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

[via cfe-commits]

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@@ -146,18 +146,104 @@ 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--vc-diff-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’."
+  ;; Use temporary buffer for output of diff.
+  (with-temp-buffer
+    ;; We could use diff.el:diff-no-select here. The reason we don't
+    ;; is diff-no-select requires extra copies on the buffers which
+    ;; induces noticeable slowdowns, especially on larger files.
+    (let ((status (call-process
+                   diff-command
+                   nil
+                   (current-buffer)
+                   nil
+                   ;; Binary diff has different behaviors that we
+                   ;; aren't interested in.
+                   "-a"
+                   ;; Print new lines in file-new formatted as
+                   ;; "--lines=<StartDiff:EndDiff> "
+                   "--changed-group-format=%(N=0?:--lines=%dF:%dM )"
+                   ;; Don't print anything for unchanged lines
+                   "--unchanged-group-format="
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goldsteinn wrote:

Its not fundementally needed, but it speeds it up as we can then skip scanning all the diffs.
Does bsd diff support `--unchanged-line-format`?

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