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+;;; test-org-drill-statistics-quality-histogram.el --- Tests for quality-histogram statistics  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; ERT tests for the org-drill statistics dashboard quality-histogram block.
+
+;;; Code:
+
+(require 'ert)
+(require 'org-drill)
+(require 'cl-lib)
+(require 'org)
+
+(defun test-org-drill-statistics--make-record (qualities)
+  "Build a minimal `org-drill-session-record' carrying QUALITIES.
+QUALITIES is a vector of ints.  Other slots are filled with inert
+defaults so the histogram tests stay focused on the qualities slot."
+  (make-org-drill-session-record
+   :start-time 0.0
+   :end-time 0.0
+   :scope nil
+   :algorithm 'sm5
+   :qualities qualities
+   :pass-percent 0
+   :new-count 0
+   :mature-count 0
+   :failed-count 0
+   :cram-mode nil))
+
+;; Normal cases.
+
+(ert-deftest test-org-drill-statistics-quality-histogram-single-record ()
+  "A single record's qualities are tallied into the right buckets."
+  (let* ((record (test-org-drill-statistics--make-record [0 3 3 5 3]))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [1 0 0 3 0 1]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-multiple-records ()
+  "Counts sum across every record in the log."
+  (let* ((r1 (test-org-drill-statistics--make-record [0 1 2]))
+         (r2 (test-org-drill-statistics--make-record [3 4 5]))
+         (r3 (test-org-drill-statistics--make-record [0 5 5]))
+         (result (org-drill-statistics--quality-histogram (list r1 r2 r3))))
+    (should (equal result [2 1 1 1 1 3]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-all-same-quality ()
+  "A record with every entry the same quality concentrates in one bucket."
+  (let* ((record (test-org-drill-statistics--make-record [4 4 4 4]))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [0 0 0 0 4 0]))))
+
+;; Boundary cases.
+
+(ert-deftest test-org-drill-statistics-quality-histogram-empty-log ()
+  "An empty log yields an all-zero histogram, never nil."
+  (let ((result (org-drill-statistics--quality-histogram '())))
+    (should (equal result [0 0 0 0 0 0]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-empty-qualities ()
+  "A record with an empty qualities vector contributes nothing."
+  (let* ((record (test-org-drill-statistics--make-record []))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [0 0 0 0 0 0]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-nil-qualities ()
+  "A record whose qualities slot is nil is skipped without error."
+  (let* ((r1 (test-org-drill-statistics--make-record nil))
+         (r2 (test-org-drill-statistics--make-record [2 2]))
+         (result (org-drill-statistics--quality-histogram (list r1 r2))))
+    (should (equal result [0 0 2 0 0 0]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-extreme-buckets ()
+  "Quality 0 and quality 5, the range endpoints, both land correctly."
+  (let* ((record (test-org-drill-statistics--make-record [0 0 5 5 5]))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [2 0 0 0 0 3]))))
+
+;; Error cases.
+
+(ert-deftest test-org-drill-statistics-quality-histogram-out-of-range-ignored ()
+  "Qualities outside 0..5 are dropped, valid ones still counted."
+  (let* ((record (test-org-drill-statistics--make-record [-1 6 3 99 2]))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [0 0 1 1 0 0]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-non-integer-ignored ()
+  "Non-integer quality entries are ignored rather than signalling."
+  (let* ((record (test-org-drill-statistics--make-record [2 nil 2.5 3]))
+         (result (org-drill-statistics--quality-histogram (list record))))
+    (should (equal result [0 0 1 1 0 0]))))
+
+(ert-deftest test-org-drill-statistics-quality-histogram-does-not-mutate-input ()
+  "The qualities vectors are read, never written."
+  (let* ((qualities (vector 1 2 3))
+         (record (test-org-drill-statistics--make-record qualities)))
+    (org-drill-statistics--quality-histogram (list record))
+    (should (equal qualities [1 2 3]))))
+
+(provide 'test-org-drill-statistics-quality-histogram)
+
+;;; test-org-drill-statistics-quality-histogram.el ends here