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+;;; test-music-config--playlist-modified-p.el --- Tests for playlist modification detection -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Unit tests for cj/music--playlist-modified-p function.
+;; Tests the logic that compares the current EMMS playlist buffer
+;; against its associated M3U file to detect unsaved changes.
+;;
+;; Test organization:
+;; - Normal Cases: Matching/differing track lists
+;; - Boundary Cases: Empty playlists, nil file, ordering
+;; - Error Cases: Missing M3U file, missing buffer
+;;
+;;; Code:
+
+(require 'ert)
+
+;; Stub missing dependencies before loading music-config
+(defvar-keymap cj/custom-keymap
+  :doc "Stub keymap for testing")
+
+;; Add EMMS elpa directory to load path for batch testing
+(let ((emms-dir (car (file-expand-wildcards
+                      (expand-file-name "elpa/emms-*" user-emacs-directory)))))
+  (when emms-dir
+    (add-to-list 'load-path emms-dir)))
+
+(require 'emms)
+(require 'emms-playlist-mode)
+(require 'music-config)
+
+;;; Test helpers
+
+(defun test-modified--setup-playlist-buffer (track-names)
+  "Create an EMMS playlist buffer with TRACK-NAMES and return it.
+Each entry in TRACK-NAMES becomes a file track in the playlist."
+  (let ((buf (get-buffer-create cj/music-playlist-buffer-name)))
+    (with-current-buffer buf
+      (emms-playlist-mode)
+      (setq emms-playlist-buffer-p t)
+      (let ((inhibit-read-only t))
+        (erase-buffer)
+        (dolist (name track-names)
+          (emms-playlist-insert-track (emms-track 'file name)))))
+    (setq emms-playlist-buffer buf)
+    buf))
+
+(defun test-modified--teardown ()
+  "Clean up test playlist buffer."
+  (when-let ((buf (get-buffer cj/music-playlist-buffer-name)))
+    (kill-buffer buf)))
+
+;;; Normal Cases
+
+(ert-deftest test-music-config--playlist-modified-p-normal-identical-tracks-returns-nil ()
+  "Identical track lists in file and playlist returns nil (not modified)."
+  (unwind-protect
+      (let* ((tracks '("/music/a.mp3" "/music/b.mp3" "/music/c.mp3"))
+             (cj/music-playlist-file "/fake/playlist.m3u"))
+        (test-modified--setup-playlist-buffer tracks)
+        (cl-letf (((symbol-function 'cj/music--m3u-file-tracks)
+                   (lambda (_file) tracks)))
+          (should-not (cj/music--playlist-modified-p))))
+    (test-modified--teardown)))
+
+(ert-deftest test-music-config--playlist-modified-p-normal-different-tracks-returns-non-nil ()
+  "Different track lists returns non-nil (modified)."
+  (unwind-protect
+      (let* ((file-tracks '("/music/a.mp3" "/music/b.mp3"))
+             (buf-tracks '("/music/a.mp3" "/music/c.mp3"))
+             (cj/music-playlist-file "/fake/playlist.m3u"))
+        (test-modified--setup-playlist-buffer buf-tracks)
+        (cl-letf (((symbol-function 'cj/music--m3u-file-tracks)
+                   (lambda (_file) file-tracks)))
+          (should (cj/music--playlist-modified-p))))
+    (test-modified--teardown)))
+
+(ert-deftest test-music-config--playlist-modified-p-normal-extra-track-returns-non-nil ()
+  "Playlist with extra track compared to file returns non-nil."
+  (unwind-protect
+      (let* ((file-tracks '("/music/a.mp3" "/music/b.mp3"))
+             (buf-tracks '("/music/a.mp3" "/music/b.mp3" "/music/c.mp3"))
+             (cj/music-playlist-file "/fake/playlist.m3u"))
+        (test-modified--setup-playlist-buffer buf-tracks)
+        (cl-letf (((symbol-function 'cj/music--m3u-file-tracks)
+                   (lambda (_file) file-tracks)))
+          (should (cj/music--playlist-modified-p))))
+    (test-modified--teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-config--playlist-modified-p-boundary-no-playlist-file-returns-nil ()
+  "Nil cj/music-playlist-file returns nil without comparing tracks."
+  (unwind-protect
+      (let ((cj/music-playlist-file nil))
+        (test-modified--setup-playlist-buffer '("/music/a.mp3"))
+        (should-not (cj/music--playlist-modified-p)))
+    (test-modified--teardown)))
+
+(ert-deftest test-music-config--playlist-modified-p-boundary-empty-file-and-playlist-returns-nil ()
+  "Both empty file tracks and empty playlist returns nil."
+  (unwind-protect
+      (let ((cj/music-playlist-file "/fake/playlist.m3u"))
+        (test-modified--setup-playlist-buffer '())
+        (cl-letf (((symbol-function 'cj/music--m3u-file-tracks)
+                   (lambda (_file) nil)))
+          (should-not (cj/music--playlist-modified-p))))
+    (test-modified--teardown)))
+
+(ert-deftest test-music-config--playlist-modified-p-boundary-order-difference-returns-non-nil ()
+  "Same tracks in different order returns non-nil."
+  (unwind-protect
+      (let* ((file-tracks '("/music/a.mp3" "/music/b.mp3"))
+             (buf-tracks '("/music/b.mp3" "/music/a.mp3"))
+             (cj/music-playlist-file "/fake/playlist.m3u"))
+        (test-modified--setup-playlist-buffer buf-tracks)
+        (cl-letf (((symbol-function 'cj/music--m3u-file-tracks)
+                   (lambda (_file) file-tracks)))
+          (should (cj/music--playlist-modified-p))))
+    (test-modified--teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-music-config--playlist-modified-p-error-missing-m3u-file-returns-nil ()
+  "When M3U file doesn't exist, m3u-file-tracks returns nil; empty playlist matches."
+  (unwind-protect
+      (let ((cj/music-playlist-file "/nonexistent/playlist.m3u"))
+        (test-modified--setup-playlist-buffer '())
+        ;; cj/music--m3u-file-tracks returns nil for nonexistent files
+        ;; empty playlist also returns nil, so equal = not modified
+        (should-not (cj/music--playlist-modified-p)))
+    (test-modified--teardown)))
+
+(ert-deftest test-music-config--playlist-modified-p-error-missing-m3u-with-tracks-returns-non-nil ()
+  "When M3U file doesn't exist but playlist has tracks, returns non-nil."
+  (unwind-protect
+      (let ((cj/music-playlist-file "/nonexistent/playlist.m3u"))
+        (test-modified--setup-playlist-buffer '("/music/a.mp3"))
+        ;; m3u-file-tracks returns nil, but playlist has tracks = modified
+        (should (cj/music--playlist-modified-p)))
+    (test-modified--teardown)))
+
+(provide 'test-music-config--playlist-modified-p)
+;;; test-music-config--playlist-modified-p.el ends here