refactor(dirvish): extract cj/--duplicate-file-name helper

The name-mangling logic in `cj/dirvish-duplicate-file' was inline -- inseparable from the dired side effects (existence check, copy, revert). Extract to `cj/--duplicate-file-name', a pure function from FILE to FILE-WITH-COPY-SUFFIX. Seven Normal/Boundary tests cover the cases I care about: typical extension, elisp file, no extension, multi-dot extensions (only the last dot counts), leading-dot dotfiles, relative paths, spaces in the base name.

The wrapper retains the dired-mode interactive shape and now reads as a thin shell over the pure helper.

2 files changed, 86 insertions, 17 deletions

diff --git a/modules/dirvish-config.el b/modules/dirvish-config.el
index fadf09ca..fa5567ad 100644
--- a/modules/dirvish-config.el
+++ b/modules/dirvish-config.el
@@ -154,33 +154,43 @@ Filters for audio files, prompts for the playlist name, and saves the resulting
 
 ;;; ------------------------ Dirvish Duplicate File Copy ------------------------
 
+(defun cj/--duplicate-file-name (file)
+  "Return FILE's path with `-copy' inserted before the extension.
+
+Pure helper used by `cj/dirvish-duplicate-file'.  Examples:
+  /tmp/report.pdf      -> /tmp/report-copy.pdf
+  /home/foo/.bashrc    -> /home/foo/.bashrc-copy
+  doc.txt              -> doc-copy.txt
+  /tmp/archive.tar.gz  -> /tmp/archive.tar-copy.gz"
+  (let* ((dir (file-name-directory file))
+         (base (file-name-base file))
+         (ext (or (file-name-extension file t) ""))
+         (new-name (concat base "-copy" ext)))
+    (if dir
+        (expand-file-name new-name dir)
+      new-name)))
+
 (defun cj/dirvish-duplicate-file ()
-  "Duplicate the file at point with '-copy' suffix before the extension.
+  "Duplicate the file at point with `-copy' suffix before the extension.
 Examples:
   report.pdf → report-copy.pdf
   script.el → script-copy.el
   README → README-copy"
   (interactive)
-  (let* ((file (dired-get-filename nil t))
-         (dir (file-name-directory file))
-         (base (file-name-base file))
-         (ext (file-name-extension file t)) ; includes the dot
-         (new-name (concat base "-copy" ext))
-         (new-path (expand-file-name new-name dir)))
+  (let ((file (dired-get-filename nil t)))
     (unless file
       (user-error "No file at point"))
     (when (file-directory-p file)
       (user-error "Cannot duplicate directories, only files"))
-
-    ;; Check if target already exists
-    (when (file-exists-p new-path)
-      (unless (y-or-n-p (format "File '%s' already exists. Overwrite? " new-name))
-        (user-error "Cancelled")))
-
-    ;; Copy the file
-    (copy-file file new-path t)
-    (revert-buffer)
-    (message "Duplicated: %s → %s" (file-name-nondirectory file) new-name)))
+    (let* ((new-path (cj/--duplicate-file-name file))
+           (new-name (file-name-nondirectory new-path)))
+      (when (file-exists-p new-path)
+        (unless (y-or-n-p (format "File '%s' already exists. Overwrite? " new-name))
+          (user-error "Cancelled")))
+      (copy-file file new-path t)
+      (revert-buffer)
+      (message "Duplicated: %s → %s"
+               (file-name-nondirectory file) new-name))))
 
 ;;; ----------------------- Dirvish Open File Manager Here ----------------------
 
diff --git a/tests/test-dirvish-config-duplicate-file-name.el b/tests/test-dirvish-config-duplicate-file-name.el
new file mode 100644
index 00000000..8e69f28b
--- /dev/null
+++ b/tests/test-dirvish-config-duplicate-file-name.el
@@ -0,0 +1,59 @@
+;;; test-dirvish-config-duplicate-file-name.el --- Tests for the duplicate-file-name helper -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/--duplicate-file-name' is the pure name-mangling half of
+;; `cj/dirvish-duplicate-file': given an absolute file path, return the
+;; new path with `-copy' inserted before the extension.  The interactive
+;; wrapper handles the dired side effects (existence check, copy, revert).
+
+;;; Code:
+
+(require 'ert)
+(require 'package)
+
+(setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
+(package-initialize)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "elpa/dirvish-2.3.0/extensions"
+                                          user-emacs-directory))
+(require 'user-constants)
+(require 'keybindings)
+(require 'dirvish-config)
+
+(ert-deftest test-cj--duplicate-file-name-with-extension ()
+  "Normal: a file with an extension gets `-copy' before the dot."
+  (should (equal (cj/--duplicate-file-name "/tmp/report.pdf")
+                 "/tmp/report-copy.pdf")))
+
+(ert-deftest test-cj--duplicate-file-name-elisp-extension ()
+  "Normal: works for elisp files (the project's daily case)."
+  (should (equal (cj/--duplicate-file-name "/home/foo/script.el")
+                 "/home/foo/script-copy.el")))
+
+(ert-deftest test-cj--duplicate-file-name-no-extension ()
+  "Boundary: an extensionless file appends `-copy' at the end."
+  (should (equal (cj/--duplicate-file-name "/dir/README")
+                 "/dir/README-copy")))
+
+(ert-deftest test-cj--duplicate-file-name-multiple-dots ()
+  "Boundary: only the last dot counts as the extension separator."
+  (should (equal (cj/--duplicate-file-name "/tmp/archive.tar.gz")
+                 "/tmp/archive.tar-copy.gz")))
+
+(ert-deftest test-cj--duplicate-file-name-dotfile ()
+  "Boundary: a leading-dot file (no real extension) keeps the dot in the base."
+  (should (equal (cj/--duplicate-file-name "/home/foo/.bashrc")
+                 "/home/foo/.bashrc-copy")))
+
+(ert-deftest test-cj--duplicate-file-name-relative-path ()
+  "Boundary: relative paths preserve their relative form."
+  (should (equal (cj/--duplicate-file-name "doc.txt")
+                 "doc-copy.txt")))
+
+(ert-deftest test-cj--duplicate-file-name-spaces-in-name ()
+  "Boundary: spaces in the base name are preserved."
+  (should (equal (cj/--duplicate-file-name "/tmp/my file.txt")
+                 "/tmp/my file-copy.txt")))
+
+(provide 'test-dirvish-config-duplicate-file-name)
+;;; test-dirvish-config-duplicate-file-name.el ends here