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+;;; test-music-config--renumber-rows.el --- Tests for playlist row numbering -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Playlist rows carry a numeric overlay prefix so the cursor stays visible
+;; when it sits on a cover-art thumbnail and each row's position in the list
+;; is readable.  The renumber walks the buffer and rebuilds the overlays; a
+;; buffer-local after-change hook debounces it behind an idle timer.  Overlays
+;; leave the buffer text untouched (EMMS owns it), so these tests drive plain
+;; temp buffers.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+(require 'music-config)
+
+(defun test-music-renumber--numbers (buffer)
+  "Return the overlay number strings in BUFFER, in position order."
+  (with-current-buffer buffer
+    (mapcar (lambda (ov) (overlay-get ov 'before-string))
+            (sort (seq-filter (lambda (ov) (overlay-get ov 'cj-music-row-number))
+                              (overlays-in (point-min) (point-max)))
+                  (lambda (a b) (< (overlay-start a) (overlay-start b)))))))
+
+;;; Normal Cases
+
+(ert-deftest test-music-renumber-rows-numbers-each-line ()
+  "Normal: every non-blank line gets a sequential number overlay."
+  (with-temp-buffer
+    (insert "track one\ntrack two\ntrack three\n")
+    (cj/music--renumber-rows (current-buffer))
+    (should (equal (mapcar #'substring-no-properties
+                           (test-music-renumber--numbers (current-buffer)))
+                   '("  1 " "  2 " "  3 ")))))
+
+(ert-deftest test-music-renumber-rows-idempotent ()
+  "Normal: renumbering twice leaves one overlay per line, not two."
+  (with-temp-buffer
+    (insert "track one\ntrack two\n")
+    (cj/music--renumber-rows (current-buffer))
+    (cj/music--renumber-rows (current-buffer))
+    (should (= 2 (length (test-music-renumber--numbers (current-buffer)))))))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-renumber-rows-skips-blank-lines ()
+  "Boundary: blank lines are not numbered and don't advance the count."
+  (with-temp-buffer
+    (insert "track one\n\ntrack two\n")
+    (cj/music--renumber-rows (current-buffer))
+    (should (equal (mapcar #'substring-no-properties
+                           (test-music-renumber--numbers (current-buffer)))
+                   '("  1 " "  2 ")))))
+
+(ert-deftest test-music-renumber-rows-empty-buffer-no-overlays ()
+  "Boundary: an empty buffer gets no overlays and no error."
+  (with-temp-buffer
+    (cj/music--renumber-rows (current-buffer))
+    (should-not (test-music-renumber--numbers (current-buffer)))))
+
+;;; Error Cases
+
+(ert-deftest test-music-renumber-rows-dead-buffer-noop ()
+  "Error: renumbering a killed buffer is a silent no-op (the debounce timer
+can fire after the playlist buffer is gone)."
+  (let ((buf (generate-new-buffer " *test-renumber-dead*")))
+    (kill-buffer buf)
+    (should-not (cj/music--renumber-rows buf))))
+
+;;; Hook wiring
+
+(ert-deftest test-music-renumber-ensure-playlist-buffer-wires-hook ()
+  "Normal: the playlist buffer gets the debounced renumber on after-change."
+  (let (created)
+    (cl-letf (((symbol-function 'emms-playlist-mode) #'ignore))
+      (unwind-protect
+          (progn
+            (setq created (cj/music--ensure-playlist-buffer))
+            (with-current-buffer created
+              (should (member #'cj/music--schedule-renumber after-change-functions))))
+        (when (buffer-live-p created) (kill-buffer created))))))
+
+(provide 'test-music-config--renumber-rows)
+;;; test-music-config--renumber-rows.el ends here