feat: add org-drill-treat-headline-as-card-p for empty-bodied cards

A drill entry with an empty body is skipped unless its card type opts into empty bodies via the DRILL-EMPTY-P slot of org-drill-card-type-alist. That left no global way to drill headline-only items, or hierarchical-notes decks where the heading is the question and the answer lives in child entries (upstream #30, #41).

I added org-drill-treat-headline-as-card-p, default nil so existing decks are unchanged. When it's on, the empty-skip gate in org-drill--entry-empty-and-not-empty-friendly-p short-circuits, so every empty-bodied entry is drilled with its heading as the question regardless of card type. I added the safe-local-variable booleanp declaration alongside the other booleans and documented the switch in org-drill.org.

Tests pin the predicate and the classify-status outcome both on and off, and confirm the per-card-type DRILL-EMPTY-P path stays independent of the new switch.

3 files changed, 110 insertions, 2 deletions

diff --git a/org-drill.el b/org-drill.el
index 2ad00f0..1a03b1f 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -414,6 +414,19 @@ even if their bodies are empty."
   :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))
@@ -925,6 +938,7 @@ regardless of whether the test was successful.")
      '(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)
@@ -3343,8 +3357,11 @@ 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)."
-  (and (org-drill-entry-empty-p)
+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)
diff --git a/org-drill.org b/org-drill.org
index d0a0857..34092d0 100644
--- a/org-drill.org
+++ b/org-drill.org
@@ -386,6 +386,10 @@ drill session, you can either:
 2. Change the entry for its card type in =org-drill-card-type-alist= so that
    items of this type will always be tested, even if they have an empty body.
    See the documentation for =org-drill-card-type-alist= for details.
+3. Set =org-drill-treat-headline-as-card-p= to =t=. This is a global switch:
+   when on, every empty-bodied drill item is tested with its heading as the
+   question, regardless of card type. Useful for hierarchical-notes decks where
+   the heading is the prompt and the answer lives in child items.
 
 
 * Running the drill session
diff --git a/tests/test-org-drill-treat-headline-as-card.el b/tests/test-org-drill-treat-headline-as-card.el
new file mode 100644
index 0000000..5b369e7
--- /dev/null
+++ b/tests/test-org-drill-treat-headline-as-card.el
@@ -0,0 +1,87 @@
+;;; test-org-drill-treat-headline-as-card.el --- Tests for headline-as-card  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `org-drill-treat-headline-as-card-p' controls whether a drill entry with an
+;; empty body is skipped (the default) or presented as a card with the heading
+;; itself as the question (upstream issues #30 and #41).
+;;
+;; The gate lives in `org-drill--entry-empty-and-not-empty-friendly-p', whose
+;; non-nil result makes `org-drill--classify-status' return nil (the entry is
+;; not a drillable card).  These tests pin both the predicate and the
+;; classification outcome, on and off, and confirm the per-card-type
+;; DRILL-EMPTY-P mechanism still works independently of the new global switch.
+
+;;; Code:
+
+(require 'ert)
+(require 'org-drill)
+
+;;;; Defcustom default
+
+(ert-deftest test-org-drill-treat-headline-as-card-defaults-off ()
+  "The defcustom ships nil so existing decks keep skipping empty entries."
+  (should (eq nil (default-value 'org-drill-treat-headline-as-card-p))))
+
+;;;; Predicate — org-drill--entry-empty-and-not-empty-friendly-p
+
+(ert-deftest test-org-drill-empty-entry-skipped-when-headline-card-off ()
+  "With the switch off, an empty-bodied drill entry is treated as
+empty-and-skippable (predicate returns non-nil)."
+  (let ((org-drill-treat-headline-as-card-p nil))
+    (with-temp-buffer
+      (insert "* A headline-only card :drill:\n")
+      (org-mode)
+      (goto-char (point-min))
+      (should (org-drill--entry-empty-and-not-empty-friendly-p)))))
+
+(ert-deftest test-org-drill-empty-entry-not-skipped-when-headline-card-on ()
+  "With the switch on, an empty-bodied drill entry is NOT skippable
+\(predicate returns nil) — the heading becomes the card."
+  (let ((org-drill-treat-headline-as-card-p t))
+    (with-temp-buffer
+      (insert "* A headline-only card :drill:\n")
+      (org-mode)
+      (goto-char (point-min))
+      (should-not (org-drill--entry-empty-and-not-empty-friendly-p)))))
+
+(ert-deftest test-org-drill-non-empty-entry-not-skipped-regardless ()
+  "An entry with a body is never empty-and-skippable, switch on or off."
+  (dolist (flag '(nil t))
+    (let ((org-drill-treat-headline-as-card-p flag))
+      (with-temp-buffer
+        (insert "* A normal card :drill:\nThe answer body.\n")
+        (org-mode)
+        (goto-char (point-min))
+        (should-not (org-drill--entry-empty-and-not-empty-friendly-p))))))
+
+(ert-deftest test-org-drill-empty-friendly-card-type-unaffected-by-switch ()
+  "A card type that opts into empty bodies via DRILL-EMPTY-P (e.g. twosided)
+is never skippable, independent of the global switch."
+  (dolist (flag '(nil t))
+    (let ((org-drill-treat-headline-as-card-p flag))
+      (with-temp-buffer
+        (insert "* A two-sided card :drill:\n"
+                ":PROPERTIES:\n:DRILL_CARD_TYPE: twosided\n:END:\n")
+        (org-mode)
+        (goto-char (point-min))
+        (should-not (org-drill--entry-empty-and-not-empty-friendly-p))))))
+
+;;;; Classification outcome — org-drill--classify-status
+
+(ert-deftest test-org-drill-classify-empty-entry-flips-with-switch ()
+  "An empty drill entry classifies as nil (skipped) with the switch off, and
+as a real status with the switch on — the user-facing effect of the feature."
+  (with-temp-buffer
+    (insert "* A headline-only card :drill:\n")
+    (org-mode)
+    (goto-char (point-min))
+    ;; due=nil reaches :unscheduled only once the empty gate is passed; session
+    ;; is unused on that path, so nil is safe here.
+    (let ((org-drill-treat-headline-as-card-p nil))
+      (should (null (org-drill--classify-status nil nil 1))))
+    (let ((org-drill-treat-headline-as-card-p t))
+      (should (org-drill--classify-status nil nil 1)))))
+
+(provide 'test-org-drill-treat-headline-as-card)
+
+;;; test-org-drill-treat-headline-as-card.el ends here