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+;;; test-calibredb-epub-config--epub-mode.el --- Tests for epub mode resolution -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests that .epub files reach nov-mode through `auto-mode-alist' alone, with
+;; no advice on `set-auto-mode'.
+;;
+;; Background: the module used to carry an :around advice on `set-auto-mode'
+;; forcing nov-mode for .epub, added to keep `magic-fallback-mode-alist' from
+;; opening the zip container in archive-mode.  It was never needed.
+;; `set-auto-mode' consults `auto-mode-alist' before `magic-fallback-mode-alist',
+;; and nov's use-package :mode registers "\\.epub\\'" there, so the alist
+;; already won.  Verified live on the daemon: a real zip-format .epub opened in
+;; nov-mode both with the advice and with it removed.
+;;
+;; The advice was not free.  `set-auto-mode' runs on every file visit, so the
+;; advice put a redundant frame and an extra failure surface on the path for
+;; every file of every type.
+;;
+;; The second test is a regression guard: it fails if the advice is ever
+;; reinstated, which is the mistake this cleanup exists to prevent.
+;;
+;; Test organization:
+;; - Normal Cases: .epub resolves to nov-mode; no advice on set-auto-mode
+;; - Boundary Cases: a path merely containing "epub", and a bare "epub" name
+;; - Error Cases: an unrelated extension does not resolve to nov-mode
+;;
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'calibredb-epub-config)
+
+(defun test-epub-mode--resolve (filename)
+  "Return the major mode `auto-mode-alist' assigns to FILENAME."
+  (assoc-default filename auto-mode-alist 'string-match))
+
+;;; Normal Cases
+
+(ert-deftest test-calibredb-epub-config-epub-resolves-to-nov-mode ()
+  "Normal: auto-mode-alist maps a .epub file to nov-mode on its own."
+  (should (eq 'nov-mode (test-epub-mode--resolve "book.epub"))))
+
+(ert-deftest test-calibredb-epub-config-no-set-auto-mode-advice ()
+  "Normal: nothing advises set-auto-mode to force nov-mode.
+Regression guard.  auto-mode-alist already wins over
+magic-fallback-mode-alist, so an advice here would be redundant work on
+every file visit."
+  (should-not (advice-member-p 'cj/force-nov-mode-for-epub 'set-auto-mode))
+  (should-not (fboundp 'cj/force-nov-mode-for-epub)))
+
+;;; Boundary Cases
+
+(ert-deftest test-calibredb-epub-config-epub-in-directory-name ()
+  "Boundary: the extension anchors at the end, so a directory named epub
+does not by itself select nov-mode."
+  (should-not (eq 'nov-mode (test-epub-mode--resolve "/home/user/epub/notes.txt"))))
+
+(ert-deftest test-calibredb-epub-config-epub-with-path ()
+  "Boundary: a full path with directories still resolves on the extension."
+  (should (eq 'nov-mode (test-epub-mode--resolve "/home/user/books/a b.epub"))))
+
+;;; Error Cases
+
+(ert-deftest test-calibredb-epub-config-other-extension-not-nov ()
+  "Error: an unrelated extension must not resolve to nov-mode."
+  (should-not (eq 'nov-mode (test-epub-mode--resolve "archive.zip")))
+  (should-not (eq 'nov-mode (test-epub-mode--resolve "notes.org"))))
+
+(provide 'test-calibredb-epub-config--epub-mode)
+;;; test-calibredb-epub-config--epub-mode.el ends here