refactor(dirvish): extract cj/--ediff-pair-from-files; fix 0-files crash

`cj/dired-ediff-files' had its pair-determination logic inline: count check, prompt fallback when only one file was marked, and the older-first ordering for `ediff-files'.  Lift it into `cj/--ediff-pair-from-files' -- pure given the file list, an injected prompt thunk, and a newer-than-p comparator -- so tests stay independent of mtimes and the dired prompt.

While extracting, surface a latent bug: with zero marked files the original code fell through to `(file-newer-than-file-p nil nil)' and crashed with a wrong-type-argument error.  Replace the crash with a clear `user-error' ("No files marked"), and add a regression test.  The 3+ files case keeps its existing user-error message.

Five Normal/Boundary/Error tests cover both ordering directions, the one-file prompt path, and both error counts.

1 files changed, 71 insertions, 0 deletions

diff --git a/tests/test-dirvish-config-ediff-pair.el b/tests/test-dirvish-config-ediff-pair.el
new file mode 100644
index 00000000..fc9e5e70
--- /dev/null
+++ b/tests/test-dirvish-config-ediff-pair.el
@@ -0,0 +1,71 @@
+;;; test-dirvish-config-ediff-pair.el --- Tests for the ediff-pair helper -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/--ediff-pair-from-files' picks the (FILE1 . FILE2) pair that
+;; `cj/dired-ediff-files' should hand to `ediff-files'.  The pair is
+;; (older . newer) so ediff renders the older revision on the left.
+;; The helper takes a prompt callback (used when only one file is
+;; marked) and a "newer than" comparator, so tests stay independent of
+;; real mtimes and the dired prompt.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(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--ediff-pair-two-files-first-is-newer ()
+  "Normal: file1 newer than file2 -> pair is (file2 . file1) so older runs first."
+  (let ((newer-fn (lambda (a _b) (string= a "/a"))))
+    (should (equal (cj/--ediff-pair-from-files
+                    '("/a" "/b") (lambda () "/unused") newer-fn)
+                   '("/b" . "/a")))))
+
+(ert-deftest test-cj--ediff-pair-two-files-second-is-newer ()
+  "Normal: file1 older than file2 -> pair preserves the input order."
+  (let ((newer-fn (lambda (_a _b) nil)))
+    (should (equal (cj/--ediff-pair-from-files
+                    '("/a" "/b") (lambda () "/unused") newer-fn)
+                   '("/a" . "/b")))))
+
+(ert-deftest test-cj--ediff-pair-one-file-calls-prompt ()
+  "Boundary: a single marked file triggers the prompt callback for the second."
+  (let ((prompt-calls 0)
+        (newer-fn (lambda (_a _b) nil)))
+    (let ((pair (cj/--ediff-pair-from-files
+                 '("/only")
+                 (lambda () (cl-incf prompt-calls) "/picked")
+                 newer-fn)))
+      (should (= prompt-calls 1))
+      (should (equal pair '("/only" . "/picked"))))))
+
+(ert-deftest test-cj--ediff-pair-three-files-signals-user-error ()
+  "Error: more than two files signals user-error without prompting."
+  (let ((prompt-calls 0))
+    (should-error
+     (cj/--ediff-pair-from-files
+      '("/a" "/b" "/c")
+      (lambda () (cl-incf prompt-calls) "/x")
+      (lambda (_a _b) nil))
+     :type 'user-error)
+    (should (zerop prompt-calls))))
+
+(ert-deftest test-cj--ediff-pair-zero-files-signals-user-error ()
+  "Error: zero files signals user-error -- regression for the latent crash
+where `cj/dired-ediff-files' fell through to `(file-newer-than-file-p nil ...)'."
+  (should-error
+   (cj/--ediff-pair-from-files
+    '() (lambda () "/x") (lambda (_a _b) nil))
+   :type 'user-error))
+
+(provide 'test-dirvish-config-ediff-pair)
+;;; test-dirvish-config-ediff-pair.el ends here