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+;;; test-music-config--append-track-to-m3u-file.el --- Tests for appending tracks to M3U files -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Unit tests for cj/music--append-track-to-m3u-file function.
+;; Tests the pure, deterministic helper that appends track paths to M3U files.
+;;
+;; Test organization:
+;; - Normal Cases: Standard append operations
+;; - Boundary Cases: Edge conditions (unicode, long paths, special chars)
+;; - Error Cases: File errors (missing, read-only, directory instead of file)
+;;
+;;; Code:
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Stub missing dependencies before loading music-config
+(defvar-keymap cj/custom-keymap
+  :doc "Stub keymap for testing")
+
+;; Load production code
+(require 'music-config)
+
+;;; Setup & Teardown
+
+(defun test-music-config--append-track-to-m3u-file-setup ()
+  "Setup test environment."
+  (cj/create-test-base-dir))
+
+(defun test-music-config--append-track-to-m3u-file-teardown ()
+  "Clean up test environment."
+  (cj/delete-test-base-dir))
+
+;;; Normal Cases
+
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-empty-file-appends-track ()
+  "Append to brand new empty M3U file."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             (track-path "/home/user/music/artist/song.mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) (concat track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-existing-with-newline-appends-track ()
+  "Append to file with existing content ending with newline."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((existing-content "/home/user/music/first.mp3\n")
+             (m3u-file (cj/create-temp-test-file-with-content existing-content "test-playlist-"))
+             (track-path "/home/user/music/second.mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string)
+                          (concat existing-content track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-existing-without-newline-appends-track ()
+  "Append to file without trailing newline adds leading newline."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((existing-content "/home/user/music/first.mp3")
+             (m3u-file (cj/create-temp-test-file-with-content existing-content "test-playlist-"))
+             (track-path "/home/user/music/second.mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string)
+                          (concat existing-content "\n" track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-normal-multiple-appends-all-succeed ()
+  "Multiple appends to same file all succeed (allows duplicates)."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             (track1 "/home/user/music/track1.mp3")
+             (track2 "/home/user/music/track2.mp3")
+             (track1-duplicate "/home/user/music/track1.mp3"))
+        (cj/music--append-track-to-m3u-file track1 m3u-file)
+        (cj/music--append-track-to-m3u-file track2 m3u-file)
+        (cj/music--append-track-to-m3u-file track1-duplicate m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (let ((content (buffer-string)))
+            (should (string= content
+                            (concat track1 "\n" track2 "\n" track1-duplicate "\n"))))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-config--append-track-to-m3u-file-boundary-very-long-path-appends-successfully ()
+  "Append very long track path without truncation."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             ;; Create a path that's ~500 chars long
+             (track-path (concat "/home/user/music/"
+                                (make-string 450 ?a)
+                                "/song.mp3")))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) (concat track-path "\n")))
+          (should (= (length (buffer-string)) (1+ (length track-path))))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-boundary-path-with-unicode-appends-successfully ()
+  "Append path with unicode characters preserves UTF-8 encoding."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             (track-path "/home/user/music/中文/artist-名前/song🎵.mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) (concat track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-boundary-path-with-spaces-appends-successfully ()
+  "Append path with spaces and special characters."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             (track-path "/home/user/music/Artist Name/Album (2024)/01 - Song's Title [Remix].mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string) (concat track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-boundary-m3u-with-comments-appends-after ()
+  "Append to M3U file containing comments and metadata."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((existing-content "#EXTM3U\n#EXTINF:-1,Radio Station\nhttp://stream.url/radio\n")
+             (m3u-file (cj/create-temp-test-file-with-content existing-content "test-playlist-"))
+             (track-path "/home/user/music/local-track.mp3"))
+        (cj/music--append-track-to-m3u-file track-path m3u-file)
+        (with-temp-buffer
+          (insert-file-contents m3u-file)
+          (should (string= (buffer-string)
+                          (concat existing-content track-path "\n")))))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-music-config--append-track-to-m3u-file-error-nonexistent-file-signals-error ()
+  "Signal error when M3U file doesn't exist."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file "/nonexistent/path/to/playlist.m3u")
+             (track-path "/home/user/music/song.mp3"))
+        (should-error (cj/music--append-track-to-m3u-file track-path m3u-file)
+                     :type 'error))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-error-readonly-file-signals-error ()
+  "Signal error when M3U file is read-only."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-file (cj/create-temp-test-file "test-playlist-"))
+             (track-path "/home/user/music/song.mp3"))
+        ;; Make file read-only
+        (set-file-modes m3u-file #o444)
+        (should-error (cj/music--append-track-to-m3u-file track-path m3u-file)
+                     :type 'error))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(ert-deftest test-music-config--append-track-to-m3u-file-error-directory-not-file-signals-error ()
+  "Signal error when path points to directory instead of file."
+  (test-music-config--append-track-to-m3u-file-setup)
+  (unwind-protect
+      (let* ((m3u-dir (cj/create-test-subdirectory "test-playlist-dir"))
+             (track-path "/home/user/music/song.mp3"))
+        (should-error (cj/music--append-track-to-m3u-file track-path m3u-dir)
+                     :type 'error))
+    (test-music-config--append-track-to-m3u-file-teardown)))
+
+(provide 'test-music-config--append-track-to-m3u-file)
+;;; test-music-config--append-track-to-m3u-file.el ends here