore-display ()
  "Restore display settings after drill session ends.
Buffer-local state (mode-line, variable-pitch-mode) is restored in
the buffer the session originated from — not the current buffer at
restore time, which may differ if the user switched away."
  (let ((target (and (buffer-live-p org-drill--saved-display-buffer)
                     org-drill--saved-display-buffer)))
    ;; Restore text size (face is global — no buffer targeting needed).
    (when org-drill--saved-text-scale
      (set-face-attribute 'default nil :height org-drill--saved-text-scale)
      (setq org-drill--saved-text-scale nil))

    ;; Restore variable-pitch-mode in the original buffer.
    (when (and org-drill-use-variable-pitch
               (not (eq org-drill--saved-variable-pitch-mode 'unbound)))
      (when target
        (with-current-buffer target
          (variable-pitch-mode (if org-drill--saved-variable-pitch-mode 1 -1))))
      (setq org-drill--saved-variable-pitch-mode nil))

    ;; Restore modeline in the original buffer.
    (when org-drill--saved-modeline-format
      (when target
        (with-current-buffer target
          (setq-local mode-line-format org-drill--saved-modeline-format)))
      (setq org-drill--saved-modeline-format nil))

    (setq org-drill--saved-display-buffer nil)))

(defun org-drill-unhide-text ()
  "Unhide text."
  (save-excursion
    (dolist (ovl (overlays-in (point-min) (point-max)))
      (when (eql 'org-drill-hidden-text-overlay (overlay-get ovl 'category))
        (delete-overlay ovl)))))

(defun org-drill-hide-clozed-text ()
  "Hide clozed text."
  (save-excursion
    (while (re-search-forward org-drill-cloze-regexp nil t)
      ;; Don't hide:
      ;; - org links, partly because they might contain inline
      ;;   images which we want to keep visible.
      ;; - LaTeX math fragments
      ;; - the contents of SRC blocks
      (unless (save-match-data
                (or (org-drill-pos-in-regexp (match-beginning 0)
                                       org-link-bracket-re 1)
                    (org-in-src-block-p)
                    (org-inside-LaTeX-fragment-p)))
        (org-drill-hide-matched-cloze-text)))))

(defun org-drill-hide-matched-cloze-text ()
  "Hide the current match with a \\='cloze\\=' visual overlay."
  (let ((ovl (make-overlay (match-beginning 0) (match-end 0)))
        (hint-sep-pos (string-match-p (regexp-quote org-drill-hint-separator)
                                      (match-string 0))))
    (overlay-put ovl 'category
                 'org-drill-cloze-overlay-defaults)
    (overlay-put ovl 'priority 9999)
    (if org-drill-cloze-length-matches-hidden-text-p
        (overlay-put ovl 'display
                     (concat "[" (make-string (max 1 (- (length (match-string 0)) 2)) ?.) "]")))
    (when (and hint-sep-pos
               (> hint-sep-pos 1))
      (let ((hint (substring-no-properties
                   (match-string 0)
                   (+ hint-sep-pos (length org-drill-hint-separator))
                   (1- (length (match-string 0))))))
        (overlay-put
         ovl 'display
         ;; If hint is like `X...' then display [X...]
         ;; otherwise display [...X]
         (format "[%s%s%s]"
                 hint
                 (if (string-match-p (regexp-quote "...") hint) "" "...")
                 (if org-drill-cloze-length-matches-hidden-text-p
                     (make-string (max 0 (- (length (match-string 0))
                                            (length hint)
                                            (if (string-match-p (regexp-quote "...") hint) 0 3)
                                            2))
                                  ?.)
                   "")))))))

(defun org-drill-hide-cloze-hints ()
  "Hide cloze hints.

The cloze regex's hint group is `\\(\\|HINTSEP.+?\\)' — an empty
alternation — so it always participates in the match.  Check for an
empty match (start = end) rather than nil match-beginning, otherwise
this function creates a zero-width overlay for every hint-less cloze."
  (save-excursion
    (while (re-search-forward org-drill-cloze-regexp nil t)
      (unless (or (save-match-data
                    (org-drill-pos-in-regexp (match-beginning 0)
                                       org-link-bracket-re 1))
                  ;; Empty match → no hint present, nothing to hide.
                  (= (match-beginning 2) (match-end 2)))
        (org-drill-hide-region (match-beginning 2) (match-end 2))))))

(defmacro org-drill-with-replaced-entry-text (text &rest body)
  "During the execution of BODY, the entire text of the current entry is
concealed by an overlay that displays the string TEXT."
  `(progn
     (org-drill-replace-entry-text ,text)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unreplace-entry-text))))

(defmacro org-drill-with-replaced-entry-text-multi (replacements &rest body)
  "During the execution of BODY, the entire text of the current entry is
concealed by an overlay that displays the overlays in REPLACEMENTS."
  `(progn
     (org-drill-replace-entry-text ,replacements t)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unreplace-entry-text))))

(defun org-drill-replace-entry-text (text &optional multi-p)
  "Make an overlay that conceals the entire text of the item, not
including properties or the contents of subheadings. The overlay shows
the string TEXT.
If MULTI-P is non-nil, TEXT must be a list of values which are legal
for the `display' text property. The text of the item will be temporarily
replaced by all of these items, in the order in which they appear in
the list.
Note: does not actually alter the item."
  (cond
   ((and multi-p
         (listp text))
    (org-drill-replace-entry-text-multi text))
   (t
    (let ((ovl (make-overlay (point-min)
                             (save-excursion
                               (outline-next-heading)
                               (point)))))
      (overlay-put ovl 'priority 9999)
      (overlay-put ovl 'category
                   'org-drill-replaced-text-overlay)
      (overlay-put ovl 'display text)))))

(defun org-drill-unreplace-entry-text ()
  "Unreplace entry text."
  (save-excursion
    (dolist (ovl (overlays-in (point-min) (point-max)))
      (when (eql 'org-drill-replaced-text-overlay (overlay-get ovl 'category))
        (delete-overlay ovl)))))

(defun org-drill-replace-entry-text-multi (replacements)
  "Make overlays that conceal the entire text of the item, not
including properties or the contents of subheadings. The overlay shows
the string TEXT.
Note: does not actually alter the item."
  (let ((ovl nil)
        (p-min (point-min))
        (p-max (save-excursion
                 (outline-next-heading)
                 (point))))
    (cl-assert (>= (- p-max p-min) (length replacements)))
    (dotimes (i (length replacements))
      (setq ovl (make-overlay (+ p-min (* 2 i))
                              (if (= i (1- (length replacements)))
                                  p-max
                                (+ p-min (* 2 i) 1))))
      (overlay-put ovl 'priority 9999)
      (overlay-put ovl 'category
                   'org-drill-replaced-text-overlay)
      (overlay-put ovl 'display (nth i replacements)))))

(defmacro org-drill-with-replaced-entry-heading (heading &rest body)
  "Display HEADING in place of current entry heading, and execute BODY."
  `(progn
     (org-drill-replace-entry-heading ,heading)
     (unwind-protect
         (progn
           ,@body)
       (org-drill-unhide-text))))

(defun org-drill-replace-entry-heading (heading)
  "Make an overlay that conceals the heading of the item. The overlay shows
the string TEXT.
Note: does not actually alter the item."
  (org-drill-hide-heading-at-point heading))

(defun org-drill-unhide-clozed-text ()
  "Show clozed text."
  (save-excursion
    (dolist (ovl (overlays-in (point-min) (point-max)))
      (when (eql 'org-drill-cloze-overlay-defaults (overlay-get ovl 'category))
        (delete-overlay ovl)))))

(defcustom org-drill-entry-text-max-lines
  100
  "Maximum number of lines of an entry's body text org-drill collects.
Used by `org-drill-get-entry-text', for example when echoing the next
Leitner item.  Raise it for decks with very long card bodies."
  :group 'org-drill-session
  :type 'integer)

(defun org-drill-get-entry-text (&optional keep-properties-p)
  "Return the text of the current entry."
  (let ((text (org-agenda-get-some-entry-text
               (point-marker) org-drill-entry-text-max-lines)))
    (if keep-properties-p
        text
      (substring-no-properties text))))

(defun org-drill-entry-empty-p ()
  "Return non-nil if the current entry is empty.

The search bound covers the whole subtree, so an entry whose answer
lives inside a child sub-heading is correctly reported as non-empty
\(upstream issue #13).  `org-end-of-subtree' is used in place of
`outline-next-heading' because the latter lands on the first child
heading, which truncates the search range before the child's body."
  (save-excursion
    (org-back-to-heading t)
    (let ((lim (save-excursion
                 (org-end-of-subtree t t) (point))))
      (if (fboundp 'org-end-of-meta-data-and-drawers)
          (org-end-of-meta-data-and-drawers) ; function removed Feb 2015
        (org-end-of-meta-data t))
      (or (>= (point) lim)
          (null (re-search-forward "[[:graph:]]" lim t))))))

;;; Presentation functions ====================================================
;;
;; Each of these is called with point on topic heading.  Each needs to show the
;; topic in the form of a 'question' or with some information 'hidden', as
;; appropriate for the card type. The user should then be prompted to press a
;; key. The function should then reveal either the 'answer' or the entire
;; topic, and should return t if the user chose to see the answer and rate their
;; recall, nil if they chose to quit.
(defun org-drill-present-simple-card (session)
  "Present a simple card."
  (org-drill-with-card-display
    (org-drill-hide-all-subheadings-except nil)
    (org-drill--show-latex-fragments)  ; overlay all LaTeX fragments with images
    (ignore-errors
      (org-display-inline-images t))
    (org-drill-hide-drawers)
    (prog1 (org-drill-presentation-prompt session)
      (org-drill-hide-subheadings-if 'org-drill-entry-p))))

(defun org-drill-present-default-answer (session reschedule-fn)
  "Present a default answer.

SESSION is the current session.
RESCHEDULE-FN is the function to reschedule."
  (prog1 (cond
          ((oref session drill-answer)
           (org-drill-with-replaced-entry-text
            (format "\nAnswer:\n\n  %s\n" (oref session drill-answer))
            (funcall reschedule-fn session)
            ))
          (t
           (org-drill-hide-subheadings-if 'org-drill-entry-p)
           (org-drill-unhide-clozed-text)
           (org-drill--show-latex-fragments)
           (ignore-errors
             (org-display-inline-images t))
           (org-drill-hide-drawers)
           ;; LaTeX preview helpers require a window-system frame
           ;; (upstream issue #44).  Skip on TTY.
           (when (display-graphic-p)
             (org-clear-latex-preview)
             (save-excursion
               (org-mark-subtree)
               (let ((beg (region-beginning))
                     (end (region-end)))
                 (org--latex-preview-region beg end))
               (deactivate-mark)))
           (org-drill-with-hidden-cloze-hints
            (funcall reschedule-fn session))))))

(defun org-drill-present-simple-card-with-typed-answer (session)
  "Present a simple card with a typed answer."
  (org-drill-with-card-display
    (org-drill-hide-all-subheadings-except nil)
    (org-drill--show-latex-fragments)  ; overlay all LaTeX fragments with images
    (ignore-errors
      (org-display-inline-images t))
    (org-drill-hide-drawers)
    (prog1 (org-drill-presentation-prompt-for-string session nil)
      (org-drill-hide-subheadings-if 'org-drill-entry-p))))

(defun org-drill--show-latex-fragments ()
  "Show LaTeX fragments as inline images.
No-op on TTY frames — `org-latex-preview' requires a window system
and otherwise raises \"Window system frame should be used\" (upstream
issue #44, 2021).  The TTY user just doesn't see preview images,
which is the right behavior."
  (when (display-graphic-p)
    (org-clear-latex-preview)
    (org-latex-preview '(16))))

(defun org-drill-present-two-sided-card (session)
  "Present a two-sided card for SESSION.
Hide every subheading, reveal one of the first two at random as the
question side, prompt, then re-hide on the way out."
  (org-drill-with-card-display
    (let ((drill-sections (org-drill-hide-all-subheadings-except nil)))
      (when drill-sections
        (save-excursion
          (goto-char (nth (cl-random (min 2 (length drill-sections)))
                          drill-sections))
          (org-fold-show-subtree)))
      (org-drill--show-latex-fragments)
      (ignore-errors
        (org-display-inline-images t))
      (org-drill-hide-drawers)
      (prog1 (org-drill-presentation-prompt session)
        (org-drill-hide-subheadings-if 'org-drill-entry-p)))))

(defun org-drill-present-multi-sided-card (session)
  "Present a multi-sided card for SESSION.
Hide every subheading, reveal one at random as the question side,
prompt, then re-hide on the way out."
  (org-drill-with-card-display
    (let ((drill-sections (org-drill-hide-all-subheadings-except nil)))
      (when drill-sections
        (save-excursion
          (goto-char (nth (cl-random (length drill-sections)) drill-sections))
          (org-fold-show-subtree)))
      (org-drill--show-latex-fragments)
      (ignore-errors
        (org-display-inline-images t))
      (org-drill-hide-drawers)
      (prog1 (org-drill-presentation-prompt session)
        (org-drill-hide-subheadings-if 'org-drill-entry-p)))))

(defun org-drill--cloze-body-bounds ()
  "Return (BODY-START . ITEM-END) for the drill item at point.
BODY-START is the position just after the property drawer or metadata,
and ITEM-END is the start of the next heading."
  (let ((body-start (let ((prop-block (org-get-property-block)))
                      (if prop-block
                          (cdr prop-block)
                        (save-excursion
                          (org-back-to-heading t)
                          (if (fboundp 'org-end-of-meta-data-and-drawers)
                              (org-end-of-meta-data-and-drawers)
                            (org-end-of-meta-data t))
                          (point)))))
        (item-end (save-excursion (outline-next-heading) (point))))
    (cons body-start item-end)))

(defun org-drill--count-cloze-matches (body-start item-end)
  "Count cloze regions between BODY-START and ITEM-END.
Matches inside an org link or a LaTeX fragment are skipped, so the count
matches the indices `org-drill--hide-cloze-by-index' will hide."
  (let ((count 0))
    (save-excursion
      (goto-char body-start)
      (while (re-search-forward org-drill-cloze-regexp item-end t)
        (unless (save-match-data
                  (or (org-drill-pos-in-regexp (match-beginning 0)
                                               org-link-bracket-re 1)
                      (org-inside-LaTeX-fragment-p)))
          (cl-incf count))))
    count))

(defun org-drill--hide-cloze-by-index (body-start item-end indices)
  "Hide the cloze regions whose 1-based position is in INDICES.
Scans between BODY-START and ITEM-END with the same link/LaTeX skip as
`org-drill--count-cloze-matches', so the indices line up."
  (save-excursion
    (goto-char body-start)
    (let ((cnt 0))
      (while (re-search-forward org-drill-cloze-regexp item-end t)
        (unless (save-match-data
                  (or (org-drill-pos-in-regexp (match-beginning 0)
                                               org-link-bracket-re 1)
                      (org-inside-LaTeX-fragment-p)))
          (cl-incf cnt)
          (when (memq cnt indices)
            (org-drill-hide-matched-cloze-text)))))))

(defun org-drill-present-multicloze-hide-n (session
                                            number-to-hide
                                            &optional
                                            force-show-first
                                            force-show-last
                                            force-hide-first)
  "Hides NUMBER-TO-HIDE pieces of text that are marked for cloze deletion,
chosen at random.
If NUMBER-TO-HIDE is negative, show only (ABS NUMBER-TO-HIDE) pieces,
hiding all the rest.
If FORCE-HIDE-FIRST is non-nil, force the first piece of text to be one of
the hidden items.
If FORCE-SHOW-FIRST is non-nil, never hide the first piece of text.
If FORCE-SHOW-LAST is non-nil, never hide the last piece of text.
If the number of text pieces in the item is less than
NUMBER-TO-HIDE, then all text pieces will be hidden (except the first or last
items if FORCE-SHOW-FIRST or FORCE-SHOW-LAST is non-nil)."
  (when (and force-hide-first force-show-first)
    (error "FORCE-HIDE-FIRST and FORCE-SHOW-FIRST are mutually exclusive"))
  (org-drill-with-hidden-comments
   (org-drill-with-hidden-cloze-hints
    (org-drill-hide-all-subheadings-except nil)
    (let* ((bounds (org-drill--cloze-body-bounds))
           (body-start (car bounds))
           (item-end (cdr bounds))
           (match-count (org-drill--count-cloze-matches body-start item-end)))
      (when (cl-minusp number-to-hide)
        (setq number-to-hide (+ match-count number-to-hide)))
      (when (cl-plusp match-count)
        (let ((positions (org-drill-shuffle
                          (cl-loop for i from 1 to match-count collect i))))
          (when force-hide-first
            ;; Force 1 into the list, and to the front of it.
            (setq positions (cons 1 (remove 1 positions))))
          (when force-show-first
            (setq positions (remove 1 positions)))
          (when force-show-last
            (setq positions (remove match-count positions)))
          (org-drill--hide-cloze-by-index
           body-start item-end
           (cl-subseq positions 0 (min number-to-hide (length positions)))))))
    (org-drill--show-latex-fragments)
    (ignore-errors
      (org-display-inline-images t))
    (org-drill-hide-drawers)
    (prog1 (org-drill-presentation-prompt session)
      (org-drill-hide-subheadings-if 'org-drill-entry-p)
      (org-drill-unhide-clozed-text)))))

(defun org-drill-present-multicloze-hide-nth (session to-hide)
  "Hide the TO-HIDE'th piece of clozed text. 1 is the first piece. If
TO-HIDE is negative, count backwards, so -1 means the last item, -2
the second to last, etc."
  (org-drill-with-hidden-comments
   (org-drill-with-hidden-cloze-hints
    (org-drill-hide-all-subheadings-except nil)
    (let* ((bounds (org-drill--cloze-body-bounds))
           (body-start (car bounds))
           (item-end (cdr bounds))
           (match-count (org-drill--count-cloze-matches body-start item-end)))
      (when (cl-minusp to-hide)
        (setq to-hide (+ 1 to-hide match-count)))
      (when (and (cl-plusp match-count) (<= to-hide match-count))
        (org-drill--hide-cloze-by-index body-start item-end (list to-hide))))
    (org-drill--show-latex-fragments)
    (ignore-errors
      (org-display-inline-images t))
    (org-drill-hide-drawers)
    (prog1 (org-drill-presentation-prompt session)
      (org-drill-hide-subheadings-if 'org-drill-entry-p)
      (org-drill-unhide-clozed-text)))))

(defun org-drill-present-multicloze-hide1 (session)
  "Hides one of the pieces of text that are marked for cloze deletion,
chosen at random."
  (org-drill-present-multicloze-hide-n session 1))

(defun org-drill-present-multicloze-hide2 (session)
  "Hides two of the pieces of text that are marked for cloze deletion,
chosen at random."
  (org-drill-present-multicloze-hide-n session 2))

(defun org-drill-present-multicloze-hide-first (session)
  "Hides the first piece of text that is marked for cloze deletion."
  (org-drill-present-multicloze-hide-nth session 1))

(defun org-drill-present-multicloze-hide-last (session)
  "Hides the last piece of text that is marked for cloze deletion."
  (org-drill-present-multicloze-hide-nth session -1))

(defun org-drill-present-multicloze-hide1-firstmore (session)
  "Commonly, hides the FIRST piece of text that is marked for
cloze deletion. Uncommonly, hide one of the other pieces of text,
chosen at random.

The definitions of \\='commonly\\=' and \\='uncommonly\\=' are determined by
the value of `org-drill-cloze-text-weight'."
  ;; The 'firstmore' and 'lastmore' functions used to randomly choose whether
  ;; to hide the 'favoured' piece of text. However even when the chance of
  ;; hiding it was set quite high (80%), the outcome was too unpredictable over
  ;; the small number of repetitions where most learning takes place for each
  ;; item. In other words, the actual frequency during the first 10 repetitions
  ;; was often very different from 80%. Hence we use modulo instead.
  (cond
   ((null org-drill-cloze-text-weight)
    ;; Behave as hide1cloze
    (org-drill-present-multicloze-hide1 session))
   ((not (and (integerp org-drill-cloze-text-weight)
              (cl-plusp org-drill-cloze-text-weight)))
    (error "Illegal value for org-drill-cloze-text-weight: %S"
           org-drill-cloze-text-weight))
   ((zerop (mod (1+ (org-drill-entry-total-repeats 0))
                org-drill-cloze-text-weight))
    ;; Uncommonly, hide any item except the first
    (org-drill-present-multicloze-hide-n session 1 t))
   (t
    ;; Commonly, hide first item
    (org-drill-present-multicloze-hide-first session))))

(defun org-drill-present-multicloze-show1-lastmore (session)
  "Commonly, hides all pieces except the last. Uncommonly, shows
any random piece. The effect is similar to \\='show1cloze\\=' except
that the last item is much less likely to be the item that is
visible.

The definitions of \\='commonly\\=' and \\='uncommonly\\=' are determined by
the value of `org-drill-cloze-text-weight'."
  (cond
   ((null org-drill-cloze-text-weight)
    ;; Behave as show1cloze
    (org-drill-present-multicloze-show1 session))
   ((not (and (integerp org-drill-cloze-text-weight)
              (cl-plusp org-drill-cloze-text-weight)))
    (error "Illegal value for org-drill-cloze-text-weight: %S"
           org-drill-cloze-text-weight))
   ((zerop (mod (1+ (org-drill-entry-total-repeats 0))
                org-drill-cloze-text-weight))
    ;; Uncommonly, show any item except the last
    (org-drill-present-multicloze-hide-n session -1 nil nil t))
   (t
    ;; Commonly, show the LAST item
    (org-drill-present-multicloze-hide-n session -1 nil t))))

(defun org-drill-present-multicloze-show1-firstless (session)
  "Commonly, hides all pieces except one, where the shown piece
is guaranteed NOT to be the first piece. Uncommonly, shows any
random piece. The effect is similar to \\='show1cloze\\=' except that
the first item is much less likely to be the item that is
visible.

The definitions of \\='commonly\\=' and \\='uncommonly\\=' are determined by
the value of `org-drill-cloze-text-weight'."
  (cond
   ((null org-drill-cloze-text-weight)
    ;; Behave as show1cloze
    (org-drill-present-multicloze-show1 session))
   ((not (and (integerp org-drill-cloze-text-weight)
              (cl-plusp org-drill-cloze-text-weight)))
    (error "Illegal value for org-drill-cloze-text-weight: %S"
           org-drill-cloze-text-weight))
   ((zerop (mod (1+ (org-drill-entry-total-repeats 0))
                org-drill-cloze-text-weight))
    ;; Uncommonly, show the first item
    (org-drill-present-multicloze-hide-n session -1 t))
   (t
    ;; Commonly, show any item, except the first
    (org-drill-present-multicloze-hide-n session -1 nil nil t))))

(defun org-drill-present-multicloze-show1 (session)
  "Similar to `org-drill-present-multicloze-hide1', but hides all
the pieces of text that are marked for cloze deletion, except for one
piece which is chosen at random."
  (org-drill-present-multicloze-hide-n session -1))

(defun org-drill-present-multicloze-show2 (session)
  "Similar to `org-drill-present-multicloze-show1', but reveals two
pieces rather than one."
  (org-drill-present-multicloze-hide-n session -2))

(defun org-drill-present-card-using-text (session question &optional answer)
  "Present the string QUESTION as the only visible content of the card.
If ANSWER is supplied, set the session slot `drill-answer' to its value."
  (if answer (setf (oref session drill-answer) answer))
  (org-drill-with-hidden-comments
   (org-drill-with-replaced-entry-text
    (concat "\n" question)
    (org-drill-hide-all-subheadings-except nil)
    (org-drill-hide-drawers)
    (ignore-errors
      (org-display-inline-images t))
    (prog1 (org-drill-presentation-prompt session)
      (org-drill-hide-subheadings-if 'org-drill-entry-p)))))

(defun org-drill-entry (session)
  "Present the current topic for interactive review, as in `org-drill'.
Review will occur regardless of whether the topic is due for review or whether
it meets the definition of a \\='review topic\\=' used by `org-drill'.

Returns a quality rating from 0 to 5, or nil if the user quit, or the symbol
EDIT if the user chose to exit the drill and edit the current item. Choosing
the latter option leaves the drill session suspended; it can be resumed
later using `org-drill-resume'.

See `org-drill' for more details."
  (org-drill-entry-f session 'org-drill-reschedule))

(defun org-drill-card-tag-caller (item tag)
  "Call the ITEM-th hook function registered for TAG.
Looks TAG up in `org-drill-card-tags-alist' and calls the function at
position ITEM in that entry.  Does nothing if TAG has no entry."
  (funcall
   (or (nth item (assoc tag org-drill-card-tags-alist))
       'ignore)))

(defun org-drill--resolve-presenter (card-type)
  "Return (PRESENTATION-FN . ANSWER-FN) for CARD-TYPE.
Looks CARD-TYPE up in `org-drill-card-type-alist'.  The mapped value is
either a bare presentation function or a list (PRESENTATION-FN ANSWER-FN
...).  ANSWER-FN defaults to `org-drill-present-default-answer'.
PRESENTATION-FN is nil when CARD-TYPE is unrecognised."
  (let ((entry (cdr (assoc card-type org-drill-card-type-alist))))
    (if (listp entry)
        (cons (cl-first entry)
              (or (cl-second entry) 'org-drill-present-default-answer))
      (cons entry 'org-drill-present-default-answer))))

(defun org-drill--classify-presentation-result (cont)
  "Classify CONT, the value returned by a card's presentation function.
Returns `quit' (nil — the user quit), `edit', `skip', or `answer' (any
other non-nil value means reveal the answer)."
  (cond
   ((not cont) 'quit)
   ((eql cont 'edit) 'edit)
   ((eql cont 'skip) 'skip)
   (t 'answer)))

(defun org-drill-entry-f (session complete-func)
  "Present the drill entry at point for SESSION and return its outcome.
Resolve the card's presenter from its DRILL_CARD_TYPE, run it, and
dispatch on the result: nil (quit), `edit', `skip', or otherwise reveal
the answer and call COMPLETE-FUNC (typically a reschedule function).
Returns the value the answer presenter returned, or `skip'/`edit'/nil."
  (org-drill-goto-drill-entry-heading)
  (let ((card-type (org-entry-get (point) "DRILL_CARD_TYPE" t)))
    ;; fontification functions in `outline-view-change-hook' (obsolete in Emacs 29.1)
    ;; can cause big slowdowns, so we no longer bind it since we require modern Emacs
    (setf (oref session drill-answer) nil)
    (org-save-outline-visibility t
      (save-restriction
        (org-narrow-to-subtree)
        (org-fold-show-subtree)
        (org-drill-hide-drawers)
        (cl-destructuring-bind (presentation-fn . answer-fn)
            (org-drill--resolve-presenter card-type)
          (let* ((tags (org-get-tags))
                 (rtn
                  (if (null presentation-fn)
                      (progn
                        (message "%s:%d: Unrecognised card type '%s', skipping..."
                                 (buffer-name) (point) card-type)
                        (sit-for 0.5)
                        'skip)
                    (mapc (apply-partially 'org-drill-card-tag-caller 1) tags)
                    (cl-case (org-drill--classify-presentation-result
                              (funcall presentation-fn session))
                      (quit (message "Quit") nil)
                      (edit 'edit)
                      (skip 'skip)
                      (answer
                       (save-excursion
                         (mapc (apply-partially 'org-drill-card-tag-caller 2) tags)
                         (funcall answer-fn session complete-func)))))))
            (mapc (apply-partially 'org-drill-card-tag-caller 3) tags)
            (cl-incf org-drill-cards-in-this-emacs)
            rtn))))))

(defun org-drill-entries-pending-p (session)
  "Return non-nil if SESSION still has entries left to drill.
True when an item is in progress or queued, unless the session's
item-count or duration limit has been reached.

When `org-drill-on-timeout-action' is `discard-current' and the
duration limit has been reached, the in-progress card and the
again-queue stop counting as pending, so the session ends now
rather than finishing them."
  (unless (and (eq org-drill-on-timeout-action 'discard-current)
               (org-drill-maximum-duration-reached-p session))
    (or (oref session again-entries)
        (oref session current-item)
        (and (not (org-drill-maximum-item-count-reached-p session))
             (not (org-drill-maximum-duration-reached-p session))
             (or (oref session new-entries)
                 (oref session failed-entries)
                 (oref session young-mature-entries)
                 (oref session old-mature-entries)
                 (oref session overdue-entries)
                 (oref session again-entries))))))

(defun org-drill-pending-entry-count (session)
  "Return the number of entries still queued in SESSION.
Counts the in-progress item plus every queue (new, failed, young,
old, overdue, again)."
  (+ (if (markerp (oref session current-item)) 1 0)
     (length (oref session new-entries))
     (length (oref session failed-entries))
     (length (oref session young-mature-entries))
     (length (oref session old-mature-entries))
     (length (oref session overdue-entries))
     (length (oref session again-entries))))

(defun org-drill-maximum-duration-reached-p (session)
  "Return trueif the current drill session has continued past its
maximum duration."
  (and org-drill-maximum-duration
       (not (oref session cram-mode))
       (oref session start-time)
       (> (- (float-time (current-time))
             (oref session start-time))
          (* org-drill-maximum-duration 60))))

(defun org-drill-maximum-item-count-reached-p (session)
  "Return trueif the current drill session has reached the
maximum number of items."
  (and org-drill-maximum-items-per-session
       (not (oref session cram-mode))
       (>= (if org-drill-item-count-includes-failed-items-p
               (+ (length (oref session done-entries))
                  (length (oref session again-entries)))
             (length (oref session done-entries)))
          org-drill-maximum-items-per-session)))

(defun org-drill-pop-next-pending-entry (session)
  "Remove and return the next entry to drill from SESSION.
Picks by priority: failed items first, then overdue, young-mature,
new and old-mature, and finally the again queue.  Returns a marker, or
nil when nothing is pending."
  (cl-block org-drill-pop-next-pending-entry
    (let ((m nil)
          (attempts 0)
          (max-attempts 1000))  ; Safety limit to prevent infinite loop
      (while (and (or (null m)
                      (not (org-drill-entry-p m)))
                  (< attempts max-attempts))
        (setq
         m
         (cond
          ;; First priority is items we failed in a prior session.
          ((and (oref session failed-entries)
                (not (org-drill-maximum-item-count-reached-p session))
                (not (org-drill-maximum-duration-reached-p session)))
           (org-drill-pop-random (oref session failed-entries)))
          ;; Next priority is overdue items.
          ((and (oref session overdue-entries)
                (not (org-drill-maximum-item-count-reached-p session))
                (not (org-drill-maximum-duration-reached-p session)))
           ;; We use `pop', not `pop-random', because we have already
           ;; sorted overdue items into a random order which takes
           ;; number of days overdue into account.
           (pop (oref session overdue-entries)))
          ;; Next priority is 'young' items.
          ((and (oref session young-mature-entries)
                (not (org-drill-maximum-item-count-reached-p session))
                (not (org-drill-maximum-duration-reached-p session)))
           (org-drill-pop-random (oref session young-mature-entries)))
          ;; Next priority is newly added items, and older entries.
          ;; We pool these into a single group.
          ((and (or (oref session new-entries)
                    (oref session old-mature-entries))
                (not (org-drill-maximum-item-count-reached-p session))
                (not (org-drill-maximum-duration-reached-p session)))
           (cond
            ((< (cl-random (+ (length (oref session new-entries))
                              (length (oref session old-mature-entries))))
                (length (oref session new-entries)))
             (org-drill-pop-random (oref session new-entries)))
            (t
             (org-drill-pop-random (oref session old-mature-entries)))))
          ;; After all the above are done, last priority is items
          ;; that were failed earlier THIS SESSION.
          ((oref session again-entries)
           (pop (oref session again-entries)))
          (t                            ; nothing left -- return nil
           (cl-return-from org-drill-pop-next-pending-entry nil))))
        (cl-incf attempts))
      ;; If we exhausted attempts trying to find a valid entry, return nil
      (if (>= attempts max-attempts)
          nil
        m))))

(defun org-drill--route-rating-result (session m result)
  "Route RESULT (return value of `org-drill-entry') for marker M.
Updates SESSION queues based on what the user did and returns one of:

  `quit'  — user quit; caller should exit the loop (and stash :quit)
  `edit'  — user wants to edit; caller should exit (and stash marker)
  `skip'  — skip this item, continue the loop
  `next'  — rating was recorded into again- or done-entries, continue"
  (cond
   ((null result)
    (message "Quit")
    (setf (oref session end-pos) :quit)
    'quit)
   ((eql result 'edit)
    (setf (oref session end-pos) (point-marker))
    'edit)
   ((eql result 'skip)
    (setf (oref session current-item) nil)
    'skip)
   (t
    (cond
     ((org-drill--quality-failed-p result)
      (when (oref session again-entries)
        (setf (oref session again-entries)
              (org-drill-shuffle (oref session again-entries))))
      (org-drill-push-end m (oref session again-entries)))
     (t
      (push m (oref session done-entries))))
    (setf (oref session current-item) nil)
    'next)))

(defun org-drill--pick-next-marker (session resuming-p)
  "Return the marker to drill next, and whether RESUMING-P stays true.
Returns a cons (M . RESUMING-P').  M is nil if no marker is available."
  (cond
   ((or (not resuming-p)
        (null (oref session current-item))
        (not (org-drill-entry-p (oref session current-item))))
    (cons (org-drill-pop-next-pending-entry session) resuming-p))
   (t                                   ; resuming a suspended session
    (cons (oref session current-item) nil))))

(defun org-drill-entries (session &optional resuming-p)
  "Return nil,t, or a list of markers representing entries that were
\\='failed\\=' and need to be presented again before the session ends.

RESUMING-P is true if we are resuming a suspended drill session."
  (cl-block org-drill-entries
    (while (org-drill-entries-pending-p session)
      (cl-destructuring-bind (m . next-resuming-p)
          (org-drill--pick-next-marker session resuming-p)
        (setq resuming-p next-resuming-p)
        (setf (oref session current-item) m)
        (unless m
          (error "Unexpectedly ran out of pending drill items"))
        (save-excursion
          (org-drill-goto-entry m)
          (cond
           ((not (org-at-heading-p))
            (error "Not at heading for entry %s" m))
           ((not (org-drill-entry-due-p session))
            (message "Entry no longer due, skipping...")
            (sit-for 0.3))
           (t
            (org-fold-show-entry)
            (when (memq (org-drill--route-rating-result
                         session m (org-drill-entry session))
                        '(quit edit))
              (cl-return-from org-drill-entries nil)))))))))

(defun org-drill--quality-percent (q qualities)
  "Percentage of QUALITIES equal to Q, rounded to integer.
Returns 0 when QUALITIES is empty (instead of dividing by zero)."
  (round (* 100 (cl-count q qualities)) (max 1 (length qualities))))

(defun org-drill--queue-tag (count label face-color)
  "Build a propertized \"COUNT LABEL\" string for the final-report queue summary."
  (propertize (format "%d %s" count label)
              'face `(:foreground ,face-color)))

(defun org-drill--build-final-report-summary (session pass-percent qualities)
  "Build the main `Session finished' message for SESSION.
PASS-PERCENT and QUALITIES come from the caller to avoid recomputing."
  (format
   "%d items reviewed. Session duration %s.
Recall of reviewed items:
 Excellent (5):     %3d%%   |   Near miss (2):      %3d%%
 Good (4):          %3d%%   |   Failure (1):        %3d%%
 Hard (3):          %3d%%   |   Abject failure (0): %3d%%

You successfully recalled %d%% of reviewed items (quality > %s)
%d/%d items still await review (%s, %s, %s, %s, %s).
Tomorrow, %d more items will become due for review.
Session finished. Press a key to continue..."
   (length (oref session done-entries))
   (format-seconds "%h:%.2m:%.2s"
                   (- (float-time (current-time))
                      (oref session start-time)))
   (org-drill--quality-percent 5 qualities)
   (org-drill--quality-percent 2 qualities)
   (org-drill--quality-percent 4 qualities)
   (org-drill--quality-percent 1 qualities)
   (org-drill--quality-percent 3 qualities)
   (org-drill--quality-percent 0 qualities)
   pass-percent
   org-drill-failure-quality
   (org-drill-pending-entry-count session)
   (+ (org-drill-pending-entry-count session)
      (oref session dormant-entry-count))
   (org-drill--queue-tag (+ (length (oref session failed-entries))
                            (length (oref session again-entries)))
                         "failed" org-drill-failed-count-color)
   (org-drill--queue-tag (length (oref session overdue-entries))
                         "overdue" org-drill-failed-count-color)
   (org-drill--queue-tag (length (oref session new-entries))
                         "new" org-drill-new-count-color)
   (org-drill--queue-tag (length (oref session young-mature-entries))
                         "young" org-drill-mature-count-color)
   (org-drill--queue-tag (length (oref session old-mature-entries))
                         "old" org-drill-mature-count-color)
   (oref session due-tomorrow-count)))

(defun org-drill--build-low-pass-warning (session pass-percent)
  "Build the WARNING prompt shown when PASS-PERCENT is below threshold."
  (format
   "%s
You failed %d%% of the items you reviewed during this session.
%d (%d%%) of all items scanned were overdue.

Are you keeping up with your items, and reviewing them
when they are scheduled? If so, you may want to consider
lowering the value of `org-drill-learn-fraction' slightly in
order to make items appear more frequently over time."
   (propertize "WARNING!" 'face 'org-warning)
   (- 100 pass-percent)
   (oref session overdue-entry-count)
   ;; Divisor is wrapped in (max 1 ...) — both counts are zero in
   ;; degenerate scopes (cram with nothing collected, pure-failure
   ;; session on empty queues), and (/ ... 0) raised arith-error
   ;; before the warning could even render.
   (round (* 100 (oref session overdue-entry-count))
          (max 1 (+ (oref session dormant-entry-count)
                    (oref session due-entry-count))))))

(defun org-drill--compute-pass-percent (qualities)
  "Return the pass percentage for QUALITIES — count above
`org-drill-failure-quality' over total, rounded.  Empty QUALITIES yields 0
(via `(max 1 ...)' on the denominator).  Single source of truth shared by
the end-of-session report and the dashboard session record."
  (round (* 100 (cl-count-if (lambda (qual)
                               (> qual org-drill-failure-quality))
                             qualities))
         (max 1 (length qualities))))

(defun org-drill-final-report (session)
  "Display the end-of-session summary for SESSION.
Reports how many items were reviewed, the pass percentage, and the
new/mature/failed counts."
  (let* ((qualities (oref session qualities))
         (pass-percent (org-drill--compute-pass-percent qualities))
         (prompt (org-drill--build-final-report-summary
                  session pass-percent qualities))
         (max-mini-window-height 0.6))
    (while (not (input-pending-p))
      (message "%s" prompt)
      (sit-for 0.5))
    (read-char-exclusive)
    (when (and qualities
               (< pass-percent (- 100 org-drill-forgetting-index)))
      (read-char-exclusive
       (org-drill--build-low-pass-warning session pass-percent)))))

(defun org-drill-session-record-from-session (session start-time end-time)
  "Build an `org-drill-session-record' from SESSION, START-TIME, END-TIME.

START-TIME and END-TIME are floats (e.g. from `float-time').  The
record's pass-percent comes from `org-drill--compute-pass-percent', the
same helper `org-drill-final-report' uses, so the two stay in lockstep.
Scope and algorithm are read from the session's start-of-session
captures (`scope-at-start' / `algorithm-at-start') so a mid-session
defcustom change doesn't misrepresent the recorded session."
  (let ((qualities (oref session qualities)))
    (make-org-drill-session-record
     :start-time start-time
     :end-time end-time
     :scope (oref session scope-at-start)
     :algorithm (oref session algorithm-at-start)
     :qualities (apply #'vector qualities)
     :pass-percent (org-drill--compute-pass-percent qualities)
     :new-count (length (oref session new-entries))
     :mature-count (+ (length (oref session young-mature-entries))
                      (length (oref session old-mature-entries)))
     :failed-count (length (oref session failed-entries))
     :cram-mode (oref session cram-mode))))

(defun org-drill-record-session (session start-time end-time)
  "Append a record for SESSION to `org-drill-session-log' and persist.
START-TIME and END-TIME are floats.  The new record lands at the head
of the log (newest first), then `persist-save' commits the log to disk."
  (push (org-drill-session-record-from-session session start-time end-time)
        org-drill-session-log)
  (persist-save 'org-drill-session-log))

(defun org-drill-free-markers (session markers)
  "MARKERS is a list of markers, all of which will be freed (set to
point nowhere). Alternatively, MARKERS can be \\='t\\=', in which case
all the markers used by Org-Drill will be freed."
  (dolist (marker (if (eql t markers)
                      (append  (oref session done-entries)
                               (oref session new-entries)
                               (oref session failed-entries)
                               (oref session again-entries)
                               (oref session overdue-entries)
                               (oref session young-mature-entries)
                               (oref session old-mature-entries))
                    markers))
    (set-marker marker nil)))

;;; overdue-data is a list of entries, each entry has the form (POS DUE AGE)
;;; where POS is a marker pointing to the start of the entry, and
;;; DUE is a number indicating how many days ago the entry was due.
;;; AGE is the number of days elapsed since item creation (nil if unknown).
;;;
;;; Sort order:
;;;   1. Split by DUE: entries with DUE > org-drill-lapse-threshold-days
;;;      are "lapsed", the rest are "not-lapsed".  This matches the gate
;;;      in `org-drill--entry-lapsed-p'.
;;;   2. Not-lapsed are sorted by DUE descending (most-overdue first).
;;;   3. Lapsed are appended after, sorted by AGE descending (oldest first).
(defun org-drill-order-overdue-entries (session)
  "Order SESSION's overdue entries into the overdue queue.
Splits off entries lapsed past `org-drill-lapse-threshold-days' (when
`org-drill--lapse-very-overdue-entries-p' is set) and sorts the rest by
how overdue they are."
  (let* ((lapsed-days (if org-drill--lapse-very-overdue-entries-p
                          org-drill-lapse-threshold-days
                          most-positive-fixnum))
         (not-lapsed (cl-remove-if (lambda (entry) (> (or (cl-second entry) 0) lapsed-days))
                                   (oref session overdue-data)))
         (lapsed (cl-remove-if-not
                  (lambda (entry) (> (or (cl-second entry) 0)
                                     lapsed-days))
                  (oref session overdue-data))))
    (setf (oref session overdue-entries)
         (mapcar 'cl-first
                  (append
                   (sort (org-drill-shuffle not-lapsed)
                         (lambda (entry other) (> (cl-second entry) (cl-second other))))
                   (sort lapsed
                         (lambda (entry other) (> (cl-third entry) (cl-third other)))))))))

(defun org-drill--entry-lapsed-p (session)
  "Return non-nil if the entry at point is lapsed (very overdue) in SESSION.
Only when `org-drill--lapse-very-overdue-entries-p' is set and the entry
is more than `org-drill-lapse-threshold-days' overdue."
  (and org-drill--lapse-very-overdue-entries-p
       (> (or (org-drill-entry-days-overdue session) 0)
          org-drill-lapse-threshold-days)))

(defun org-drill-entry-days-since-creation (session &optional use-last-interval-p)
  "If USE-LAST-INTERVAL-P is non-nil, and DATE_ADDED is missing, use the
value of DRILL_LAST_INTERVAL instead (as the item's age must be at least
that many days)."
  (let ((timestamp (org-entry-get (point) "DATE_ADDED")))
    (cond
     (timestamp
      (condition-case nil
          (- (org-time-stamp-to-now timestamp))
        (error nil)))
     (use-last-interval-p
      (+ (or (org-drill-entry-days-overdue session) 0)
         (string-to-number (or (org-entry-get (point) "DRILL_LAST_INTERVAL") "0"))))
     (t nil))))

(defun org-drill--entry-empty-and-not-empty-friendly-p ()
  "Non-nil when the entry has an empty body AND its card type doesn't
treat empty bodies as meaningful.

A card type that wants empty bodies is one whose entry in
`org-drill-card-type-alist' has a non-nil third element (the
DRILL-EMPTY-P slot).  When `org-drill-treat-headline-as-card-p' is
non-nil, no empty entry is treated as skippable — the heading itself
is the card."
  (and (not org-drill-treat-headline-as-card-p)
       (org-drill-entry-empty-p)
       (let* ((card-type (org-entry-get (point) "DRILL_CARD_TYPE" nil))
              (card-def (cdr (assoc card-type org-drill-card-type-alist))))
         (or (null card-type)
             (not (cl-third card-def))))))

(defun org-drill--classify-status (session due last-int)
  "Return the status keyword for the entry at point.
SESSION is needed for cram-mode and overdue-factor.  DUE is the
days-overdue value already computed by `entry-days-overdue', and
LAST-INT is the entry's last interval (defaulted to 1)."
  (cond
   ((not (org-drill-entry-p)) nil)
   ((org-drill--entry-empty-and-not-empty-friendly-p) nil)
   ((null due) :unscheduled)            ; usually a skipped leech
   ((cl-minusp due) :future)
   ;; Mature entries that were failed last time are :failed regardless
   ;; of how young, old, or overdue they are.  The 9999 default keeps
   ;; never-rated cards out of the failure bucket.
   ((org-drill--quality-failed-p (org-drill-entry-last-quality 9999)) :failed)
   ((org-drill-entry-new-p) :new)
   ;; Overdue overrides the young/old distinction.
   ((org-drill-entry-overdue-p session due last-int) :overdue)
   ((<= (org-drill-entry-last-interval 9999) org-drill-days-before-old) :young)
   (t :old)))

(defun org-drill-entry-status (session)
  "Return alist (STATUS DUE AGE) where DUE is the number of days overdue,
zero being due today, -1 being scheduled 1 day in the future.
AGE is the number of days elapsed since the item was created (nil if unknown).
STATUS is one of the following values:
- nil, if the item is not a drill entry, or has an empty body
- :unscheduled
- :future
- :new
- :failed
- :overdue
- :young
- :old
"
  (save-excursion
    (unless (org-at-heading-p)
      (org-back-to-heading))
    (let ((due (org-drill-entry-days-overdue session))
          (age (org-drill-entry-days-since-creation session t))
          (last-int (org-drill-entry-last-interval 1)))
      (list (org-drill--classify-status session due last-int) due age))))

(defun org-drill-progress-message (collected scanned)
  "Show a progress meter while scanning for due items.
COLLECTED is how many drill items have been gathered, SCANNED how many
entries have been examined.  Updates only every 50 entries."
  (when (zerop (% scanned 50))
    (let* ((meter-width 40)
           (current-meter-char (if (cl-oddp (floor scanned (* 50 meter-width))) ?| ?.))
           (alternate-meter-char (if (eql current-meter-char ?.) ?| ?.)))
      (message "Collecting due drill items:%4d %s%s"
               collected
               (make-string (% (ceiling scanned 50) meter-width)
                            alternate-meter-char)
               (make-string (- meter-width (% (ceiling scanned 50) meter-width))
                            current-meter-char)))))

(defun org-drill-map-entry-function (session)
  "Classify the drill entry at point and file it into SESSION's queues.
Called once per entry while scanning the drill scope, updating the
progress meter as it goes."
  (org-drill-progress-message
   (+ (length (oref session new-entries))
      (length (oref session overdue-entries))
      (length (oref session young-mature-entries))
      (length (oref session old-mature-entries))
      (length (oref session failed-entries)))
   (cl-incf (oref session cnt)))
  (when (org-drill-entry-p)
    ;; Per-entry processing is wrapped in `condition-case' so a single
    ;; bad entry doesn't kill the whole collection scan.  Upstream
    ;; issue #53 reported that a new (no-ID) entry triggered a
    ;; "hash-table-p, nil" error that stopped the scan, leaving every
    ;; subsequent entry uncollected — the user had to re-run org-drill
    ;; once per item.  Catching the error here lets the scan continue.
    (condition-case err
        (progn
          (org-drill-id-get-create-with-warning session)
          (cl-destructuring-bind (status due age)
              (org-drill-entry-status session)
            (cl-case status
              (:unscheduled
               (cl-incf (oref session dormant-entry-count)))
              (:future
               (cl-incf (oref session dormant-entry-count))
               (if (eq -1 due)
                   (cl-incf (oref session due-tomorrow-count))))
              (:new
               (push (point-marker) (oref session new-entries)))
              (:failed
               (push (point-marker) (oref session failed-entries)))
              (:young
               (push (point-marker) (oref session young-mature-entries)))
              (:overdue
               (push (list (point-marker) due age) (oref session overdue-data)))
              (:old
               (push (point-marker) (oref session old-mature-entries))))))
      (error
       (message "org-drill: error processing entry at %s (%s); skipping"
                (point) err)))))

(defun org-drill-id-get-create-with-warning (session)
  "Return the entry's :ID:, creating one if absent.
The first time an ID has to be created in SESSION, warn the user that
the one-time ID-creation pass is slow, and flip the session's
warned-about-id-creation flag so the warning shows only once."
  (when (and (not (oref session warned-about-id-creation))
             (null (org-id-get)))
    (message (concat "Creating unique IDs for items "
                     "(slow, but only happens once)"))
    (sit-for 0.5)
    (setf (oref session warned-about-id-creation) t))
  (org-id-get-create))

;;;###autoload
(defun org-drill--prepare-fresh-session (session cram)
  "Reset SESSION's queues and counters for a fresh (non-resume) run.
CRAM, if non-nil, starts the session in cram mode."
  (org-drill-free-markers session t)
  (setf (oref session cram-mode) cram
        (oref session current-item) nil
        (oref session done-entries) nil
        (oref session dormant-entry-count) 0
        (oref session due-entry-count) 0
        (oref session due-tomorrow-count) 0
        (oref session overdue-entry-count) 0
        (oref session new-entries) nil
        (oref session overdue-entries) nil
        (oref session young-mature-entries) nil
        (oref session old-mature-entries) nil
        (oref session failed-entries) nil
        (oref session again-entries) nil
        (oref session undo-stack) nil
        (oref session start-time) (float-time (current-time))
        (oref session scope-at-start) org-drill-scope
        (oref session algorithm-at-start) org-drill-spaced-repetition-algorithm))

(defun org-drill--collect-entries (session scope drill-match)
  "Scan buffers in SCOPE for drill entries matching DRILL-MATCH.
Populates SESSION's queues via `org-drill-map-entry-function', then
sorts the overdue queue."
  (let ((org-trust-scanner-tags t))
    (org-drill-map-entries
     (apply-partially #'org-drill-map-entry-function session)
     scope drill-match)
    (org-drill-order-overdue-entries session)
    (setf (oref session overdue-entry-count)
          (length (oref session overdue-entries)))))

(defun org-drill--queues-empty-p (session)
  "Non-nil when SESSION has no current-item and every drill queue is empty."
  (and (null (oref session current-item))
       (null (oref session new-entries))
       (null (oref session failed-entries))
       (null (oref session overdue-entries))
       (null (oref session young-mature-entries))
       (null (oref session old-mature-entries))))

(defun org-drill--show-resume-hint ()
  "Show the post-suspend `you can resume...' message."
  (let ((keystr (org-drill-command-keybinding-to-string 'org-drill-resume)))
    (message
     "You can continue the drill session with the command `org-drill-resume'.%s"
     (if keystr (format "\nYou can run this command by pressing %s." keystr)
       ""))))

(defun org-drill--show-end-message (session)
  "Display the appropriate end-of-session message and side-effects.
If SESSION was suspended (end-pos is set), shows the resume hint.
Otherwise runs `org-drill-final-report', persists the SM5 matrix,
records the session for the stats dashboard, and optionally saves
buffers."
  (cond
   ((oref session end-pos)
    (when (markerp (oref session end-pos))
      (org-drill-goto-entry (oref session end-pos))
      (org-reveal)
      (org-fold-show-entry))
    (org-drill--show-resume-hint))
   (t
    (org-drill-final-report session)
    (when (eql 'sm5 org-drill-spaced-repetition-algorithm)
      (persist-save 'org-drill-sm5-optimal-factor-matrix))
    ;; Recording is protective of the end-of-session flow: a recorder
    ;; bug or a persist-save failure must not block the final-report
    ;; dismissal or the SM5 save above.  Trap the error and surface it
    ;; via `message' so silent data loss leaves a forensic trail.
    (condition-case err
        (org-drill-record-session session
                                  (oref session start-time)
                                  (float-time (current-time)))
      (error
       (message "org-drill: failed to record session for the stats log (%s)"
                err)))
    (when org-drill-save-buffers-after-drill-sessions-p
      (save-some-buffers))
    (message "Drill session finished!")
    (sit-for 1)
    (message nil))))

;;;###autoload
(defun org-drill (&optional scope drill-match resume-p cram)
  "Begin an interactive \\='drill session\\='. The user is asked to
review a series of topics (headers). Each topic is initially
presented as a \\='question\\=', often with part of the topic content
hidden. The user attempts to recall the hidden information or
answer the question, then presses a key to reveal the answer. The
user then rates his or her recall or performance on that
topic. This rating information is used to reschedule the topic
for future review.

Org-drill proceeds by:

- Finding all topics (headings) in SCOPE which have either been
  used and rescheduled before, or which have a tag that matches
  `org-drill-question-tag'.

- All matching topics which are either unscheduled, or are
  scheduled for the current date or a date in the past, are
  considered to be candidates for the drill session.

- If `org-drill-maximum-items-per-session' is set, a random
  subset of these topics is presented. Otherwise, all of the
  eligible topics will be presented.

SCOPE determines the scope in which to search for
questions.  It accepts the same values as `org-drill-scope',
which see.

DRILL-MATCH, if supplied, is a string specifying a tags/property/
todo query. Only items matching the query will be considered.
It accepts the same values as `org-drill-match', which see.

If RESUME-P is non-nil, resume a suspended drill session rather
than starting a new one.

CRAM, if non-nil, will start a new session in cram mode. If
resuming a suspended session, this parameter is ignored."
  (interactive)
  (let ((session (if resume-p
                     org-drill-last-session
                   (setq org-drill-last-session (org-drill-session)))))
    ;; Clear stale `end-pos' on resume — see upstream issue #33.
    (when resume-p
      (setf (oref session end-pos) nil))
    (unless resume-p
      (org-drill--prepare-fresh-session session cram))
    (unwind-protect
        (save-excursion
          (unless resume-p
            (org-drill--collect-entries session scope drill-match))
          (setf (oref session due-entry-count)
                (org-drill-pending-entry-count session))
          (unless resume-p
            (org-drill--setup-display)
            (run-hooks 'org-drill-before-session-hook))
          (cond
           ((org-drill--queues-empty-p session)
            (message "I did not find any pending drill items."))
           (t
            (org-drill-entries session resume-p)
            (message "Drill session finished!")
            (sit-for 1)
            (message nil))))
      ;; Cleanup runs whether the body succeeded, errored, or was
      ;; suspended via cl-return-from in `org-drill-entries'.
      (org-drill--restore-display)
      (run-hooks 'org-drill-after-session-hook)
      (org-drill-free-markers session (oref session done-entries))
      (unless (oref session end-pos)
        (setf (oref session cram-mode) nil)))
    (org-drill--show-end-message session)))

;;;###autoload
(defun org-drill-cram (&optional scope drill-match)
  "Run an interactive drill session in \\='cram mode\\='. In cram mode,
all drill items are considered to be due for review, unless they
have been reviewed within the last `org-drill-cram-hours'
hours."
  (interactive)
  (org-drill scope drill-match nil t))

;;;###autoload
(defun org-drill-cram-tree ()
  "Run  an interactive drill session in \\='cram mode\\=' using subtree at point.

See also, `org-drill-cram' and `org-drill-tree'."
  (interactive)
  (org-drill-cram 'tree))

;;;###autoload
(defun org-drill-tree ()
  "Run an interactive drill session using drill items within the
subtree at point."
  (interactive)
  (org-drill 'tree))

;;;###autoload
(defun org-drill-directory ()
  "Run an interactive drill session using drill items from all org
files in the same directory as the current file."
  (interactive)
  (org-drill 'directory))

;;;###autoload
(defun org-drill-again (&optional scope drill-match)
  "Run a new drill session, but try to use leftover due items that
were not reviewed during the last session, rather than scanning for
unreviewed items. If there are no leftover items in memory, a full
scan will be performed."
  (interactive)
  (let ((session org-drill-last-session))
    (unless session
      (user-error "No previous drill session to continue — run `org-drill' first"))
    (setf (oref session cram-mode) nil)
    (cond
     ((cl-plusp (org-drill-pending-entry-count session))
      (org-drill-free-markers session (oref session done-entries))
      (if (markerp (oref session current-item))
          (set-marker (oref session current-item) nil))
      (setf (oref session start-time) (float-time (current-time))
            (oref session scope-at-start) org-drill-scope
            (oref session algorithm-at-start) org-drill-spaced-repetition-algorithm)
      (setf (oref session done-entries) nil
            (oref session current-item) nil)
      (org-drill scope drill-match t))
     (t
      (org-drill scope drill-match)))))

;;;###autoload
(defun org-drill-resume ()
  "Resume a suspended drill session. Sessions are suspended by
exiting them with the `edit' or `quit' options."
  (interactive)
  (let ((session org-drill-last-session))
    (unless session
      (user-error "No suspended drill session to resume — run `org-drill' first"))
    (cond
     ((org-drill-entries-pending-p session)
      (org-drill nil nil t))
     ((and (cl-plusp (org-drill-pending-entry-count session))
           ;; Current drill session is finished, but there are still
           ;; more items which need to be reviewed.
           (y-or-n-p (format
                      "You have finished the drill session.  However, %d items still
need reviewing.  Start a new drill session? "
                      (org-drill-pending-entry-count session))))
      (org-drill-again))
     (t
      (message "You have finished the drill session.")))))


;;;###autoload
(defun org-drill-relearn-item ()
  "Make the current item due for revision, and set its last interval to 0.
Makes the item behave as if it has been failed, without actually recording a
failure. This command can be used to \\='reset\\=' repetitions for an item."
  (interactive)
  (org-drill-smart-reschedule 4 0))


(defun org-drill-strip-entry-data ()
  "Remove all org-drill scheduling data from the entry at point.
Deletes every property in `org-drill-scheduling-properties' and clears
the entry's schedule."
  (dolist (prop org-drill-scheduling-properties)
    (org-delete-property prop))
  (org-schedule '(4)))


;;;###autoload
(defun org-drill-strip-all-data (&optional scope)
  "Delete scheduling data from every drill entry in scope. This
function may be useful if you want to give your collection of
entries to someone else.  Scope defaults to the current buffer,
and is specified by the argument SCOPE, which accepts the same
values as `org-drill-scope'."
  (interactive)
  (when (yes-or-no-p
         "Delete scheduling data from ALL items in scope: are you sure?")
    (cond
     ((null scope)
      ;; Scope is the current buffer. This means we can use
      ;; `org-delete-property-globally', which is faster.
      (dolist (prop org-drill-scheduling-properties)
        (org-delete-property-globally prop))
      (org-drill-map-entries (lambda () (org-schedule '(4))) scope))
     (t
      (org-drill-map-entries 'org-drill-strip-entry-data scope)))
    (message "Done.")))


(defvar-local org-drill--installed-cloze-keywords nil
  "The exact cloze font-lock keyword list installed by `org-drill-mode' in
this buffer, kept so the mode can remove precisely what it added.")

(define-minor-mode org-drill-mode
  "Minor mode that highlights cloze deletions in the current buffer.

Enabling the mode adds cloze fontification to this buffer only, so the
highlighting does not leak into unrelated Org buffers.  That scoping also
prevents Org priority cookies such as `[#A]' — which match the cloze
`[...]' pattern — from being fontified as clozes in non-drill buffers.

The mode controls only the persistent display of clozes in the source
buffer.  Running drill sessions (`org-drill', `org-drill-cram', etc.) does
not depend on it.

Highlighting is applied only when `org-drill-use-visible-cloze-face-p' is
non-nil; otherwise the mode is a no-op for fontification."
  :lighter " Drill"
  :group 'org-drill
  ;; Recompute the buffer-local cloze regexp/keywords in case the cloze
  ;; delimiters were redefined locally.
  (setq-local org-drill-cloze-regexp (org-drill--compute-cloze-regexp))
  (setq-local org-drill-cloze-keywords (org-drill--compute-cloze-keywords))
  ;; Remove any previously-installed keywords first so repeated toggles
  ;; don't stack duplicates.
  (when org-drill--installed-cloze-keywords
    (font-lock-remove-keywords nil org-drill--installed-cloze-keywords)
    (setq org-drill--installed-cloze-keywords nil))
  (when (and org-drill-mode org-drill-use-visible-cloze-face-p)
    (setq org-drill--installed-cloze-keywords org-drill-cloze-keywords)
    (font-lock-add-keywords nil org-drill--installed-cloze-keywords 'append))
  (when font-lock-mode
    (save-restriction
      (widen)
      (font-lock-flush)
      (font-lock-ensure))))

(defun org-drill-buffer-has-cards-p ()
  "Return non-nil if the current buffer contains a drill card — a heading
tagged with `org-drill-question-tag' or `org-drill-leitner-tag'.

The tag may sit on the heading itself, on an ancestor heading (the
ancestor line carries the literal tag, so the per-heading scan still
matches), or be applied file-wide via `#+FILETAGS:'.  The file-tag case
is checked separately because the tag never appears on a heading line
there — the upstream bug where filetag-only decks failed to auto-enable
`org-drill-mode'."
  (save-excursion
    (save-restriction
      (widen)
      (let ((case-fold-search t)
            (tags (concat "\\(?:"
                          (regexp-quote org-drill-question-tag)
                          "\\|"
                          (regexp-quote org-drill-leitner-tag)
                          "\\)")))
        (or
         ;; Per-heading tag (also catches inheritance from a tagged ancestor).
         (progn
           (goto-char (point-min))
           (re-search-forward (concat "^\\*+ .*:" tags ":") nil t))
         ;; File-wide tag via #+FILETAGS:, in either the space-separated or
         ;; colon-delimited syntax.  The [: \t] boundaries keep a value like
         ;; `drilldown' from matching `drill'.
         (progn
           (goto-char (point-min))
           (re-search-forward
            (concat "^#\\+FILETAGS:.*[: \t]" tags "\\(?:[: \t]\\|$\\)")
            nil t)))))))

(defun org-drill-maybe-enable-mode ()
  "Enable `org-drill-mode' when appropriate for the current buffer.
Intended for `org-mode-hook': turns the mode on when
`org-drill-auto-enable-mode' is non-nil and the buffer holds drill cards."
  (when (and org-drill-auto-enable-mode
             (derived-mode-p 'org-mode)
             (org-drill-buffer-has-cards-p))
    (org-drill-mode 1)))

(add-hook 'org-mode-hook #'org-drill-maybe-enable-mode)


;;; Synching card collections =================================================
(defvar org-drill-dest-id-table (make-hash-table :test 'equal))

(defun org-drill-copy-entry-to-other-buffer (dest &optional path)
  "Copy the subtree at point to the buffer DEST. The copy will receive
the tag \\='imported\\='."
  (cl-block org-drill-copy-entry-to-other-buffer
    (save-excursion
      (let ((m nil))
        (cl-flet ((paste-tree-here (&optional level)
                                   (org-paste-subtree level)
                                   (org-drill-strip-entry-data)
                                   (org-toggle-tag "imported" 'on)
                                   (org-drill-map-entries
                                    (lambda ()
                                      (let ((id (org-id-get)))
                                        (org-drill-strip-entry-data)
                                        (unless (gethash id org-drill-dest-id-table)
                                          (puthash id (point-marker)
                                                   org-drill-dest-id-table))))
                                    'tree)))
          (unless path
            (setq path (org-get-outline-path)))
          (org-copy-subtree)
          (switch-to-buffer dest)
          (setq m
                (condition-case nil
                    (org-find-olp path t)
                  (error                ; path does not exist in DEST
                   (cl-return-from org-drill-copy-entry-to-other-buffer
                     (cond
                      ((cdr path)
                       (org-drill-copy-entry-to-other-buffer
                        dest (butlast path)))
                      (t
                       ;; We've looked all the way up the path
                       ;; Default to appending to the end of DEST
                       (goto-char (point-max))
                       (newline)
                       (paste-tree-here)))))))
          (goto-char m)
          (outline-next-heading)
          (newline)
          (forward-line -1)
          (paste-tree-here (1+ (or (org-current-level) 0)))
          )))))

(defun org-drill--build-dest-id-table (dest)
  "Populate `org-drill-dest-id-table' with id→marker for every entry in DEST."
  (clrhash org-drill-dest-id-table)
  (with-current-buffer dest
    (org-drill-map-entries
     (lambda ()
       (let ((this-id (org-id-get)))
         (when this-id
           (puthash this-id (point-marker) org-drill-dest-id-table))))
     'file)))

(defun org-drill--copy-scheduling-to-marker (marker)
  "Copy the current entry's scheduling state to MARKER's position.
The destination's previous data is stripped first.  Skips zero-
total-repeats items (i.e., never-rated cards) — those have no
scheduling information worth migrating."
  (let ((state (org-drill-get-item-data))
        (last-reviewed (org-entry-get (point) "DRILL_LAST_REVIEWED"))
        (last-quality (org-entry-get (point) "DRILL_LAST_QUALITY"))
        (scheduled-time (org-get-scheduled-time (point))))
    (with-current-buffer (marker-buffer marker)
      (save-excursion
        (goto-char marker)
        (org-drill-strip-entry-data)
        (unless (zerop (org-drill-card-state-total-repeats state))
          (org-drill-store-item-data state)
          (if last-quality
              (org-set-property "DRILL_LAST_QUALITY" last-quality)
            (org-delete-property "DRILL_LAST_QUALITY"))
          (if last-reviewed
              (org-set-property "DRILL_LAST_REVIEWED" last-reviewed)
            (org-delete-property "DRILL_LAST_REVIEWED"))
          (when scheduled-time
            (org-schedule nil scheduled-time)))))))

(defun org-drill--migrate-from-source (src dest ignore-new-items-p)
  "Walk SRC's entries, migrating scheduling data into matching DEST entries.
Removes matched IDs from `org-drill-dest-id-table' as it goes, so the
caller can clean up unmatched DEST entries afterwards.  Entries with
no DEST match are copied into DEST as new items unless IGNORE-NEW-ITEMS-P."
  (with-current-buffer src
    (org-drill-map-entries
     (lambda ()
       (let ((id (org-id-get)))
         (cond
          ((or (null id) (not (org-drill-entry-p))) nil)
          ((gethash id org-drill-dest-id-table)
           (let ((marker (gethash id org-drill-dest-id-table)))
             (org-drill--copy-scheduling-to-marker marker)
             (remhash id org-drill-dest-id-table)
             (set-marker marker nil)))
          (t
           ;; Item in SRC has an ID but no match in DEST — new item.
           (unless ignore-new-items-p
             (org-drill-copy-entry-to-other-buffer dest))))))
     'file)))

(defun org-drill--strip-unmatched-dest-entries ()
  "Strip scheduling data from every entry left in `org-drill-dest-id-table'.
Anything still in the table after migration didn't have a matching
SRC entry — its scheduling info came from a different deck and the
deck owner is opting in to a clean migration."
  (maphash (lambda (_id m)
             (goto-char m)
             (org-drill-strip-entry-data)
             (set-marker m nil))
           org-drill-dest-id-table))

;;;###autoload
(defun org-drill-merge-buffers (src &optional dest ignore-new-items-p)
  "SRC and DEST are two org mode buffers containing drill items.
For each drill item in DEST that shares an ID with an item in SRC,
overwrite scheduling data in DEST with data taken from the item in SRC.
This is intended for use when two people are sharing a set of drill items,
one person has made some updates to the item set, and the other person
wants to migrate to the updated set without losing their scheduling data.

By default, any drill items in SRC which do not exist in DEST are
copied into DEST. We attempt to place the copied item in the
equivalent location in DEST to its location in SRC, by matching
the heading hierarchy. However if IGNORE-NEW-ITEMS-P is non-nil,
we simply ignore any items that do not exist in DEST, and do not
copy them across."
  (interactive "bImport scheduling info from which buffer?")
  (unless dest
    (setq dest (current-buffer)))
  (setq src (get-buffer src)
        dest (get-buffer dest))
  (when (yes-or-no-p
         (format
          (concat "About to overwrite all scheduling data for drill items in `%s' "
                  "with information taken from matching items in `%s'. Proceed? ")
          (buffer-name dest) (buffer-name src)))
    (org-drill--build-dest-id-table dest)
    (org-drill--migrate-from-source src dest ignore-new-items-p)
    (with-current-buffer dest
      (org-drill--strip-unmatched-dest-entries))))



;;; Card types for learning languages =========================================

;;; `conjugate' card type =====================================================
;;; See spanish.org for usage

(defvar org-drill-verb-tense-alist
  '(("present" "tomato")
    ("simple present" "tomato")
    ("present indicative" "tomato")
    ;; past tenses
    ("past" "purple")
    ("simple past" "purple")
    ("preterite" "purple")
    ("imperfect" "darkturquoise")
    ("present perfect" "royalblue")
    ;; future tenses
    ("future" "green")
    ;; moods (backgrounds).
    ("indicative" nil)                  ; default
    ("subjunctive" "medium blue")
    ("conditional" "grey30")
    ("negative imperative" "red4")
    ("positive imperative" "darkgreen")
    )
  "Alist where each entry has the form (TENSE COLOUR), where
TENSE is a string naming a tense in which verbs can be
conjugated, and COLOUR is a string specifying a foreground colour
which will be used by `org-drill-present-verb-conjugation' and
`org-drill-show-answer-verb-conjugation' to fontify the verb and
the name of the tense.")

(defun org-drill--read-property-string (raw)
  "Return the first datum read from property value RAW, or nil if RAW is nil.
Some card properties are stored as quoted Lisp strings, so a single
`read' strips one layer: \"\\\"hablar\\\"\" becomes \"hablar\"."
  (and raw (car (read-from-string raw))))

(defun org-drill--face-from-alist (key alist default)
  "Return the colour mapped to KEY in ALIST, case-insensitively, or DEFAULT.
ALIST entries are (NAME COLOUR ...) as in `org-drill-verb-tense-alist' and
`org-drill-noun-gender-alist'."
  (or (cl-second (assoc-string key alist t)) default))

(defun org-drill-get-verb-conjugation-info ()
  "Auxiliary function used by `org-drill-present-verb-conjugation' and
`org-drill-show-answer-verb-conjugation'."
  (let ((infinitive (org-entry-get (point) "VERB_INFINITIVE" t))
        (inf-hint (org-entry-get (point) "VERB_INFINITIVE_HINT" t))
        (translation (org-entry-get (point) "VERB_TRANSLATION" t))
        (tense (org-entry-get (point) "VERB_TENSE" nil))
        (mood (org-entry-get (point) "VERB_MOOD" nil))
        (highlight-face nil))
    (unless (and infinitive translation (or tense mood))
      (error "Missing information for verb conjugation card (%s, %s, %s, %s) at %s"
             infinitive translation tense mood (point)))
    (setq tense (if tense (downcase (org-drill--read-property-string tense)))
          mood (if mood (downcase (org-drill--read-property-string mood)))
          infinitive (org-drill--read-property-string infinitive)
          inf-hint (org-drill--read-property-string inf-hint)
          translation (org-drill--read-property-string translation))
    (setq highlight-face
          (list :foreground
                (org-drill--face-from-alist tense org-drill-verb-tense-alist "hotpink")
                :background
                (org-drill--face-from-alist mood org-drill-verb-tense-alist "black")))
    (setq infinitive (propertize infinitive 'face highlight-face))
    (setq translation (propertize translation 'face highlight-face))
    (if tense (setq tense (propertize tense 'face highlight-face)))
    (if mood (setq mood (propertize mood 'face highlight-face)))
    (list infinitive inf-hint translation tense mood)))

(defun org-drill--format-tense-mood (tense mood)
  "Return a human-readable label for a verb's TENSE and MOOD.
Either argument may be nil.  Returns nil when both are nil."
  (cond
   ((and tense mood)
    (format "%s tense, %s mood" tense mood))
   (tense
    (format "%s tense" tense))
   (mood
    (format "%s mood" mood))))

(defun org-drill-present-verb-conjugation (session)
  "Present a drill entry whose card type is \\='conjugate\\='."
  (cl-destructuring-bind (infinitive inf-hint translation tense mood)
      (org-drill-get-verb-conjugation-info)
    (org-drill-present-card-using-text
     session
     (cond
      ((zerop (cl-random 2))
       (format "\nTranslate the verb\n\n%s\n\nand conjugate for the %s.\n\n"
               infinitive (org-drill--format-tense-mood tense mood)))

      (t
       (format "\nGive the verb that means\n\n%s %s\n
and conjugate for the %s.\n\n"
               translation
               (if inf-hint (format "  [HINT: %s]" inf-hint) "")
               (org-drill--format-tense-mood tense mood)))))))

(defun org-drill-show-answer-verb-conjugation (session reschedule-fn)
 "Show the answer for a drill item whose card type is \\='conjugate\\='.
RESCHEDULE-FN must be a function that calls `org-drill-reschedule' and
returns its return value."
  (cl-destructuring-bind (infinitive _inf-hint translation tense mood)
      (org-drill-get-verb-conjugation-info)
    (org-drill-with-replaced-entry-heading
     (format "%s of %s ==> %s\n\n"
             (capitalize (org-drill--format-tense-mood tense mood))
             infinitive translation)
     (org-drill-hide-drawers)
     (funcall reschedule-fn session))))


;;; `decline_noun' card type ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


(defvar org-drill-noun-gender-alist
  '(("masculine" "dodgerblue")
    ("masc" "dodgerblue")
    ("male" "dodgerblue")
    ("m" "dodgerblue")
    ("feminine" "orchid")
    ("fem" "orchid")
    ("female" "orchid")
    ("f" "orchid")
    ("neuter" "green")
    ("neutral" "green")
    ("neut" "green")
    ("n" "green")
    ))

(defun org-drill-get-noun-info ()
  "Auxiliary function used by `org-drill-present-noun-declension' and
`org-drill-show-answer-noun-declension'."
  (let ((noun (org-entry-get (point) "NOUN" t))
        (noun-hint (org-entry-get (point) "NOUN_HINT" t))
        (noun-root (org-entry-get (point) "NOUN_ROOT" t))
        (noun-gender (org-entry-get (point) "NOUN_GENDER" t))
        (translation (org-entry-get (point) "NOUN_TRANSLATION" t))
        (highlight-face nil))
    (unless (and noun translation)
      (error "Missing information for `decline_noun' card (%s, %s, %s, %s) at %s"
             noun translation noun-hint noun-root (point)))
    (setq noun-root (org-drill--read-property-string noun-root)
          noun (org-drill--read-property-string noun)
          noun-gender (downcase (org-drill--read-property-string noun-gender))
          noun-hint (org-drill--read-property-string noun-hint)
          translation (org-drill--read-property-string translation))
    (setq highlight-face
          (list :foreground
                (org-drill--face-from-alist noun-gender
                                            org-drill-noun-gender-alist "red")))
    (setq noun (propertize noun 'face highlight-face))
    (setq translation (propertize translation 'face highlight-face))
    (list noun noun-root noun-gender noun-hint translation)))

(defun org-drill-present-noun-declension (session)
  "Present a drill entry whose card type is \\='decline_noun\\='."
  (cl-destructuring-bind (noun _noun-root noun-gender noun-hint translation)
      (org-drill-get-noun-info)
    (let* ((props (org-entry-properties (point)))
           (definite
             (cond
              ((assoc "DECLINE_DEFINITE" props)
               (propertize (if (org-entry-get (point) "DECLINE_DEFINITE")
                               "definite" "indefinite")
                           'face 'warning))
              (t nil)))
           (plural
            (cond
             ((assoc "DECLINE_PLURAL" props)
              (propertize (if (org-entry-get (point) "DECLINE_PLURAL")
                              "plural" "singular")
                          'face 'warning))
             (t nil))))
      (org-drill-present-card-using-text
       session
       (cond
        ((zerop (cl-random 2))
         (format "\nTranslate the noun\n\n%s (%s)\n\nand list its declensions%s.\n\n"
                 noun noun-gender
                 (if (or plural definite)
                     (format " for the %s %s form" definite plural)
                   "")))
        (t
         (format "\nGive the noun that means\n\n%s %s\n
and list its declensions%s.\n\n"
                 translation
                 (if noun-hint (format "  [HINT: %s]" noun-hint) "")
                 (if (or plural definite)
                     (format " for the %s %s form" definite plural)
                   ""))))))))

(defun org-drill-show-answer-noun-declension (session reschedule-fn)
  "Show the answer for a drill item whose card type is \\='decline_noun\\='.
RESCHEDULE-FN must be a function that calls `org-drill-reschedule' and
returns its return value."
  (cl-destructuring-bind (noun _noun-root noun-gender _noun-hint translation)
      (org-drill-get-noun-info)
    (org-drill-with-replaced-entry-heading
     (format "Declensions of %s (%s) ==> %s\n\n"
             noun noun-gender translation)
     (org-drill-hide-drawers)
     (funcall reschedule-fn session))))

;;; `spanish_verb' card type ==================================================
;;; Not very interesting, but included to demonstrate how a presentation
;;; function can manipulate which subheading are hidden versus shown.

(defconst org-drill--spanish-verb-prompts
  '(("Infinitive" . "Translate this Spanish verb, and conjugate it for the *present* tense.")
    ("English"    . "For the *present* tense, conjugate the Spanish translation of this English verb.")
    ("Infinitive" . "Translate this Spanish verb, and conjugate it for the *past* tense.")
    ("English"    . "For the *past* tense, conjugate the Spanish translation of this English verb.")
    ("Infinitive" . "Translate this Spanish verb, and conjugate it for the *future perfect* tense.")
    ("English"    . "For the *future perfect* tense, conjugate the Spanish translation of this English verb."))
  "Six (REVEAL . PROMPT) pairs for `org-drill-present-spanish-verb'.
Each card-presentation chooses one at random, hiding all subheadings
except REVEAL and showing PROMPT in the rating prompt.")

(defun org-drill-present-spanish-verb (session)
  "Present a Spanish-verb conjugation card for SESSION.
Pick one of the six (reveal . prompt) pairs in
`org-drill--spanish-verb-prompts' at random, reveal that subheading, and
prompt with the matching question."
  (org-drill-with-card-display
    (let* ((choice (nth (cl-random (length org-drill--spanish-verb-prompts))
                        org-drill--spanish-verb-prompts))
           (reveal (car choice))
           (prompt (cdr choice)))
      (org-drill-hide-all-subheadings-except (list reveal))
      (org-drill-hide-drawers)
      (prog1 (org-drill-presentation-prompt session prompt)
        (org-drill-hide-subheadings-if 'org-drill-entry-p)))))

;; org-drill :explain: implementations
(defun org-drill-get-explain-text (&optional existing-text)
  "Fetch the explaination texts for this entry.

Explaination text is found in parent entries with an :explain:
tag. If there are multiple parents entries with such a tag, all
of them are returned.

Returns a list of strings."
  (save-excursion
    (save-restriction
      (widen)
      (if (>= 1 (funcall outline-level))
          existing-text
        (outline-up-heading 1 t)
        (if (org-drill-explain-entry-p t)
            (org-drill-get-explain-text
             (cons
              (org-drill-get-entry-text)
              existing-text))
          existing-text)))))

(defvar org-drill-explain-overlay nil)

(defun org-drill-explain-entry-p (&optional no-inherit)
  "Return non-nilif an entry is associated with explanation"
  (member "explain" (org-get-tags nil no-inherit)))

(defun org-drill-end-of-entry-pos ()
  "Return the buffer position at the end of the current entry's subtree."
  (save-excursion
    (org-end-of-subtree)
    (point)))

(defun org-drill-explain-answer-presenter ()
  "Append the entry's explanation text as an overlay after the subtree.
The explanation is gathered from `:explain:'-tagged ancestors and stored
in `org-drill-explain-overlay' so `org-drill-explain-cleaner' can remove it."
  (save-excursion
    (when org-drill-explain-overlay
      (delete-overlay org-drill-explain-overlay))
    (let* ((end (org-drill-end-of-entry-pos))
           (ov (make-overlay
                end end
                (current-buffer))))
      (overlay-put ov 'after-string
                   (format "\n\nExplanation:\n\n%s"
                           (mapconcat 'identity
                                      (org-drill-get-explain-text) "\n\n")))
      (setq org-drill-explain-overlay ov))))

(defun org-drill-explain-cleaner ()
  "Delete the explanation overlay left by `org-drill-explain-answer-presenter'."
  (when org-drill-explain-overlay
      (delete-overlay org-drill-explain-overlay)))

;;; Leitner Learning
(defvar org-drill-leitner-boxed-entries nil
  "All leitner entries that are currently in an active box.")

(defvar org-drill-leitner-unboxed-entries nil
  "All leitner entries that are not in a box.")

(defvar org-drill-leitner-promote-to-drill-p t)

(defvar org-drill-leitner-completed 0
  "The number of entries that have been completed this time.")

(defun org-drill-sm-or-leitner ()
  "Resume or start a drill, choosing the Leitner or SM path automatically.
Reuse the last session if one is pending; otherwise scan and dispatch to
the Leitner or spaced-repetition flow as appropriate."
  (interactive)
  ;; org-drill-again uses org-drill-pending-entry-count to decide
  ;; whether it needs to scan or not.
  (let* ((session
          (or org-drill-last-session (org-drill-session)))
         (pending
          (org-drill-pending-entry-count session)))
    (unless (cl-plusp pending)
      (org-drill-map-entries
       (apply-partially 'org-drill-map-entry-function session)
       nil nil))
    ;; if the overdue entries are not ones we have just created
    (if (> (org-drill-pending-entry-count session) org-drill-leitner-completed)
        ;; we should have scanned previously if we need to
        (progn
          (message "Org Drill: Starting SM learning")
          (sit-for 0.5)
          (setq org-drill-last-session session)
          (org-drill-again))
      (message "Org Drill: Starting leitner learning")
      (sit-for 0.5)
      (org-drill-leitner session))))

(defun org-drill-leitner (&optional session)
  "Perform Leitner learning"
  (interactive)
  (let ((org-drill-leitner-boxed-entries nil)
        (org-drill-leitner-unboxed-entries nil)
        (session (setq org-drill-last-session
                       (or session
                           (org-drill-session))))
        (count 0))
    (org-drill-all-leitner-capture)
    ;; make sure we have enough (or at least the maximum number we
    ;; can) of boxed entities
    (when (<
           (length org-drill-leitner-boxed-entries)
           (- org-drill-maximum-items-per-session count))
      (org-drill-leitner-start-box
       (- org-drill-maximum-items-per-session
          (length org-drill-leitner-boxed-entries)
          count))
      (setq org-drill-leitner-boxed-entries nil)
      (setq org-drill-leitner-unboxed-entries nil)
      (org-drill-all-leitner-capture))
    (pcase
        (catch 'user-exit
          (seq-map
           (lambda (loc)
             (org-drill-goto-entry loc)
             (let ((r (org-drill-leitner-entry session)))
               ;; short circuit if necessary
               (unless (eq t r)
                 (throw 'user-exit (list r loc)))))
           (org-drill-shuffle
            (seq-take org-drill-leitner-boxed-entries
                      org-drill-maximum-items-per-session))))
      (`(quit ,_)  t)
      (`(edit ,loc)
       (org-drill-goto-entry loc)
       (org-reveal)
       (org-fold-show-entry))
      (`,_
       (message "Finished Leitner Learning: %s complete today, %s in process, %s to start"
                org-drill-leitner-completed
                (length org-drill-leitner-boxed-entries)
                (length org-drill-leitner-unboxed-entries))))))

;; take from John Kitchen
(defun org-drill-swap (LIST el1 el2)
  "in LIST swap indices EL1 and EL2 in place"
  (let ((tmp (elt LIST el1)))
    (setf (elt LIST el1) (elt LIST el2))
    (setf (elt LIST el2) tmp)))

(defun org-drill-shuffle (LIST)
  "Return a random permutation of the elements in LIST.
The shuffle runs over a temporary vector with a Fisher-Yates pass,
so the cost is linear in the length of LIST rather than quadratic
\(the previous version indexed a list with `elt' on every swap)."
  (cl-check-type LIST list)
  (let ((vec (vconcat LIST)))
    (cl-loop for i from (1- (length vec)) downto 1
             do (let ((j (random (1+ i))))
                  (cl-rotatef (aref vec i) (aref vec j))))
    (append vec nil)))

(defun org-drill-leitner-start-box (number)
  "Box some items for the first time."
  (message "Starting %s new items" number)
  (sit-for 0.25)
  (seq-map
   (lambda (loc)
     (org-drill-goto-entry loc)
     (message "New leitner entry: %s" (org-drill-get-entry-text))
     (sit-for 0.5)
     (org-set-property "DRILL_LEITNER_BOX" "1"))
   (seq-take
    (org-drill-shuffle (seq-copy org-drill-leitner-unboxed-entries))
    number)))

(defun org-drill-map-leitner (func &optional scope)
  "Return all entries marked with leitner tag."
  (org-map-entries
   func (concat "+" "leitner")
   (org-drill-current-scope scope)))

(defun org-drill-all-leitner-capture (&optional scope)
  "Capture all items marked with a leitner tag"
  (let ((org-drill-question-tag org-drill-leitner-tag))
    (org-drill-map-leitner
     (apply-partially #'org-drill-map-leitner-capture (org-drill-session))
     scope)
    (setq org-drill-leitner-boxed-entries
          (nreverse org-drill-leitner-boxed-entries)
          org-drill-leitner-unboxed-entries
          (nreverse org-drill-leitner-unboxed-entries))))

(defun org-drill-map-leitner-capture (session)
  "Capture this entry if it is a valid leitner entry"
  ;; This bit is all rather shared with org-drill-map-entry-function
  (org-drill-progress-message
   (+ (length org-drill-leitner-unboxed-entries)
      (length org-drill-leitner-boxed-entries))
   (cl-incf (oref session cnt)))
  (when (org-drill-entry-p)
    (org-drill-id-get-create-with-warning session)
    (let ((leitner-box (org-entry-get (point) "DRILL_LEITNER_BOX" nil)))
      (cond
       ;; Entries we have not looked at yet
       ((null leitner-box)
        (push (point-marker) org-drill-leitner-unboxed-entries))
       ;; Entries we have finished with
       ((> (string-to-number leitner-box) 5) nil)
       ((and
         (>= (string-to-number leitner-box) 0)
         (<= (string-to-number leitner-box) 5))
        (push (point-marker)
              org-drill-leitner-boxed-entries))))))

(defun org-drill-leitner-entry (session)
  "Interactive drill for the current entry."
  (let ((org-drill-question-tag org-drill-leitner-tag))
    (org-drill-entry-f session #'org-drill-leitner-rebox)))

(defun org-drill-leitner-rebox (session)
  "Return qualityrating (0-5), or nil if the user quit."
  (let ((ch (org-drill--read-rating-key
             (oref session typed-answer)
             "0-2 Means you have forgotten the item.
3-5 Means you have remembered the item.

0 - Completely forgot. (Back to Zero)
1 - Even after seeing the answer, it still took a bit to sink in (Back to one)
2 - After seeing the answer, you remembered it (Remain in current Box)
3 - It took you awhile, but you finally remembered. (Forward One)
4 - After a little bit of thought you remembered. (Forward One)
5 - You remembered the item really easily. (Forward One)")))
    (cond
     ((and (>= ch ?0) (<= ch ?5))
      (let ((current-box
             ;; Default to 0 if the property isn't set — `org-entry-get'
             ;; returns nil and `string-to-number' would error on nil.
             ;; Box 0 is sensible: a downgrade stays at 0, a promotion
             ;; goes to 1.
             (string-to-number
              (or (org-entry-get (point) "DRILL_LEITNER_BOX" nil)
                  "0"))))
        (cond
         ((or (= ch ?0))
          (message "Refiled down to box: 1")
          (org-set-property "DRILL_LEITNER_BOX" "1"))
         ((or (= ch ?1))
          (let ((box
                 (format
                  "%s"
                  (if (eq current-box 1)
                      1
                    (- current-box 1)))))
            (message "Refiled down to box: %s" box)
            (sit-for 0.3)
            (org-set-property
             "DRILL_LEITNER_BOX" box)))
         ((= ch ?2)
          ;; neither promote nor demote
          (message "Remaining in box: %s" current-box)
          (sit-for 0.3))
         ((or (= ch ?3) (= ch ?4)(= ch ?5))
          (org-drill-leitner-promote current-box)))
        t))
     ((= ch org-drill--edit-key)
      'edit)
     ((= ch org-drill--quit-key)
      'quit)
     (t nil))))

(defun org-drill-leitner-promote (current-box)
  "Promote the current entry to drill or otherwise"
  (when (eq current-box 5)
    (org-toggle-tag "leitner" 'off)
    (when org-drill-leitner-promote-to-drill-p
      (org-toggle-tag "drill" 'on))
    (cl-incf org-drill-leitner-completed))
  (org-set-property
   "DRILL_LEITNER_BOX"
   (format
    "%s"
    (+ current-box 1)))
  (message "Refiled to box: %s" (+ current-box 1))
  (sit-for 0.3))

;;; Test functions
(defun org-drill-test-display ()
  "Developer helper: present the entry at point as a drill card.
Temporarily tags the entry so it counts as a drill item, runs the
presenter through `org-drill-entry-f', then removes the tag again."
  (interactive)
  ;; set tag to anything
  (org-toggle-tag "zysygy")
  (unwind-protect
      (let ((org-drill-question-tag "zysygy"))
        (org-drill-entry-f (org-drill-session)
                           #'org-drill-test-display-rescheduler))
    (org-toggle-tag "zysygy")))

(defun org-drill-test-display-rescheduler (_session)
  "Stand-in reschedule function for `org-drill-test-display'.
Runs the answer hook and waits for a keypress instead of actually
rescheduling, so the developer helper can show the answer side."
  (run-hooks 'org-drill-display-answer-hook)
  ;; Normally, the rescheduler waits for input at this point
  (read-key-sequence "Press anything to continue"))

(defun org-drill-leitner-vs-drill-entries ()
  "Report how many entries in scope are Leitner items versus drill items.
A diagnostic command for decks that mix the two systems."
  (interactive)
  (let
      ((number-drill-entries 0)
       (org-drill-leitner-unboxed-entries nil)
       (org-drill-leitner-boxed-entries nil))
    (org-drill-all-leitner-capture)
    (org-drill-map-entries
     (lambda ()
       (setq number-drill-entries (+ 1 number-drill-entries)))
     nil nil)
    (message "There are %s drill entries\nThere are %s leitner entries\nA total of %s entries."
             number-drill-entries
             (+ (length org-drill-leitner-boxed-entries)
                (length org-drill-leitner-unboxed-entries))
             (+ number-drill-entries
                (+ (length org-drill-leitner-boxed-entries)
                   (length org-drill-leitner-unboxed-entries))))))

;;; Statistics dashboard: step-2 aggregation and rendering layer.
;;
;; Aggregators, section renderers, and the dashboard UI shell built on
;; the step-1 session-record/session-log data layer above.

;;; Statistics dashboard: customization and shared primitives

;; The `org-drill-statistics' defgroup already exists in org-drill.el
;; (around line 87) as a sibling of `org-drill-session'.  These
;; defcustoms attach to it; do not redefine the group.

(defcustom org-drill-statistics-trend-days 90
  "Number of days the statistics dashboard trend section spans.
Daily and weekly aggregates are computed over the most recent window
of this many days, ending today.  Weekly aggregates additionally cap at
the last 12 weeks."
  :type 'integer
  :group 'org-drill-statistics)

(defcustom org-drill-statistics-forecast-days 7
  "Number of days the statistics dashboard forecast section spans.
Cards are bucketed into this many upcoming days by their SCHEDULED
day, starting today."
  :type 'integer
  :group 'org-drill-statistics)

(defcustom org-drill-statistics-attention-row-limit 10
  "Maximum rows shown per needs-attention table on the dashboard.
Tables that would exceed this length are truncated and gain a
\"+N more\" footer naming the number of hidden rows."
  :type 'integer
  :group 'org-drill-statistics)

(defcustom org-drill-statistics-leech-quality-threshold 2.5
  "Average-quality ceiling below which a card counts as a leech candidate.
A card is flagged when its DRILL_FAILURE_COUNT is at least
`org-drill-leech-failure-threshold' and its DRILL_AVERAGE_QUALITY is
strictly less than this value."
  :type 'number
  :group 'org-drill-statistics)

;;; Shared pure primitives for the aggregation helpers.
;;
;; These are small, side-effect-free building blocks reused by the
;; trend, forecast, distribution, and needs-attention aggregators.  Day
;; numbers are absolute integer day counts (`time-to-days'), so they
;; can be compared and subtracted directly without timezone surprises.

(defun org-drill-statistics--today-day ()
  "Return today's absolute day number as an integer.
This is `time-to-days' of the current time.  It is factored out so
tests can redefine it to a fixed day without mocking the clock."
  (time-to-days (current-time)))

(defun org-drill-statistics--record-day (record)
  "Return the integer day number of RECORD's start time.
RECORD is an `org-drill-session-record'; its start-time slot is a float
as produced by `float-time'.  The result is an absolute day number, the
same scale as `org-drill-statistics--today-day'."
  (time-to-days
   (seconds-to-time (org-drill-session-record-start-time record))))

(defun org-drill-statistics--filter-log (log &optional algorithm)
  "Return the records in LOG whose algorithm matches ALGORITHM.
LOG is a list of `org-drill-session-record'.  When ALGORITHM is nil,
return LOG unchanged (all algorithms).  Otherwise keep only records
whose algorithm slot is `eq' to ALGORITHM.  The original list is not
modified."
  (if (null algorithm)
      log
    (cl-remove-if-not
     (lambda (record)
       (eq (org-drill-session-record-algorithm record) algorithm))
     log)))

(defun org-drill-statistics--log-since (log cutoff-float)
  "Return the records in LOG started at or after CUTOFF-FLOAT.
LOG is a list of `org-drill-session-record'.  CUTOFF-FLOAT is a float
in the same units as `float-time' (and the records' start-time slot).
A record is kept when its start-time is greater than or equal to
CUTOFF-FLOAT.  The original list is not modified."
  (cl-remove-if-not
   (lambda (record)
     (>= (org-drill-session-record-start-time record) cutoff-float))
   log))

(defcustom org-drill-statistics-distribution-bar-width 40
  "Maximum width, in block characters, of a distribution histogram bar.
The quality with the highest count fills this many blocks; every other
quality's bar is scaled proportionally against it.  A larger value makes
the histogram wider on the dashboard."
  :type 'integer
  :group 'org-drill-statistics)

(defun org-drill-statistics--render-distribution (histogram)
  "Return the quality-distribution section as a string.
HISTOGRAM is a 6-element vector of integer counts for qualities 0..5, as
produced by `org-drill-statistics--quality-histogram': element I is the
number of times quality I was rated.

The result begins with the section subheading \"** Quality
Distribution\" and contains one row per quality 0..5.  Each row shows the
quality number, a horizontal bar of repeated block characters, the
absolute count, and that quality's percentage of the total ratings.  The
longest bar is `org-drill-statistics-distribution-bar-width' blocks wide
and the others scale against the largest count, so the bars stay
proportional.  A quality with a zero count renders an empty bar but is
still listed, so all six rows are always present.

When the histogram is empty (every count zero, no ratings recorded), the
percentages are all 0 and a short note replaces the bars' meaning, but
the six rows are still emitted so the layout is stable.  This function is
pure: it returns a string and performs no buffer or display side
effects."
  (let* ((counts (append histogram nil))
         (total (apply #'+ counts))
         (max-count (apply #'max counts))
         (width (max 1 org-drill-statistics-distribution-bar-width))
         (block ?\x2588)
         (lines nil))
    (dotimes (q 6)
      (let* ((count (nth q counts))
             (percent (if (> total 0)
                          (round (* 100.0 (/ (float count) total)))
                        0))
             (bar-len (if (> max-count 0)
                          (round (* (/ (float count) max-count) width))
                        0))
             (bar (make-string bar-len block))
             (padded (concat bar
                             (make-string (max 0 (- width (length bar)))
                                          ?\s))))
        (push (format "%d  %s  %4d  %3d%%"
                      q padded count percent)
              lines)))
    (concat "** Quality Distribution\n"
            (if (> total 0)
                (format "Total ratings: %d\n" total)
              "No quality ratings recorded yet.\n")
            (mapconcat #'identity (nreverse lines) "\n")
            "\n")))

(defun org-drill-statistics--reviews-by-day (log &optional days)
  "Return a vector of review COUNTS per day over the last DAYS.
LOG is a list of `org-drill-session-record'; order does not matter
here.  DAYS defaults to `org-drill-statistics-trend-days'.  A
non-positive DAYS is clamped to 1.

The result has one slot per day, oldest to newest, covering the window
that ends today and spans DAYS days inclusive: slot 0 is the oldest day
in the window, the final slot is today.  A day's count is the sum of
\(length qualities) across every record whose start day falls in the
window.  Records that fall outside the window, in the past or the
future, are ignored.  Empty days are zero-filled.  A record with a nil
qualities slot contributes zero.

Day numbers come from `org-drill-statistics--record-day' and
`org-drill-statistics--today-day', so the window aligns on absolute
calendar days and the helper stays testable by redefining those."
  (let* ((days (max 1 (or days org-drill-statistics-trend-days)))
         (today (org-drill-statistics--today-day))
         (start-day (- today (1- days)))
         (counts (make-vector days 0)))
    (dolist (record log counts)
      (let* ((day (org-drill-statistics--record-day record))
             (idx (- day start-day)))
        (when (and (>= idx 0) (< idx days))
          (let ((qualities (org-drill-session-record-qualities record)))
            (aset counts idx
                  (+ (aref counts idx)
                     (if qualities (length qualities) 0)))))))))

(defun org-drill-statistics--pass-rate-by-day (log &optional days)
  "Return a vector of daily pass rates over the last DAYS, oldest first.
LOG is a list of `org-drill-session-record', newest first as stored in
`org-drill-session-log'.  DAYS defaults to `org-drill-statistics-trend-days'.

The result is a vector of length DAYS.  Element 0 is the oldest day in
the window and the final element is today, so the vector reads left to
right in chronological order, matching sparkline rendering.

Each element is an integer pass rate, 0 to 100, for that day, or nil
when no qualities were recorded that day.  A day's pass rate is the
percentage of that day's qualities that are passes across every record
started that day.  A quality is a pass when it is strictly greater than
`org-drill-failure-quality'.  Records outside the window are ignored.

The function is pure with respect to LOG: it reads only the record
slots and the failure-quality threshold.  Today is taken from
`org-drill-statistics--today-day', which tests may redefine to a fixed
day."
  (let* ((days (or days org-drill-statistics-trend-days))
         (days (max 1 days))
         (today (org-drill-statistics--today-day))
         (oldest (- today (1- days)))
         (passes (make-vector days 0))
         (totals (make-vector days 0))
         (threshold org-drill-failure-quality))
    (dolist (record log)
      (let* ((day (org-drill-statistics--record-day record))
             (idx (- day oldest)))
        (when (and (>= idx 0) (< idx days))
          (let ((qualities (org-drill-session-record-qualities record)))
            (when qualities
              (dotimes (qi (length qualities))
                (let ((q (aref qualities qi)))
                  (aset totals idx (1+ (aref totals idx)))
                  (when (> q threshold)
                    (aset passes idx (1+ (aref passes idx)))))))))))
    (let ((result (make-vector days nil)))
      (dotimes (i days)
        (let ((total (aref totals i)))
          (when (> total 0)
            (aset result i (round (* 100.0 (/ (float (aref passes i))
                                              total)))))))
      result)))

(defconst org-drill-statistics--sparkline-chars "▁▂▃▄▅▆▇█"
  "Eight-level quadrant-block charset for sparklines, low to high.
Index 0 is the shortest block, index 7 the full block.  Used by
`org-drill-statistics--sparkline' to map scaled values to glyphs.")

(defun org-drill-statistics--sparkline (numbers &optional max)
  "Render NUMBERS as a quadrant-block sparkline string.
NUMBERS is a sequence (list or vector) of non-negative numbers, with
nil permitted for a missing data point.  Each non-nil value is scaled
against MAX and mapped to one of eight block glyphs in
`org-drill-statistics--sparkline-chars', low to high.  Each nil value
renders as a single space, preserving column alignment.

MAX is the value mapped to the tallest block.  When MAX is nil it
defaults to the largest non-nil value in NUMBERS.  A value at or above
MAX renders as the full block; intermediate values scale linearly.

Edge cases handled without error:
- Empty NUMBERS returns the empty string.
- All-nil NUMBERS returns a string of spaces, one per entry.
- A MAX of zero (all values zero, or an explicit zero MAX) renders
  every non-nil value as the lowest block, since no value exceeds the
  ceiling and division by zero is avoided.

Values are clamped to the inclusive 0..MAX range before scaling, so a
value above MAX still maps to the full block rather than overflowing
the charset."
  (let* ((seq (append numbers nil))
         (effective-max
          (or max
              (let ((non-nil (delq nil (copy-sequence seq))))
                (if non-nil (apply #'max non-nil) 0))))
         (top (1- (length org-drill-statistics--sparkline-chars))))
    (mapconcat
     (lambda (n)
       (cond
        ((null n) " ")
        ((or (null effective-max) (<= effective-max 0))
         (substring org-drill-statistics--sparkline-chars 0 1))
        (t
         (let* ((clamped (max 0 (min n effective-max)))
                (index (round (* (/ (float clamped) effective-max) top))))
           (substring org-drill-statistics--sparkline-chars
                      index (1+ index))))))
     seq
     "")))

;;; Weekly trend aggregation for the statistics dashboard.

(defun org-drill-statistics--week-start-day (day)
  "Return the absolute day number of the Monday starting DAY's week.
DAY is an absolute day number as produced by `time-to-days'.  Weeks
start on Monday, matching ISO week conventions.  The result is itself an
absolute day number and is `<=' DAY.  Pure integer arithmetic, so it is
deterministic across timezones."
  (- day (mod (- day 1) 7)))

(defun org-drill-statistics--avg-duration-min (records)
  "Return the mean session duration of RECORDS in minutes, as a float.
RECORDS is a list of `org-drill-session-record'.  Each duration is
end-time minus start-time, divided by 60.  Returns 0.0 when RECORDS is
empty."
  (if (null records)
      0.0
    (/ (cl-reduce
        (lambda (acc record)
          (+ acc
             (/ (- (org-drill-session-record-end-time record)
                   (org-drill-session-record-start-time record))
                60.0)))
        records
        :initial-value 0.0)
       (float (length records)))))

(defun org-drill-statistics--weekly-aggregates (log &optional weeks)
  "Return per-week aggregate statistics for the most recent WEEKS.
LOG is a list of `org-drill-session-record' in any order.  WEEKS
defaults to 12 and bounds how many weeks the result spans, ending with
the week that contains today.  WEEKS must be a positive integer.

The result is a list of plists, oldest week first, one per week in the
window even when a week has no sessions.  Each plist has these keys:

  :week-start        absolute day number of that week's Monday
  :reviews           total quality ratings entered across the week
  :pass-percent      pooled pass percentage (0 when the week has none)
  :avg-duration-min  mean session duration in minutes, as a float

Weeks start on Monday.  REVIEWS counts every entry in each record's
qualities vector, so a week's reviews is the sum over its sessions.
PASS-PERCENT is recomputed from the week's pooled qualities via
`org-drill--compute-pass-percent', not averaged from the stored
per-session percentages, so multi-session weeks stay correctly weighted.
This function is pure: it reads no org entries and does not modify LOG."
  (setq weeks (or weeks 12))
  (unless (and (integerp weeks) (>= weeks 1))
    (error "WEEKS must be a positive integer, got %s" weeks))
  (let* ((this-week (org-drill-statistics--week-start-day
                     (org-drill-statistics--today-day)))
         (oldest (- this-week (* 7 (1- weeks))))
         (buckets (make-vector weeks nil)))
    (dolist (record log)
      (let* ((wk (org-drill-statistics--week-start-day
                  (org-drill-statistics--record-day record)))
             (idx (/ (- wk oldest) 7)))
        (when (and (>= idx 0) (< idx weeks)
                   (zerop (mod (- wk oldest) 7)))
          (aset buckets idx (cons record (aref buckets idx))))))
    (let ((result nil))
      (dotimes (i weeks)
        (let* ((idx (- weeks 1 i))
               (records (aref buckets idx))
               (week-start (+ oldest (* 7 idx)))
               (qualities (apply #'vconcat
                                 (mapcar #'org-drill-session-record-qualities
                                         records))))
          (push (list :week-start week-start
                      :reviews (length qualities)
                      :pass-percent
                      (org-drill--compute-pass-percent qualities)
                      :avg-duration-min
                      (org-drill-statistics--avg-duration-min records))
                result)))
      result)))

(defun org-drill-statistics--quality-histogram (log)
  "Return a 6-element vector of quality counts across every record in LOG.
LOG is a list of `org-drill-session-record'.  Element I of the result is
the total number of times quality I was entered across all records'
qualities vectors, for I in 0..5.  Qualities outside the 0..5 range are
ignored defensively so a malformed record cannot corrupt the histogram.
A record whose qualities slot is nil contributes nothing.  The original
log and its records are not modified."
  (let ((histogram (make-vector 6 0)))
    (dolist (record log)
      (let ((qualities (org-drill-session-record-qualities record)))
        (when qualities
          (mapc
           (lambda (quality)
             (when (and (integerp quality) (>= quality 0) (<= quality 5))
               (aset histogram quality (1+ (aref histogram quality)))))
           (append qualities nil)))))
    histogram))

;;; Statistics dashboard: card-population overview aggregator.

(defun org-drill-statistics--overview-counts (&optional scope)
  "Return a card-population overview plist for the drill entries in SCOPE.
SCOPE is a value understood by `org-drill-current-scope' (a symbol such
as `file', `directory', `agenda', or a list of files); nil means the
current value of `org-drill-scope'.  The plist has four keys:

  :total   every genuine drill card seen (any non-nil entry status).
  :new     cards never reviewed (status :new).
  :mature  cards with an established interval (status :young, :old, or
           :overdue), counted together as the mature population.
  :lapsed  cards whose last review failed (status :failed).

The mapping is driven by `org-drill-entry-status', which classifies the
entry at point into one of nil, :unscheduled, :future, :new, :failed,
:overdue, :young, or :old.  Cards classified :unscheduled or :future are
real drill cards, so they count toward :total, but they are neither new,
mature, nor lapsed, so they are absent from those three buckets.  Entries
whose status is nil (non-drill headings and skippable empty cards) are
ignored entirely and do not count toward :total.

A fresh, non-cram `org-drill-session' is created for the classification
so the result reflects the cards' own scheduling state rather than any
in-progress drill session."
  (let ((session (org-drill-session))
        (total 0)
        (new 0)
        (mature 0)
        (lapsed 0))
    (org-drill-map-entries
     (lambda ()
       (let ((status (car (org-drill-entry-status session))))
         (when status
           (cl-incf total)
           (cl-case status
             (:new (cl-incf new))
             ((:young :old :overdue) (cl-incf mature))
             (:failed (cl-incf lapsed))))))
     scope)
    (list :total total :new new :mature mature :lapsed lapsed)))

;;; Statistics dashboard: needs-attention selectors (aggregation 7/8)
;;
;; These three selectors return lists of (HEADING . POS) cons cells for
;; the dashboard's needs-attention section: leech candidates,
;; long-overdue cards, and forgotten-new cards.  POS is the integer
;; buffer position of the entry's heading (`point' at the heading), so
;; the renderer can build a follow link.
;;
;; The org traversal is isolated in a single collector,
;; `org-drill-statistics--collect-attention-data', which gathers one
;; plist per drill entry.  Every predicate and sort then operates on
;; that plain data, so the classification and ordering logic is unit
;; testable with a `with-temp-buffer' fixture and without touching the
;; clock.  Day-based fields are stored as integers relative to a
;; supplied "today" day number, again so tests stay deterministic.

(cl-defstruct (org-drill-statistics--entry-attention-data
               (:constructor org-drill-statistics--make-entry-attention-data)
               (:copier nil))
  "Per-entry data used by the needs-attention selectors.
HEADING is the cleaned outline heading string.  POS is the integer
buffer position of the heading.  FAILURE-COUNT is DRILL_FAILURE_COUNT as
an integer (0 when absent).  AVG-QUALITY is DRILL_AVERAGE_QUALITY as a
float, or nil when absent.  DAYS-SINCE-REVIEW is the integer day count
since DRILL_LAST_REVIEWED, or nil when never reviewed.  DAYS-SINCE-ADDED
is the integer day count since DATE_ADDED, or nil when absent.
TOTAL-REPEATS is DRILL_TOTAL_REPEATS as an integer (0 when absent)."
  heading pos failure-count avg-quality
  days-since-review days-since-added total-repeats)

;; Short aliases for the long struct accessors, used by the sort
;; comparators below so the comparator lines stay readable.
(defalias 'org-drill-statistics--ad-avg
  #'org-drill-statistics--entry-attention-data-avg-quality)
(defalias 'org-drill-statistics--ad-fails
  #'org-drill-statistics--entry-attention-data-failure-count)
(defalias 'org-drill-statistics--ad-review
  #'org-drill-statistics--entry-attention-data-days-since-review)
(defalias 'org-drill-statistics--ad-added
  #'org-drill-statistics--entry-attention-data-days-since-added)

(defun org-drill-statistics--collect-attention-data (&optional scope today-day)
  "Collect `org-drill-statistics--entry-attention-data' for each drill entry.
SCOPE is passed through to `org-drill-map-entries' to bound the
traversal, defaulting to `org-drill-scope'.  TODAY-DAY is the absolute
day number treated as today, defaulting to
`org-drill-statistics--today-day'; it is factored out so callers and
tests can pin the reference day.

The point is on each drill entry's heading while its data is read, so
this must run inside an org buffer.  Returns a list of structs, one per
drill entry, in document order."
  (let ((today (or today-day (org-drill-statistics--today-day))))
    (org-drill-map-entries
     (lambda ()
       (org-drill-statistics--entry-attention-data-at-point today))
     scope)))

(defun org-drill-statistics--entry-attention-data-at-point (today-day)
  "Read needs-attention data for the drill entry at point.
TODAY-DAY is the absolute day number treated as today.  Returns an
`org-drill-statistics--entry-attention-data' struct.  This reads the
DRILL_* and DATE_ADDED properties via `org-entry-get' and the heading
via `org-get-heading', so the point must be on a drill heading."
  (let* ((pos (point))
         (heading (org-get-heading t t t t))
         (failure-raw (org-entry-get pos "DRILL_FAILURE_COUNT"))
         (avg-raw (org-entry-get pos "DRILL_AVERAGE_QUALITY"))
         (last-raw (org-entry-get pos "DRILL_LAST_REVIEWED"))
         (added-raw (org-entry-get pos "DATE_ADDED"))
         (repeats-raw (org-entry-get pos "DRILL_TOTAL_REPEATS")))
    (org-drill-statistics--make-entry-attention-data
     :heading (or heading "")
     :pos pos
     :failure-count (if failure-raw (string-to-number failure-raw) 0)
     :avg-quality (and avg-raw (string-to-number avg-raw))
     :days-since-review
     (org-drill-statistics--days-since-org-timestamp last-raw today-day)
     :days-since-added
     (org-drill-statistics--days-since-org-timestamp added-raw today-day)
     :total-repeats (if repeats-raw (string-to-number repeats-raw) 0))))

(defun org-drill-statistics--days-since-org-timestamp (timestamp today-day)
  "Return integer days from org TIMESTAMP string to TODAY-DAY.
TIMESTAMP is an org timestamp string (active or inactive) or nil.
TODAY-DAY is an absolute day number as from `time-to-days'.  Returns the
difference TODAY-DAY minus the timestamp's day, so a value reviewed
today yields 0 and an older review yields a positive count.  Returns nil
when TIMESTAMP is nil or cannot be parsed."
  (when (and timestamp (not (string-empty-p timestamp)))
    (condition-case nil
        (- today-day
           (time-to-days
            (apply #'encode-time (org-parse-time-string timestamp))))
      (error nil))))

;;; Pure predicates and the row cap.

(defun org-drill-statistics--leech-candidate-p (data)
  "Non-nil when DATA describes a leech candidate.
DATA is an `org-drill-statistics--entry-attention-data'.  A card is a
leech candidate when its failure count is at least
`org-drill-leech-failure-threshold' and its average quality is
known and strictly below `org-drill-statistics-leech-quality-threshold'.
A card with no recorded average quality is not flagged, since the
quality criterion cannot be evaluated."
  (let ((avg (org-drill-statistics--entry-attention-data-avg-quality data)))
    (and (>= (org-drill-statistics--entry-attention-data-failure-count data)
             org-drill-leech-failure-threshold)
         avg
         (< avg org-drill-statistics-leech-quality-threshold))))

(defun org-drill-statistics--long-overdue-p (data)
  "Non-nil when DATA describes a long-overdue card.
DATA is an `org-drill-statistics--entry-attention-data'.  A card is
long overdue when it has a recorded last-review date and that review is
more than `org-drill-lapse-threshold-days' days ago.  A card that was
never reviewed has no overdue measure and is not flagged here."
  (let ((days (org-drill-statistics--entry-attention-data-days-since-review
               data)))
    (and days (> days org-drill-lapse-threshold-days))))

(defun org-drill-statistics--forgotten-new-p (data)
  "Non-nil when DATA describes a forgotten-new card.
DATA is an `org-drill-statistics--entry-attention-data'.  A card is
forgotten-new when it was added at least 14 days ago and has never been
repeated, that is its total repeats are zero.  A card with no recorded
add date is not flagged, since its age cannot be evaluated."
  (let ((added (org-drill-statistics--entry-attention-data-days-since-added
                data)))
    (and added
         (>= added 14)
         (= (org-drill-statistics--entry-attention-data-total-repeats data)
            0))))

(defun org-drill-statistics--cap-rows (rows)
  "Cap ROWS at `org-drill-statistics-attention-row-limit' entries.
ROWS is a list.  When it is no longer than the limit, return it
unchanged.  Otherwise return its first LIMIT elements; the renderer is
responsible for the trailing \"+N more\" footer using the original
length.  ROWS is not modified."
  (let ((limit org-drill-statistics-attention-row-limit))
    (if (<= (length rows) limit)
        rows
      (cl-subseq rows 0 limit))))

;;; The three public selectors.
;;
;; Each filters the collected per-entry data with its predicate, sorts
;; per the spec, maps to (HEADING . POS), and caps the row count.

(defun org-drill-statistics--leech-candidates (&optional scope)
  "Return capped leech candidates as a list of (HEADING . POS).
SCOPE bounds the traversal, defaulting to `org-drill-scope'.  Candidates
satisfy `org-drill-statistics--leech-candidate-p' and are sorted worst
first by ascending average quality, breaking ties by descending failure
count.  The list is capped at `org-drill-statistics-attention-row-limit'
rows; the full count is available to the caller via a fresh scan if a
\"+N more\" footer is needed."
  (let* ((data (cl-remove-if-not
                #'org-drill-statistics--leech-candidate-p
                (org-drill-statistics--collect-attention-data scope)))
         (sorted
          (sort data
                (lambda (a b)
                  (let ((qa (org-drill-statistics--ad-avg a))
                        (qb (org-drill-statistics--ad-avg b)))
                    (if (= qa qb)
                        (> (org-drill-statistics--ad-fails a)
                           (org-drill-statistics--ad-fails b))
                      (< qa qb)))))))
    (org-drill-statistics--cap-rows
     (mapcar (lambda (d)
               (cons (org-drill-statistics--entry-attention-data-heading d)
                     (org-drill-statistics--entry-attention-data-pos d)))
             sorted))))

(defun org-drill-statistics--long-overdue (&optional scope)
  "Return capped long-overdue cards as a list of (HEADING . POS).
SCOPE bounds the traversal, defaulting to `org-drill-scope'.  Cards
satisfy `org-drill-statistics--long-overdue-p' and are sorted most
overdue first, by descending days since last review.  The list is capped
at `org-drill-statistics-attention-row-limit' rows."
  (let* ((data (cl-remove-if-not
                #'org-drill-statistics--long-overdue-p
                (org-drill-statistics--collect-attention-data scope)))
         (sorted
          (sort data
                (lambda (a b)
                  (> (org-drill-statistics--ad-review a)
                     (org-drill-statistics--ad-review b))))))
    (org-drill-statistics--cap-rows
     (mapcar (lambda (d)
               (cons (org-drill-statistics--entry-attention-data-heading d)
                     (org-drill-statistics--entry-attention-data-pos d)))
             sorted))))

(defun org-drill-statistics--forgotten-new (&optional scope)
  "Return capped forgotten-new cards as a list of (HEADING . POS).
SCOPE bounds the traversal, defaulting to `org-drill-scope'.  Cards
satisfy `org-drill-statistics--forgotten-new-p' and are sorted oldest
first, by descending days since they were added.  The list is capped at
`org-drill-statistics-attention-row-limit' rows."
  (let* ((data (cl-remove-if-not
                #'org-drill-statistics--forgotten-new-p
                (org-drill-statistics--collect-attention-data scope)))
         (sorted
          (sort data
                (lambda (a b)
                  (> (org-drill-statistics--ad-added a)
                     (org-drill-statistics--ad-added b))))))
    (org-drill-statistics--cap-rows
     (mapcar (lambda (d)
               (cons (org-drill-statistics--entry-attention-data-heading d)
                     (org-drill-statistics--entry-attention-data-pos d)))
             sorted))))

;;; Forecast aggregator (helper 8/8).
;;
;; The forecast answers: of the scheduled cards in scope, how many come
;; due on each of the next DAYS days, starting today?  The org-reading
;; part (collecting SCHEDULED day numbers from entries in scope) is kept
;; separate from the pure bucketing math so the math is unit-testable
;; without an org buffer.

(defun org-drill-statistics--bucket-forecast-days
    (scheduled-days today-day days)
  "Bucket SCHEDULED-DAYS into a DAYS-long forecast vector.
SCHEDULED-DAYS is a list of absolute integer day numbers (the scale of
`time-to-days'), one per scheduled card.  TODAY-DAY is today's absolute
day number.  DAYS is the number of forecast buckets.

Return a list of DAYS integers.  Element I is the number of entries in
SCHEDULED-DAYS whose day equals TODAY-DAY plus I, so index 0 counts
cards due today and the last index counts cards due TODAY-DAY plus DAYS
minus one.  Cards scheduled in the past or beyond the window are not
counted.  When DAYS is zero or negative the result is the empty list."
  (let ((buckets (if (> days 0) (make-vector days 0) (vector))))
    (when (> days 0)
      (dolist (day scheduled-days)
        (let ((offset (- day today-day)))
          (when (and (>= offset 0) (< offset days))
            (aset buckets offset (1+ (aref buckets offset)))))))
    (append buckets nil)))

(defun org-drill-statistics--scheduled-days (&optional scope)
  "Return the SCHEDULED day numbers of drill entries in SCOPE.
SCOPE accepts the same values as `org-drill-scope'; nil uses the
current value.  Walk the drill entries in scope with
`org-drill-map-entries' and collect, for each entry that carries a
SCHEDULED time, its absolute integer day number (`time-to-days' of the
scheduled time).  Entries without a SCHEDULED time are skipped.  The
returned list is in entry-traversal order and may contain duplicate
days (several cards due the same day)."
  (delq nil
        (org-drill-map-entries
         (lambda ()
           (let ((scheduled (org-get-scheduled-time (point))))
             (when scheduled
               (time-to-days scheduled))))
         scope)))

(defun org-drill-statistics--forecast (&optional scope days)
  "Return the upcoming-due forecast for drill entries in SCOPE.
SCOPE accepts the same values as `org-drill-scope'; nil uses the
current value.  DAYS is the number of forecast buckets and defaults to
`org-drill-statistics-forecast-days'.

The result is a list of DAYS integers.  Element I counts the cards in
scope whose SCHEDULED date falls on today plus I, so index 0 is due
today and the last index is due DAYS minus one days out.  Cards
scheduled in the past or beyond the window are not counted, and cards
with no SCHEDULED time are ignored."
  (let ((days (or days org-drill-statistics-forecast-days)))
    (org-drill-statistics--bucket-forecast-days
     (org-drill-statistics--scheduled-days scope)
     (org-drill-statistics--today-day)
     days)))

;;; Statistics dashboard: needs-attention section renderer (render 4/5).
;;
;; `org-drill-statistics--render-attention' returns the section text as a
;; string so it is unit testable without a live dashboard buffer.  It runs
;; a single org scan via `org-drill-statistics--collect-attention-data',
;; then filters, sorts, caps, and footers each of the three categories.
;; Doing one scan (rather than calling the three public selectors, which
;; each scan and cap) gives the pre-cap totals needed for the "+N more"
;; footers for free.

(defun org-drill-statistics--card-link (heading pos)
  "Return an org bracket link to a drill card heading.
HEADING is the card's outline heading string, used as the link
description.  POS is the integer buffer position of the heading, carried
in the link path so the dashboard's RET handler can jump to the card.
The path has the form \"org-drill-card:POS\".  Any closing bracket in
HEADING is replaced so a literal \"]]\" cannot terminate the link early.
An empty or nil HEADING falls back to a position-based description."
  (let* ((desc (if (and heading (not (string-empty-p heading)))
                   heading
                 (format "card at %d" pos)))
         (safe (replace-regexp-in-string "]" "}" desc)))
    (format "[[org-drill-card:%d][%s]]" pos safe)))

(defun org-drill-statistics--render-attention-table
    (title rows total empty-note)
  "Render one needs-attention subsection as an org string.
TITLE is the third-level heading text for the subsection.  ROWS is a
list of (HEADING . POS) cons cells, already capped at
`org-drill-statistics-attention-row-limit' and already sorted.  TOTAL is
the full count of matching cards before capping, used for the
\"+N more\" footer.  EMPTY-NOTE is the line shown when ROWS is empty.

The returned string ends with a trailing newline.  When ROWS is
non-empty it contains a single-column org table of card links, followed
by a \"+N more\" footer line naming the hidden count when TOTAL exceeds
the number of rows shown."
  (let ((shown (length rows)))
    (concat
     (format "*** %s\n" title)
     (if (null rows)
         (format "%s\n" empty-note)
       (concat
        "| Card |\n"
        "|------|\n"
        (mapconcat
         (lambda (row)
           (format "| %s |"
                   (org-drill-statistics--card-link (car row) (cdr row))))
         rows
         "\n")
        "\n"
        (let ((hidden (- total shown)))
          (if (> hidden 0)
              (format "+%d more\n" hidden)
            "")))))))

(defun org-drill-statistics--render-attention (&optional scope)
  "Return the needs-attention dashboard section as an org string.
SCOPE bounds the drill traversal and accepts the same values as
`org-drill-scope'; nil uses the current value.  This must run inside an
org buffer because it scans drill entries via
`org-drill-statistics--collect-attention-data'.

The section opens with a \"** Needs attention\" heading and contains
three subsections, each an org table of card links:

  Leech candidates   cards failing repeatedly with low average quality,
                     worst first.
  Long overdue       cards whose last review is well past the lapse
                     threshold, most overdue first.
  Forgotten new      cards added long ago and never repeated, oldest
                     first.

Each table is capped at `org-drill-statistics-attention-row-limit' rows;
a category with more matches than the cap gains a \"+N more\" footer
naming the hidden count.  A category with no matches shows a short note
instead of a table.  This function returns a string and does not print,
switch buffers, or move point beyond the traversal itself.

A single collection pass feeds all three categories, so the full
pre-cap totals are available for the footers without rescanning."
  (let* ((data (org-drill-statistics--collect-attention-data scope))
         (leech-all (cl-remove-if-not
                     #'org-drill-statistics--leech-candidate-p data))
         (overdue-all (cl-remove-if-not
                       #'org-drill-statistics--long-overdue-p data))
         (new-all (cl-remove-if-not
                   #'org-drill-statistics--forgotten-new-p data))
         (leech-sorted
          (sort (copy-sequence leech-all)
                (lambda (a b)
                  (let ((qa (org-drill-statistics--ad-avg a))
                        (qb (org-drill-statistics--ad-avg b)))
                    (if (= qa qb)
                        (> (org-drill-statistics--ad-fails a)
                           (org-drill-statistics--ad-fails b))
                      (< qa qb))))))
         (overdue-sorted
          (sort (copy-sequence overdue-all)
                (lambda (a b)
                  (> (org-drill-statistics--ad-review a)
                     (org-drill-statistics--ad-review b)))))
         (new-sorted
          (sort (copy-sequence new-all)
                (lambda (a b)
                  (> (org-drill-statistics--ad-added a)
                     (org-drill-statistics--ad-added b)))))
         (to-rows
          (lambda (structs)
            (mapcar
             (lambda (d)
               (cons (org-drill-statistics--entry-attention-data-heading d)
                     (org-drill-statistics--entry-attention-data-pos d)))
             (org-drill-statistics--cap-rows structs)))))
    (concat
     "** Needs attention\n"
     (org-drill-statistics--render-attention-table
      "Leech candidates"
      (funcall to-rows leech-sorted)
      (length leech-all)
      "No leech candidates.")
     (org-drill-statistics--render-attention-table
      "Long overdue"
      (funcall to-rows overdue-sorted)
      (length overdue-all)
      "No long-overdue cards.")
     (org-drill-statistics--render-attention-table
      "Forgotten new"
      (funcall to-rows new-sorted)
      (length new-all)
      "No forgotten-new cards."))))

;;; Statistics dashboard: overview section renderer (render 1/5).
;;
;; Pure string builder.  It reads card-population counts via
;; `org-drill-statistics--overview-counts' (which traverses the org
;; entries in scope) and the newest session record from LOG, then
;; returns the section text.  It never prints, switches buffers, or
;; mutates state, so it is testable with a plain string assertion.

(defun org-drill-statistics--format-last-session (record)
  "Return the one-line \"Last session\" recap string for RECORD.
RECORD is an `org-drill-session-record', or nil when no session has
been logged.  When nil, return a line stating there is no session yet.
Otherwise the line names the session date, its duration in minutes, the
number of cards reviewed (length of the qualities vector), and the
pass percentage.  Pure: it reads only RECORD's slots."
  (if (null record)
      "Last session: none recorded yet."
    (let* ((start (org-drill-session-record-start-time record))
           (end (org-drill-session-record-end-time record))
           (qualities (org-drill-session-record-qualities record))
           (reviewed (if qualities (length qualities) 0))
           (duration-min (max 0 (round (/ (- end start) 60.0))))
           (pass (org-drill-session-record-pass-percent record))
           (date (format-time-string "%Y-%m-%d" (seconds-to-time start))))
      (format
       "Last session: %s, %d min, %d card%s reviewed, %d%% pass."
       date duration-min reviewed (if (= reviewed 1) "" "s") pass))))

(defun org-drill-statistics--render-overview (&optional scope log)
  "Return the Overview section of the statistics dashboard as a string.
SCOPE is passed to `org-drill-statistics--overview-counts' to bound the
card-population traversal; nil uses the current `org-drill-scope'.  LOG
is a list of `org-drill-session-record' newest first, defaulting to
`org-drill-session-log'; its head supplies the \"Last session\" recap.

The returned string is an org fragment: a \"** Overview\" subheading, a
4-column org table with a header row (Total cards, New, Mature, Lapsed)
and one data row from `org-drill-statistics--overview-counts', then a
blank line and the one-line last-session recap.  This function is pure
with respect to its inputs apart from the org traversal that
`org-drill-statistics--overview-counts' performs over SCOPE; it does
not print, switch buffers, or modify any state."
  (let* ((log (or log org-drill-session-log))
         (counts (org-drill-statistics--overview-counts scope))
         (total (plist-get counts :total))
         (new (plist-get counts :new))
         (mature (plist-get counts :mature))
         (lapsed (plist-get counts :lapsed))
         (recap (org-drill-statistics--format-last-session (car log))))
    (concat
     "** Overview\n"
     "| Total cards | New | Mature | Lapsed |\n"
     "|-------------+-----+--------+--------|\n"
     (format "| %d | %d | %d | %d |\n" total new mature lapsed)
     "\n"
     recap "\n")))

;;; Statistics dashboard: trends render helper (render 2/5).
;;
;; `org-drill-statistics--render-trends' returns the Trends section as a
;; string: a subheading, two sparkline lines (reviews per day and pass
;; rate per day over the trend window), then an org table of the last 12
;; weekly aggregates.  It is pure: it reads only LOG and the dashboard
;; defcustoms, calls the aggregation helpers, and returns a string.  It
;; never prints, inserts, or switches buffers, so it stays unit testable.

(defun org-drill-statistics--format-week-start (day)
  "Format absolute DAY number as a YYYY-MM-DD date string.
DAY is an absolute day number on the `time-to-days' scale, as stored in
the :week-start slot of `org-drill-statistics--weekly-aggregates'.  The
result is the Gregorian calendar date of that day, zero-padded.  Pure
integer-to-string conversion via `calendar-gregorian-from-absolute', so
it is timezone independent and deterministic."
  (let ((gregorian (calendar-gregorian-from-absolute day)))
    (format "%04d-%02d-%02d"
            (nth 2 gregorian)
            (nth 0 gregorian)
            (nth 1 gregorian))))

(defun org-drill-statistics--render-weekly-table (weekly)
  "Render WEEKLY aggregates as an org-mode table string.
WEEKLY is a list of plists as returned by
`org-drill-statistics--weekly-aggregates', oldest week first.  The table
has a header row, an org separator row, and one body row per week with
columns: Week (the Monday date), Reviews, Pass %, and Avg min (mean
session duration in minutes, one decimal place).  The returned string
ends with a trailing newline.  Pure string assembly: no buffer or org
table alignment is performed, so columns are single-space padded inside
the pipes and org realigns them when the buffer renders."
  (let ((rows
         (mapcar
          (lambda (week)
            (format "| %s | %d | %d | %.1f |"
                    (org-drill-statistics--format-week-start
                     (plist-get week :week-start))
                    (plist-get week :reviews)
                    (plist-get week :pass-percent)
                    (plist-get week :avg-duration-min)))
          weekly)))
    (concat "| Week | Reviews | Pass % | Avg min |\n"
            "|------+---------+--------+---------|\n"
            (if rows
                (concat (mapconcat #'identity rows "\n") "\n")
              ""))))

(defun org-drill-statistics--render-trends (log &optional algorithm)
  "Return the Trends dashboard section for LOG as a string.
LOG is a list of `org-drill-session-record', newest first as stored in
`org-drill-session-log'.  ALGORITHM, when non-nil, restricts the section
to records whose algorithm slot is `eq' to it, via
`org-drill-statistics--filter-log'; nil includes every algorithm.

The section is a string containing, in order:

  - an \"* Trends\" org subheading,
  - a reviews-per-day sparkline over the last
    `org-drill-statistics-trend-days' days, labelled with the window,
  - a pass-rate-per-day sparkline over the same window,
  - an org table of the last 12 weekly aggregates (week-start date,
    reviews, pass percentage, mean session duration in minutes).

The sparklines come from `org-drill-statistics--sparkline' fed by
`org-drill-statistics--reviews-by-day' and
`org-drill-statistics--pass-rate-by-day'; the pass-rate sparkline is
scaled against a fixed ceiling of 100 so the glyph heights read as
absolute percentages rather than relative to the window's own peak.  The
table comes from `org-drill-statistics--weekly-aggregates'.

This function is pure: it reads only LOG and the dashboard defcustoms,
returns a string, and does not print, insert, or switch buffers."
  (let* ((filtered (org-drill-statistics--filter-log log algorithm))
         (days org-drill-statistics-trend-days)
         (reviews (org-drill-statistics--reviews-by-day filtered days))
         (pass-rates (org-drill-statistics--pass-rate-by-day filtered days))
         (weekly (org-drill-statistics--weekly-aggregates filtered 12)))
    (concat
     "* Trends\n"
     (format "Reviews/day (last %d): %s\n"
             days
             (org-drill-statistics--sparkline reviews))
     (format "Pass rate/day (last %d): %s\n"
             days
             (org-drill-statistics--sparkline pass-rates 100))
     "\n"
     (org-drill-statistics--render-weekly-table weekly))))

;;; Statistics dashboard: forecast section renderer (render 5/5).
;;
;; Pure string builder for the dashboard's Forecast section.  It calls
;; `org-drill-statistics--forecast' for the per-day counts and lays them
;; out as a one-line org table: a header row labelling each upcoming day
;; (Today, +1, +2, ...) and a counts row beneath it.  The function does
;; no printing and switches no buffers, so it is unit testable by
;; asserting on the returned string.

(defun org-drill-statistics--render-forecast (&optional scope days)
  "Return the Forecast dashboard section as an org-formatted string.
SCOPE accepts the same values as `org-drill-scope'; nil uses the current
value.  DAYS is the number of forecast buckets and defaults to
`org-drill-statistics-forecast-days'.  Reading SCOPE walks org entries,
so this must run inside an org buffer when SCOPE resolves to live cards.

The returned string starts with a \"** Forecast\" subheading, then a
one-line org table.  The table's header row labels each upcoming day:
the first column is \"Today\", and each later column is \"+N\" for N days
out.  The counts row beneath holds the number of cards due on each day,
taken from `org-drill-statistics--forecast'.  A trailing newline ends
the section so sections concatenate cleanly.

When DAYS resolves to zero or fewer the forecast is empty; the section
then carries a short note in place of the table."
  (let* ((counts (org-drill-statistics--forecast scope days))
         (n (length counts)))
    (if (zerop n)
        (concat "** Forecast\n"
                "No forecast window configured.\n")
      (let* ((labels (cons "Today"
                           (mapcar (lambda (i) (format "+%d" i))
                                   (number-sequence 1 (1- n)))))
             (header (concat "| " (mapconcat #'identity labels " | ") " |"))
             (values (concat "| "
                             (mapconcat (lambda (c) (format "%d" c))
                                        counts " | ")
                             " |")))
        (concat "** Forecast\n"
                header "\n"
                values "\n")))))

;;; Statistics dashboard: buffer-wide filter state and the UI shell.
;;
;; The dashboard is one read-only org-mode buffer assembled from a
;; filter header line plus the five `org-drill-statistics--render-*'
;; section strings.  A single buffer-wide filter (scope, range,
;; algorithm) drives every section; the cycle commands rotate one filter
;; dimension and re-render in place.
;;
;; Integration seam with the render helpers: the renderers read the
;; three buffer-local filter variables below.
;; `org-drill-statistics--render-all'
;; binds the resolved log and scope/algorithm into those vars before
;; calling the renderers, so each renderer sees a consistent window.  See
;; the integration notes for the exact contract each renderer is expected
;; to honor.

(declare-function org-drill-statistics--render-overview "org-drill")
(declare-function org-drill-statistics--render-trends "org-drill")
(declare-function org-drill-statistics--render-distribution "org-drill")
(declare-function org-drill-statistics--render-attention "org-drill")
(declare-function org-drill-statistics--render-forecast "org-drill")

(defconst org-drill-statistics--buffer-name "*Org Drill Statistics*"
  "Name of the read-only buffer holding the statistics dashboard.")

(defvar org-drill-statistics-range-presets
  '(("last 90d" . 90)
    ("last 30d" . 30)
    ("last 7d"  . 7)
    ("all time" . nil))
  "Range filter presets for the dashboard, in cycle order.
Each element is a cons (LABEL . DAYS).  LABEL is the human string shown
in the header line.  DAYS is the size of the trailing window in days, or
nil for the whole log.  The first element is the default range and the
cycle wraps back to it after the last.")

(defvar-local org-drill-statistics--scope nil
  "Buffer-local active scope filter for the dashboard.
A value understood by `org-drill-current-scope', or nil to use the
current `org-drill-scope'.  Set by `org-drill-statistics' and rotated by
`org-drill-statistics-cycle-scope'.  Read by the render helpers to bound
their org traversal.")

(defvar-local org-drill-statistics--range "last 90d"
  "Buffer-local active range-filter label for the dashboard.
One of the LABEL strings in `org-drill-statistics-range-presets'.  Set
by `org-drill-statistics' and rotated by
`org-drill-statistics-cycle-range'.  The matching DAYS value bounds the
session log the trend and distribution sections aggregate over.")

(defvar-local org-drill-statistics--algorithm nil
  "Buffer-local active algorithm filter for the dashboard.
A drill algorithm symbol to restrict the session log to, or nil for all
algorithms.  Set by `org-drill-statistics' and rotated by
`org-drill-statistics-cycle-algorithm'.  Read by the render helpers via
the filtered log they are handed.")

(defun org-drill-statistics--range-cutoff-float (label)
  "Return the float-time cutoff for range LABEL, or nil for all time.
LABEL is a key in `org-drill-statistics-range-presets'.  When its DAYS
value is non-nil, return the `float-time' of the instant DAYS days before
now, so records at or after the cutoff fall in the window.  When DAYS is
nil, or LABEL is unknown, return nil meaning no lower bound."
  (let ((days (cdr (assoc label org-drill-statistics-range-presets))))
    (when days
      (- (float-time) (* days 86400.0)))))

(defun org-drill-statistics--filtered-log (range-label algorithm)
  "Return `org-drill-session-log' narrowed by RANGE-LABEL and ALGORITHM.
RANGE-LABEL is a key in `org-drill-statistics-range-presets'; its DAYS
value bounds the trailing window, and a nil DAYS keeps the whole log.
ALGORITHM is an algorithm symbol to keep, or nil for all algorithms.
The original log is not modified."
  (let* ((by-algo (org-drill-statistics--filter-log
                   org-drill-session-log algorithm))
         (cutoff (org-drill-statistics--range-cutoff-float range-label)))
    (if cutoff
        (org-drill-statistics--log-since by-algo cutoff)
      by-algo)))

(defun org-drill-statistics--header-line (scope range algorithm)
  "Return the dashboard filter header line for SCOPE, RANGE and ALGORITHM.
SCOPE is the active scope value, or nil for the current `org-drill-scope'.
RANGE is a range-preset label string.  ALGORITHM is an algorithm symbol,
or nil for all algorithms.  The result is a single line summarizing the
three active filters, matching the spec's header format."
  (format "Scope: %s    Range: %s    Algorithm: %s"
          (or scope org-drill-scope)
          range
          (or algorithm "all")))

(defun org-drill-statistics--render-all (scope range algorithm)
  "Assemble the full dashboard body string for the active filters.
SCOPE, RANGE and ALGORITHM are the resolved filter values.  This binds
the resolved filtered log and the scope/algorithm into the buffer-local
filter variables so each `org-drill-statistics--render-*' helper reads a
consistent window, then concatenates the header line and the five
section strings.  The log and card scans run while these bindings are in
effect.  Returns the assembled buffer contents as a string."
  (let* ((org-drill-statistics--scope scope)
         (org-drill-statistics--range range)
         (org-drill-statistics--algorithm algorithm)
         (log (org-drill-statistics--filtered-log range algorithm))
         (hist (org-drill-statistics--quality-histogram log)))
    (concat
     (org-drill-statistics--header-line scope range algorithm)
     "\n\n"
     (org-drill-statistics--render-overview scope log)
     "\n"
     ;; LOG is already algorithm-filtered, so pass nil to avoid
     ;; filtering a second time.
     (org-drill-statistics--render-trends log nil)
     "\n"
     (org-drill-statistics--render-distribution hist)
     "\n"
     (org-drill-statistics--render-attention scope)
     "\n"
     (org-drill-statistics--render-forecast scope))))

(defvar org-drill-statistics-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map (kbd "q")   #'quit-window)
    (define-key map (kbd "g")   #'org-drill-statistics-refresh)
    (define-key map (kbd "e")   #'org-drill-statistics-export-csv)
    (define-key map (kbd "s")   #'org-drill-statistics-cycle-scope)
    (define-key map (kbd "r")   #'org-drill-statistics-cycle-range)
    (define-key map (kbd "a")   #'org-drill-statistics-cycle-algorithm)
    (define-key map (kbd "RET") #'org-open-at-point)
    map)
  "Keymap for `org-drill-statistics-mode'.
Bindings: q bury, g refresh, e export CSV, s cycle scope, r cycle range,
a cycle algorithm, RET follow the card link at point.  These avoid the
read-only org-mode bindings the dashboard relies on for link following
and navigation.")

(define-minor-mode org-drill-statistics-mode
  "Minor mode active in the org-drill statistics dashboard buffer.
Installs `org-drill-statistics-mode-map' so q, g, e, s, r, a and RET
drive the dashboard.  Enabled by `org-drill-statistics' after the buffer
is put in `org-mode'; not intended to be toggled by hand."
  :lighter " OrgDrillStats"
  :keymap org-drill-statistics-mode-map)

(defun org-drill-statistics--render (buffer scope range algorithm)
  "Render the dashboard for SCOPE, RANGE, ALGORITHM into BUFFER.
BUFFER is the target buffer.  Its read-only state is lifted for the
write, the assembled body replaces the contents, point returns to the
top, and the buffer is left read-only.  The three filter values are
stored buffer-locally so refresh and the cycle commands can read and
rotate them.  Returns BUFFER."
  (with-current-buffer buffer
    (let ((inhibit-read-only t)
          (body (org-drill-statistics--render-all scope range algorithm)))
      (erase-buffer)
      (insert body)
      (goto-char (point-min)))
    (setq org-drill-statistics--scope scope
          org-drill-statistics--range range
          org-drill-statistics--algorithm algorithm)
    (setq buffer-read-only t)
    buffer))

;;;###autoload
(defun org-drill-statistics ()
  "Open the org-drill statistics dashboard.
Builds the read-only org-mode buffer `*Org Drill Statistics*' from the
filter header and the five render sections, using the current
`org-drill-scope', the default range, and all algorithms, then switches
to it.  Refresh and the s/r/a cycle commands re-render in place."
  (interactive)
  (let ((buffer (get-buffer-create org-drill-statistics--buffer-name))
        (scope org-drill-scope)
        (range (caar org-drill-statistics-range-presets))
        (algorithm nil))
    (with-current-buffer buffer
      (org-mode)
      (org-drill-statistics-mode 1))
    (org-drill-statistics--render buffer scope range algorithm)
    (switch-to-buffer buffer)
    buffer))

(defun org-drill-statistics-refresh ()
  "Re-render the dashboard in place, preserving the active filters.
Reads the buffer-local scope, range, and algorithm filters and rebuilds
every section against the current log and card state.  Bound to g."
  (interactive)
  (org-drill-statistics--render
   (current-buffer)
   org-drill-statistics--scope
   org-drill-statistics--range
   org-drill-statistics--algorithm))

(defun org-drill-statistics-cycle-scope ()
  "Cycle the dashboard scope filter and refresh.
Rotates through a small fixed set of common scopes, then re-renders.
Bound to s."
  (interactive)
  (let* ((scopes '(file directory agenda agenda-with-archives))
         (current org-drill-statistics--scope)
         (tail (cdr (member current scopes)))
         (next (or (car tail) (car scopes))))
    (setq org-drill-statistics--scope next)
    (org-drill-statistics-refresh)
    (message "Scope: %s" next)))

(defun org-drill-statistics-cycle-range ()
  "Cycle the dashboard range filter and refresh.
Rotates through `org-drill-statistics-range-presets' in order, wrapping
after the last preset, then re-renders.  Bound to r."
  (interactive)
  (let* ((labels (mapcar #'car org-drill-statistics-range-presets))
         (current org-drill-statistics--range)
         (tail (cdr (member current labels)))
         (next (or (car tail) (car labels))))
    (setq org-drill-statistics--range next)
    (org-drill-statistics-refresh)
    (message "Range: %s" next)))

(defun org-drill-statistics-cycle-algorithm ()
  "Cycle the dashboard algorithm filter and refresh.
Rotates through every algorithm symbol seen in `org-drill-session-log'
plus an all-algorithms state (nil), then re-renders.  When the log has
no records the filter stays at all-algorithms.  Bound to a."
  (interactive)
  (let* ((algorithms
          (delete-dups
           (delq nil
                 (mapcar #'org-drill-session-record-algorithm
                         org-drill-session-log))))
         ;; nil (all) leads the cycle, then each known algorithm.
         (states (cons nil algorithms))
         (current org-drill-statistics--algorithm)
         (tail (cdr (member current states)))
         (next (if (member current states)
                   (car tail)
                 nil)))
    (setq org-drill-statistics--algorithm next)
    (org-drill-statistics-refresh)
    (message "Algorithm: %s" (or next "all"))))

;;; CSV export for the statistics dashboard (step 3).
;;
;; The row builders are pure: they turn the session log and the scope's
;; drill entries into lists of field strings.  `--write-csv' renders those
;; through `--csv-row' and writes a file.  The command threads the active
;; dashboard filters (scope, range, algorithm) into all three views.

(defun org-drill-statistics--csv-quote (field)
  "Return FIELD as a CSV field string, quoted when required (RFC 4180).
FIELD is coerced to a string with `format' when it is not already one.
It is wrapped in double quotes when it contains a comma, a double quote,
a carriage return, or a newline, and any embedded double quote is doubled."
  (let ((s (if (stringp field) field (format "%s" field))))
    (if (string-match-p "[\",\r\n]" s)
        (concat "\"" (replace-regexp-in-string "\"" "\"\"" s) "\"")
      s)))

(defun org-drill-statistics--csv-row (fields)
  "Join FIELDS into one CSV record line, quoting each field as needed.
FIELDS is a list; each element passes through `org-drill-statistics--csv-quote'.
No trailing newline is added."
  (mapconcat #'org-drill-statistics--csv-quote fields ","))

(defun org-drill-statistics--write-csv (path header rows)
  "Write HEADER then ROWS as CSV to PATH.
HEADER is a list of column names.  ROWS is a list of field-lists.  Each is
rendered with `org-drill-statistics--csv-row' and terminated by a newline."
  (with-temp-file path
    (insert (org-drill-statistics--csv-row header) "\n")
    (dolist (row rows)
      (insert (org-drill-statistics--csv-row row) "\n"))))

(defconst org-drill-statistics--sessions-csv-header
  '("start" "end" "scope" "algorithm" "qualities" "pass_percent"
    "new" "mature" "failed" "cram")
  "Column header for sessions.csv, tracking the session-record slots.")

(defun org-drill-statistics--session-row (record)
  "Return RECORD as a list of CSV field strings for sessions.csv.
RECORD is an `org-drill-session-record'.  Start and end times render as
local \"YYYY-MM-DD HH:MM:SS\"; the qualities vector renders space-joined."
  (list
   (format-time-string
    "%Y-%m-%d %H:%M:%S"
    (seconds-to-time (org-drill-session-record-start-time record)))
   (format-time-string
    "%Y-%m-%d %H:%M:%S"
    (seconds-to-time (org-drill-session-record-end-time record)))
   (format "%s" (org-drill-session-record-scope record))
   (format "%s" (org-drill-session-record-algorithm record))
   (mapconcat #'number-to-string
              (append (or (org-drill-session-record-qualities record) []) nil)
              " ")
   (number-to-string (org-drill-session-record-pass-percent record))
   (number-to-string (org-drill-session-record-new-count record))
   (number-to-string (org-drill-session-record-mature-count record))
   (number-to-string (org-drill-session-record-failed-count record))
   (if (org-drill-session-record-cram-mode record) "t" "nil")))

(defun org-drill-statistics--sessions-rows (log)
  "Return one CSV field-list per record in LOG, preserving LOG order."
  (mapcar #'org-drill-statistics--session-row log))

(defconst org-drill-statistics--cards-csv-header
  '("heading" "last_interval" "repeats_since_fail" "total_repeats"
    "failure_count" "average_quality" "ease" "last_quality"
    "last_reviewed" "status")
  "Column header for cards.csv.")

(defun org-drill-statistics--card-row (session)
  "Return the drill entry at point as a list of CSV fields for cards.csv.
SESSION is a transient `org-drill-session' used only to classify status.
A missing property renders as the empty string."
  (list
   (org-get-heading t t t t)
   (or (org-entry-get (point) "DRILL_LAST_INTERVAL") "")
   (or (org-entry-get (point) "DRILL_REPEATS_SINCE_FAIL") "")
   (or (org-entry-get (point) "DRILL_TOTAL_REPEATS") "")
   (or (org-entry-get (point) "DRILL_FAILURE_COUNT") "")
   (or (org-entry-get (point) "DRILL_AVERAGE_QUALITY") "")
   (or (org-entry-get (point) "DRILL_EASE") "")
   (or (org-entry-get (point) "DRILL_LAST_QUALITY") "")
   (or (org-entry-get (point) "DRILL_LAST_REVIEWED") "")
   (format "%s" (or (car (org-drill-entry-status session)) ""))))

(defun org-drill-statistics--cards-rows (&optional scope)
  "Return one CSV field-list per drill entry in SCOPE for cards.csv.
SCOPE is a value understood by `org-drill-current-scope'; nil uses the
current `org-drill-scope'.  A fresh non-cram session classifies status."
  (let ((session (org-drill-session))
        (rows nil))
    (org-drill-map-entries
     (lambda () (push (org-drill-statistics--card-row session) rows))
     scope)
    (nreverse rows)))

(defconst org-drill-statistics--daily-csv-header
  '("date" "reviews" "passes" "fails" "pass_percent" "duration_min")
  "Column header for daily.csv.")

(defun org-drill-statistics--daily-rows (log &optional days)
  "Return one CSV field-list per day over the last DAYS for daily.csv.
LOG is a list of `org-drill-session-record'.  DAYS defaults to
`org-drill-statistics-trend-days'; a non-positive value is clamped to 1.
Rows are oldest-first, one per day, the final row being today.  Each row
is (DATE REVIEWS PASSES FAILS PASS_PERCENT DURATION_MIN).  A pass is a
quality above `org-drill-failure-quality'.  Empty days report zeros."
  (let* ((days (max 1 (or days org-drill-statistics-trend-days)))
         (today (org-drill-statistics--today-day))
         (start-day (- today (1- days)))
         (reviews (make-vector days 0))
         (passes (make-vector days 0))
         (fails (make-vector days 0))
         (duration (make-vector days 0.0)))
    (dolist (record log)
      (let* ((day (org-drill-statistics--record-day record))
             (idx (- day start-day)))
        (when (and (>= idx 0) (< idx days))
          (let ((qs (org-drill-session-record-qualities record)))
            (when qs
              (mapc
               (lambda (q)
                 (aset reviews idx (1+ (aref reviews idx)))
                 (if (> q org-drill-failure-quality)
                     (aset passes idx (1+ (aref passes idx)))
                   (aset fails idx (1+ (aref fails idx)))))
               qs)))
          (aset duration idx
                (+ (aref duration idx)
                   (/ (- (org-drill-session-record-end-time record)
                         (org-drill-session-record-start-time record))
                      60.0))))))
    (let ((rows nil))
      (dotimes (i days)
        (let* ((g (calendar-gregorian-from-absolute (+ start-day i)))
               (date (format "%04d-%02d-%02d" (nth 2 g) (nth 0 g) (nth 1 g)))
               (r (aref reviews i)))
          (push (list date
                      (number-to-string r)
                      (number-to-string (aref passes i))
                      (number-to-string (aref fails i))
                      (number-to-string
                       (if (> r 0)
                           (round (* 100.0 (/ (float (aref passes i)) r)))
                         0))
                      (number-to-string (round (aref duration i))))
                rows)))
      (nreverse rows))))

;;;###autoload
(defun org-drill-statistics-export-csv (directory)
  "Export the statistics views to CSV files in DIRECTORY.
Writes sessions.csv (one row per session record), cards.csv (one row per
drill entry in the active scope), and daily.csv (one row per day in the
active range).  When called from the dashboard buffer the active
scope/range/algorithm filters apply; otherwise the buffer-local defaults
do.  Interactively, prompts for DIRECTORY."
  (interactive
   (list (read-directory-name "Export statistics CSV to directory: ")))
  (let* ((scope org-drill-statistics--scope)
         (range (or org-drill-statistics--range
                    (caar org-drill-statistics-range-presets)))
         (algorithm org-drill-statistics--algorithm)
         (log (org-drill-statistics--filtered-log range algorithm))
         (days (cdr (assoc range org-drill-statistics-range-presets))))
    (make-directory directory t)
    (org-drill-statistics--write-csv
     (expand-file-name "sessions.csv" directory)
     org-drill-statistics--sessions-csv-header
     (org-drill-statistics--sessions-rows log))
    (org-drill-statistics--write-csv
     (expand-file-name "cards.csv" directory)
     org-drill-statistics--cards-csv-header
     (org-drill-statistics--cards-rows scope))
    (org-drill-statistics--write-csv
     (expand-file-name "daily.csv" directory)
     org-drill-statistics--daily-csv-header
     (org-drill-statistics--daily-rows log days))
    (when (called-interactively-p 'interactive)
      (message "Exported sessions.csv, cards.csv, daily.csv to %s" directory))
    directory))

(provide 'org-drill)
;;; org-drill.el ends here