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+;;; test-music-config--valid-file-p.el --- Tests for music file validation -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Unit tests for cj/music--valid-file-p function.
+;; Tests the pure, deterministic helper that validates music file extensions.
+;;
+;; Test organization:
+;; - Normal Cases: Valid music extensions (case-insensitive)
+;; - Boundary Cases: Edge conditions (no extension, dots in path, empty strings)
+;; - Error Cases: Invalid extensions, nil input
+;;
+;;; Code:
+
+(require 'ert)
+
+;; Stub missing dependencies before loading music-config
+(defvar-keymap cj/custom-keymap
+  :doc "Stub keymap for testing")
+
+;; Load production code
+(require 'music-config)
+
+;;; Normal Cases
+
+(ert-deftest test-music-config--valid-file-p-normal-mp3-extension-returns-true ()
+  "Validate mp3 file extension returns non-nil."
+  (should (cj/music--valid-file-p "/path/to/song.mp3")))
+
+(ert-deftest test-music-config--valid-file-p-normal-flac-extension-returns-true ()
+  "Validate flac file extension returns non-nil."
+  (should (cj/music--valid-file-p "/path/to/song.flac")))
+
+(ert-deftest test-music-config--valid-file-p-normal-all-extensions-return-true ()
+  "Validate all configured music extensions return non-nil."
+  ;; Test each extension from cj/music-file-extensions
+  (dolist (ext '("aac" "flac" "m4a" "mp3" "ogg" "opus" "wav"))
+    (should (cj/music--valid-file-p (format "/path/to/song.%s" ext)))))
+
+(ert-deftest test-music-config--valid-file-p-normal-uppercase-extension-returns-true ()
+  "Validate uppercase extension returns non-nil (case-insensitive)."
+  (should (cj/music--valid-file-p "/path/to/song.MP3")))
+
+(ert-deftest test-music-config--valid-file-p-normal-mixed-case-extension-returns-true ()
+  "Validate mixed-case extension returns non-nil (case-insensitive)."
+  (should (cj/music--valid-file-p "/path/to/song.Mp3"))
+  (should (cj/music--valid-file-p "/path/to/song.FLaC")))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-config--valid-file-p-boundary-dots-in-path-returns-true ()
+  "Validate file with dots in directory path uses only last extension."
+  (should (cj/music--valid-file-p "/path/with.dots/in.directory/song.mp3")))
+
+(ert-deftest test-music-config--valid-file-p-boundary-multiple-extensions-uses-last ()
+  "Validate file with multiple extensions uses rightmost extension."
+  (should (cj/music--valid-file-p "/path/to/song.backup.mp3"))
+  (should (cj/music--valid-file-p "/path/to/song.old.flac")))
+
+(ert-deftest test-music-config--valid-file-p-boundary-just-filename-with-extension-returns-true ()
+  "Validate bare filename without path returns non-nil."
+  (should (cj/music--valid-file-p "song.mp3")))
+
+(ert-deftest test-music-config--valid-file-p-boundary-no-extension-returns-nil ()
+  "Validate file without extension returns nil."
+  (should-not (cj/music--valid-file-p "/path/to/song")))
+
+(ert-deftest test-music-config--valid-file-p-boundary-dot-at-end-returns-nil ()
+  "Validate file ending with dot (empty extension) returns nil."
+  (should-not (cj/music--valid-file-p "/path/to/song.")))
+
+(ert-deftest test-music-config--valid-file-p-boundary-empty-string-returns-nil ()
+  "Validate empty string returns nil."
+  (should-not (cj/music--valid-file-p "")))
+
+;;; Error Cases
+
+(ert-deftest test-music-config--valid-file-p-error-nil-input-returns-nil ()
+  "Validate nil input returns nil gracefully."
+  (should-not (cj/music--valid-file-p nil)))
+
+(ert-deftest test-music-config--valid-file-p-error-non-music-extension-returns-nil ()
+  "Validate non-music file extension returns nil."
+  (should-not (cj/music--valid-file-p "/path/to/document.txt"))
+  (should-not (cj/music--valid-file-p "/path/to/readme.md")))
+
+(ert-deftest test-music-config--valid-file-p-error-image-extension-returns-nil ()
+  "Validate image file extension returns nil."
+  (should-not (cj/music--valid-file-p "/path/to/cover.jpg"))
+  (should-not (cj/music--valid-file-p "/path/to/artwork.png")))
+
+(ert-deftest test-music-config--valid-file-p-error-video-extension-returns-nil ()
+  "Validate video file extension returns nil (mp4 not in list, only m4a)."
+  (should-not (cj/music--valid-file-p "/path/to/video.mp4"))
+  (should-not (cj/music--valid-file-p "/path/to/clip.mkv")))
+
+(provide 'test-music-config--valid-file-p)
+;;; test-music-config--valid-file-p.el ends here