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+;;; test-org-drill-undo.el --- Tests for undo/retry of the last rating  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Undo lets the user take back a misclicked rating.  Before a rating is
+;; applied, the entry's scheduling state is snapshotted onto the session's
+;; undo-stack (capped at org-drill-undo-limit).  org-drill-undo-last-rating
+;; restores that state, drops the recorded quality, and re-queues the card so
+;; it comes around again.
+
+;;; Code:
+
+(require 'ert)
+(require 'org-drill)
+
+;;;; defcustoms
+
+(ert-deftest test-org-drill-undo-limit-defaults-to-3 ()
+  (should (eq 3 (default-value 'org-drill-undo-limit))))
+
+(ert-deftest test-org-drill-undo-key-is-customizable-and-defaults-to-u ()
+  (should (custom-variable-p 'org-drill--undo-key))
+  (should (eq ?u (default-value 'org-drill--undo-key))))
+
+;;;; snapshot / restore round-trip
+
+(ert-deftest test-org-drill-snapshot-restore-round-trips-scheduling-data ()
+  "Restoring a snapshot puts changed properties back to their captured values."
+  (with-temp-buffer
+    (insert "* Card :drill:\n")
+    (org-mode)
+    (goto-char (point-min))
+    (org-set-property "DRILL_LAST_QUALITY" "4")
+    (org-set-property "DRILL_EASE" "2.5")
+    (let ((snap (org-drill--snapshot-entry-data)))
+      (org-set-property "DRILL_LAST_QUALITY" "1")
+      (org-set-property "DRILL_EASE" "1.8")
+      (org-drill--restore-entry-data snap)
+      (should (equal "4" (org-entry-get (point) "DRILL_LAST_QUALITY")))
+      (should (equal "2.5" (org-entry-get (point) "DRILL_EASE"))))))
+
+(ert-deftest test-org-drill-restore-deletes-properties-absent-at-snapshot ()
+  "A property that did not exist when snapshotted is removed on restore."
+  (with-temp-buffer
+    (insert "* Card :drill:\n")
+    (org-mode)
+    (goto-char (point-min))
+    (let ((snap (org-drill--snapshot-entry-data)))
+      (org-set-property "DRILL_LAST_QUALITY" "3")
+      (org-drill--restore-entry-data snap)
+      (should (null (org-entry-get (point) "DRILL_LAST_QUALITY"))))))
+
+;;;; org-drill-undo-last-rating
+
+(ert-deftest test-org-drill-undo-last-rating-restores-pops-and-requeues ()
+  "Undo restores the entry data, drops the recorded quality, and re-queues
+the card onto the again list."
+  (with-temp-buffer
+    (insert "* Card :drill:\n")
+    (org-mode)
+    (goto-char (point-min))
+    (org-set-property "DRILL_LAST_QUALITY" "4")
+    (let ((session (org-drill-session)))
+      (push (org-drill--snapshot-entry-data) (oref session undo-stack))
+      (org-set-property "DRILL_LAST_QUALITY" "1")
+      (push 1 (oref session qualities))
+      (org-drill-undo-last-rating session)
+      (should (equal "4" (org-entry-get (point) "DRILL_LAST_QUALITY")))
+      (should (null (oref session qualities)))
+      (should (= 1 (length (oref session again-entries)))))))
+
+(ert-deftest test-org-drill-undo-last-rating-empty-stack-is-noop ()
+  "Undo with nothing recorded does not error and changes no session state."
+  (let ((session (org-drill-session)))
+    (org-drill-undo-last-rating session)
+    (should (null (oref session qualities)))
+    (should (null (oref session again-entries)))))
+
+;;;; undo-key routing through the rating prompt
+
+(ert-deftest test-org-drill-reschedule-undo-key-takes-back-previous-rating ()
+  "Pressing the undo key at the prompt restores the previous card and
+re-queues it, then the prompt loop continues (here, a quit ends it).
+
+Components integrated:
+- org-drill-reschedule (entry point, real)
+- org-drill--read-rating-key (MOCKED — returns undo key then quit key)
+- org-drill-undo-last-rating + snapshot/restore (real)
+
+Validates the undo key is handled in the prompt loop rather than treated
+as a rating, and that the loop keeps prompting afterward."
+  (with-temp-buffer
+    (insert "* Prev :drill:\n* Current :drill:\n")
+    (org-mode)
+    (let ((session (org-drill-session)))
+      (goto-char (point-min))
+      (org-set-property "DRILL_LAST_QUALITY" "5")
+      (push (org-drill--snapshot-entry-data) (oref session undo-stack))
+      (org-set-property "DRILL_LAST_QUALITY" "0")  ; the misrating to take back
+      (push 0 (oref session qualities))
+      (goto-char (point-min))
+      (re-search-forward "Current")
+      (let ((keys (list org-drill--undo-key org-drill--quit-key)))
+        (cl-letf (((symbol-function 'org-drill--read-rating-key)
+                   (lambda (&rest _) (pop keys)))
+                  ((symbol-function 'sit-for) #'ignore))
+          (should (null (org-drill-reschedule session)))))
+      (goto-char (point-min))
+      (should (equal "5" (org-entry-get (point) "DRILL_LAST_QUALITY")))
+      (should (null (oref session qualities)))
+      (should (= 1 (length (oref session again-entries)))))))
+
+(provide 'test-org-drill-undo)
+
+;;; test-org-drill-undo.el ends here