C: Auto-format with clang-format or uncrustify on save

.emacs.d/lisp/init-cc.el

@@ -70,12 +70,7 @@ restored."
     (when makefile-dir
       (compile (format "make -k -C %s clean" (shell-quote-argument makefile-dir))))))
 
-;;; It is tempting to add `ambrevar/cc-fmt' to the hook:
-;; (add-hook 'before-save-hook 'ambrevar/cc-prettify nil t)
-;;; Unlike Go however, there is no formatting standard and thus this would break
-;;; the formatting rules of every third-party C file that does not follow the
-;;; same style.
-(defun ambrevar/cc-prettify ()
+(defun ambrevar/cc-format-with-uncrustify (&optional cfg-file)
   "Run uncrustify(1) on current buffer or region."
   (interactive)
   (let  (status
@@ -85,13 +80,51 @@ restored."
         (setq start (region-beginning) end (region-end))
       (setq start (point-min) end (point-max)))
     (setq status
-          (call-process-region start end "uncrustify" nil formatbuf nil "-lc" "-q" "-c" (expand-file-name ".uncrustify.cfg" (getenv "HOME"))))
+          (call-process-region start end "uncrustify" nil formatbuf nil "-lc" "-q" "-c"
+                               (or cfg-file
+                                   (expand-file-name ".uncrustify.cfg" (getenv "HOME")))))
     (if (/= status 0)
         (error "error running uncrustify")
       (delete-region start end)
       (insert-buffer-substring formatbuf)
       (kill-buffer formatbuf))))
 
+(defun ambrevar/cc-format-file-lookup ()
+  "Find .clang-format or .uncrustify.cfg in parent folder up to Git root.
+Return nil if non is found or if not a Git repository."
+  (unless (require 'magit nil 'noerror)
+    (error "Magit is missing"))
+  (when (or (magit-get-current-branch) (magit-get-current-tag))
+    (let ((git-root (magit-rev-parse "--show-toplevel"))
+          (default-directory default-directory))
+      (while (and (string= (magit-rev-parse "--show-toplevel") git-root)
+                  (not (file-exists-p ".clang-format"))
+                  (not (file-exists-p ".uncrustify.cfg")))
+        (cd ".."))
+      (or (and (file-exists-p ".clang-format")
+               (expand-file-name ".clang-format" default-directory))
+          (and (file-exists-p ".uncrustify.cfg")
+               (expand-file-name ".uncrustify.cfg" default-directory))))))
+
+(defun ambrevar/cc-format ()
+  "Format C file.
+It uses `ambrevar/cc-format-file-lookup' to find the format rules.
+This is suitable for a `before-save-hook'."
+  (interactive)
+  (let ((cfg-file (ambrevar/cc-format-file-lookup)))
+    (cond
+     ((and (string= (file-name-base cfg-file) ".clang-format")
+           (require 'clang-format nil 'noerror))
+      (clang-format-buffer))
+     ((and (string= (file-name-base cfg-file) ".uncrustify.cfg")
+           (executable-find "uncrustify"))
+      (ambrevar/cc-format-with-uncrustify cfg-file)))))
+
+(defun ambrevar/cc-turn-on-format ()
+  "Add `ambrevar/cc-format' to `before-save-hook' locally.
+You can add your mode hook (e.g. `c-mode-hook')."
+  (add-hook 'before-save-hook 'ambrevar/cc-format nil 'local))
+
 ;;; GMP documentation
 (with-eval-after-load "info-look"
   (let ((mode-value (assoc 'c-mode (assoc 'symbol info-lookup-alist))))
@@ -147,6 +180,7 @@ restored."
 (dolist (hook '(c-mode-hook c++-mode-hook))
   (when (require 'company nil t)
     (add-hook hook 'company-mode))
+  (add-hook hook 'ambrevar/cc-turn-on-format)
   (add-hook hook 'ambrevar/cc-set-compiler))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;