feat(emacs): Configure Ediff

- Restore window configuration on quit.
- Very simple, half-broken helper for resolving Syncthing conflicts.
  - Doesn't automatically delete the conflict file after resolve. }:\

users/crumb/programs/emacs/lib/syd-file.el

@@ -94,4 +94,45 @@ If FORCE-P, delete without confirmation."
  :desc "Open current file as root.  See `doom/sudo-this-file'."
  :interactive t)
 
+(defun syd--make-syncthing-merge-finalise-hook (file-name conflict-file-name)
+  (lambda ()
+    (let ((merge-result-file (read-file-name
+                              (format "Write merge to (default: %s):"
+                                      file-name)
+                              nil file-name))
+          (delete-conflict-p (yes-or-no-p (format "Delete conflict file? (%s)"
+                                                  conflict-file-name)))))
+    (when merge-result-file
+      (with-current-buffer ediff-buffer-C
+        (set-visited-file-name merge-result-file)
+        (save-buffer))
+      (kill-buffer ediff-buffer-C))
+    (when delete-conflict-p
+      (kill-buffer (find-buffer-visiting conflict-file-name))
+      (delete-file conflict-file-name))))
+
+(defun syd--read-syncthing-conflict-file (&optional directory)
+  (let ((conflict-files (directory-files-recursively
+                         (or directory default-directory)
+                         (rx ".sync-conflict-"))))
+    (completing-read "Conflict file: " conflict-files nil t)))
+
+(defun syd--syncthing-conflict-file-base-name (conflict-file)
+  (replace-regexp-in-string (rx ".sync-conflict-" (* (not ?.)))
+                            ""
+                            conflict-file))
+
+(defun syd-resolve-syncthing-conflict (conflict-file)
+  (interactive (list (syd--read-syncthing-conflict-file)))
+  (require 'ediff)
+  (let* ((base-file (syd--syncthing-conflict-file-base-name conflict-file))
+         (ediff-after-quit-hook-internal
+          ;; Override Ediff's "save and quit" hook with our own.
+          (cons (syd--make-syncthing-merge-finalise-hook base-file conflict-file)
+                (remq #'ediff-write-merge-buffer-and-maybe-kill
+                      (ensure-list ediff-quit-merge-hook)))))
+    (ediff-merge-files
+     base-file
+     conflict-file)))
+
 (provide 'syd-file)