test: add coverage for org-drill-smart-reschedule

6 ERT tests covering all four days-ahead branches:

- 0 → unschedule (treat as new again)
- negative → schedule today (current-time)
- positive → schedule N days ahead
- nil → use the algorithm-computed next-interval (locks in the
  numberp guard fix)

Plus property side-effects: writes DRILL_LAST_INTERVAL / EASE /
TOTAL_REPEATS via store-item-data, and TOTAL_REPEATS increments on
each call.

1 files changed, 119 insertions, 0 deletions

diff --git a/tests/test-org-drill-smart-reschedule.el b/tests/test-org-drill-smart-reschedule.el
new file mode 100644
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+++ b/tests/test-org-drill-smart-reschedule.el
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+;;; test-org-drill-smart-reschedule.el --- Tests for smart-reschedule  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for `org-drill-smart-reschedule', the function that runs after a
+;; user rates a card.  It updates the card's DRILL_* properties via
+;; store-item-data and then sets a fresh SCHEDULED stamp.
+;;
+;; Three days-ahead branches:
+;;
+;; - 0   → unschedule (treat as new again)
+;; - <0  → schedule for today (right now)
+;; - >0  → schedule N days from today
+;; - nil → use the algorithm-computed next-interval (this branch was
+;;         broken before the fix — `(= 0 nil)' errored).
+;;
+;; The user-facing contract: rate a card, see the next-review date
+;; advance to a sensible point.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'org)
+(require 'org-drill)
+
+;;;; Helpers
+
+(defmacro with-fresh-drill-entry (&rest body)
+  (declare (indent 0))
+  `(with-temp-buffer
+     (let ((org-startup-folded nil))
+       (insert "* Question :drill:\n")
+       (org-mode)
+       (goto-char (point-min))
+       ,@body)))
+
+(defmacro with-fixed-now (&rest body)
+  `(cl-letf (((symbol-function 'current-time)
+              (lambda () (encode-time 0 0 12 5 5 2026))))
+     ,@body))
+
+(defun current-scheduled-time-string ()
+  "Return SCHEDULED stamp on entry at point or nil."
+  (org-entry-get (point) "SCHEDULED"))
+
+;;;; Branch: days-ahead = 0 (unschedule)
+
+(ert-deftest test-org-drill-smart-reschedule-zero-days-ahead-unschedules ()
+  "Passing 0 removes the SCHEDULED stamp — the entry is treated as new again."
+  (with-fresh-drill-entry
+    (org-schedule nil "2026-05-01")
+    (with-fixed-now
+      (org-drill-smart-reschedule 4 0))
+    (should (null (current-scheduled-time-string)))))
+
+;;;; Branch: days-ahead < 0 (today)
+
+(ert-deftest test-org-drill-smart-reschedule-negative-schedules-today ()
+  "Negative days-ahead schedules for current-time (today)."
+  (with-fresh-drill-entry
+    (with-fixed-now
+      (org-drill-smart-reschedule 4 -1))
+    (let ((scheduled (current-scheduled-time-string)))
+      (should scheduled)
+      ;; Scheduled stamp matches today's date.
+      (should (string-match-p "2026-05-05" scheduled)))))
+
+;;;; Branch: days-ahead > 0 (N days from today)
+
+(ert-deftest test-org-drill-smart-reschedule-positive-schedules-n-days-ahead ()
+  "Positive days-ahead schedules exactly N days into the future."
+  (with-fresh-drill-entry
+    (with-fixed-now
+      (org-drill-smart-reschedule 4 7))
+    (let ((scheduled (current-scheduled-time-string)))
+      (should scheduled)
+      ;; 2026-05-05 + 7 days = 2026-05-12.
+      (should (string-match-p "2026-05-12" scheduled)))))
+
+;;;; Branch: days-ahead = nil (use algorithm)
+
+(ert-deftest test-org-drill-smart-reschedule-nil-days-ahead-uses-algorithm ()
+  "Without days-ahead, fall back to the scheduler's computed next-interval.
+Pre-fix this branch crashed with `Wrong type argument: number-or-marker-p, nil'
+because the cond compared nil with `=' before the type-guard."
+  (with-fresh-drill-entry
+    (with-fixed-now
+      ;; Should not error.  The scheduled date should be in the future
+      ;; (some sensible interval).
+      (org-drill-smart-reschedule 5)
+      (let ((scheduled (current-scheduled-time-string)))
+        (should scheduled)))))
+
+;;;; Property side-effects
+
+(ert-deftest test-org-drill-smart-reschedule-writes-drill-properties ()
+  "Rescheduling also writes DRILL_* properties via store-item-data."
+  (with-fresh-drill-entry
+    (with-fixed-now
+      (org-drill-smart-reschedule 5 7))
+    (should (org-entry-get (point) "DRILL_LAST_INTERVAL"))
+    (should (org-entry-get (point) "DRILL_TOTAL_REPEATS"))
+    (should (org-entry-get (point) "DRILL_EASE"))))
+
+(ert-deftest test-org-drill-smart-reschedule-increments-total-repeats ()
+  "Each rating increments DRILL_TOTAL_REPEATS."
+  (with-fresh-drill-entry
+    (with-fixed-now
+      (org-drill-smart-reschedule 4 7)
+      (let ((after-first (string-to-number
+                          (org-entry-get (point) "DRILL_TOTAL_REPEATS"))))
+        (org-drill-smart-reschedule 4 7)
+        (let ((after-second (string-to-number
+                             (org-entry-get (point) "DRILL_TOTAL_REPEATS"))))
+          (should (= (1+ after-first) after-second)))))))
+
+(provide 'test-org-drill-smart-reschedule)
+
+;;; test-org-drill-smart-reschedule.el ends here