[clang] [emacs][clang-format] Add elisp API for clang-format on git diffs (PR #112792)

[Campbell Barton via cfe-commits]

================
@@ -132,18 +132,97 @@ is a zero-based file offset, assuming ‘utf-8-unix’ coding."
     (lambda (byte &optional _quality _coding-system)
       (byte-to-position (1+ byte)))))
 
-;;;###autoload
-(defun clang-format-region (start end &optional style assume-file-name)
-  "Use clang-format to format the code between START and END according to STYLE.
-If called interactively uses the region or the current statement if there is no
-no active region. If no STYLE is given uses `clang-format-style'. Use
-ASSUME-FILE-NAME to locate a style config file, if no ASSUME-FILE-NAME is given
-uses the function `buffer-file-name'."
-  (interactive
-   (if (use-region-p)
-       (list (region-beginning) (region-end))
-     (list (point) (point))))
+(defun clang-format--git-diffs-get-diff-lines (file-orig file-new)
+  "Return all line regions that contain diffs between FILE-ORIG and
+FILE-NEW.  If there is no diff 'nil' is returned. Otherwise the
+return is a 'list' of lines in the format '--lines=<start>:<end>'
+which can be passed directly to 'clang-format'"
+  ;; Temporary buffer for output of diff.
+  (with-temp-buffer
+    (let ((status (call-process
+                   "diff"
+                   nil
+                   (current-buffer)
+                   nil
+                   ;; Binary diff has different behaviors that we
+                   ;; aren't interested in.
+                   "-a"
+                   ;; Printout changes as only the line groups.
+                   "--changed-group-format=--lines=%dF:%dL "
+                   ;; Ignore unchanged content.
+                   "--unchanged-group-format="
+                   file-orig
+                   file-new
+                   )
+                  )
+          (stderr (concat (if (zerop (buffer-size)) "" ": ")
+                          (buffer-substring-no-properties
+                           (point-min) (line-end-position)))))
+      (when (stringp status)
----------------
ideasman42 wrote:

\*picky\* I think a `(cond ...)` would read better than  unless & if statements.

example:
```elisp
(cond 
  ((stringp status) ...snip...)
  ((= status 0) nil)
  ((= status 1) ...snip...)
  (t (error ...snip...)))
```

https://github.com/llvm/llvm-project/pull/112792