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+;;; test-music-config--safe-filename.el --- Tests for filename sanitization -*- coding: utf-8; lexical-binding: t; -*-
+;;
+;; Author: Craig Jennings <c@cjennings.net>
+;;
+;;; Commentary:
+;; Unit tests for cj/music--safe-filename function.
+;; Tests the pure helper that sanitizes filenames by replacing invalid chars.
+;;
+;; Test organization:
+;; - Normal Cases: Valid filenames unchanged, spaces replaced
+;; - Boundary Cases: Special chars, unicode, slashes, consecutive invalid chars
+;; - Error Cases: 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--safe-filename-normal-alphanumeric-unchanged ()
+  "Validate alphanumeric filename remains unchanged."
+  (should (string= (cj/music--safe-filename "MyPlaylist123")
+                   "MyPlaylist123")))
+
+(ert-deftest test-music-config--safe-filename-normal-with-hyphens-unchanged ()
+  "Validate filename with hyphens remains unchanged."
+  (should (string= (cj/music--safe-filename "my-playlist-name")
+                   "my-playlist-name")))
+
+(ert-deftest test-music-config--safe-filename-normal-with-underscores-unchanged ()
+  "Validate filename with underscores remains unchanged."
+  (should (string= (cj/music--safe-filename "my_playlist_name")
+                   "my_playlist_name")))
+
+(ert-deftest test-music-config--safe-filename-normal-spaces-replaced ()
+  "Validate spaces are replaced with underscores."
+  (should (string= (cj/music--safe-filename "My Favorite Songs")
+                   "My_Favorite_Songs")))
+
+;;; Boundary Cases
+
+(ert-deftest test-music-config--safe-filename-boundary-special-chars-replaced ()
+  "Validate special characters are replaced with underscores."
+  (should (string= (cj/music--safe-filename "playlist@#$%^&*()")
+                   "playlist_________")))
+
+(ert-deftest test-music-config--safe-filename-boundary-unicode-replaced ()
+  "Validate unicode characters are replaced with underscores."
+  (should (string= (cj/music--safe-filename "中文歌曲")
+                   "____")))
+
+(ert-deftest test-music-config--safe-filename-boundary-mixed-valid-invalid ()
+  "Validate mixed valid and invalid characters."
+  (should (string= (cj/music--safe-filename "Rock & Roll")
+                   "Rock___Roll")))
+
+(ert-deftest test-music-config--safe-filename-boundary-dots-replaced ()
+  "Validate dots are replaced with underscores."
+  (should (string= (cj/music--safe-filename "my.playlist.name")
+                   "my_playlist_name")))
+
+(ert-deftest test-music-config--safe-filename-boundary-slashes-replaced ()
+  "Validate slashes are replaced with underscores."
+  (should (string= (cj/music--safe-filename "folder/file")
+                   "folder_file")))
+
+(ert-deftest test-music-config--safe-filename-boundary-consecutive-invalid-chars ()
+  "Validate consecutive invalid characters each become underscores."
+  (should (string= (cj/music--safe-filename "test!!!name")
+                   "test___name")))
+
+(ert-deftest test-music-config--safe-filename-boundary-empty-string-unchanged ()
+  "Validate empty string remains unchanged."
+  (should (string= (cj/music--safe-filename "")
+                   "")))
+
+(ert-deftest test-music-config--safe-filename-boundary-only-invalid-chars ()
+  "Validate string with only invalid characters becomes all underscores."
+  (should (string= (cj/music--safe-filename "!@#$%")
+                   "_____")))
+
+;;; Error Cases
+
+(ert-deftest test-music-config--safe-filename-error-nil-input-signals-error ()
+  "Validate nil input signals error."
+  (should-error (cj/music--safe-filename nil)
+                :type 'wrong-type-argument))
+
+(provide 'test-music-config--safe-filename)
+;;; test-music-config--safe-filename.el ends here