Implement prefix arg for 'c-ts-mode-toggle-comment-style'

* lisp/progmodes/c-ts-mode.el (c-ts-mode-toggle-comment-style):
Actually implement the optional numeric arg mentioned in the
docstring.

Copyright-paperwork-exempt: yes

1 files changed, 13 insertions, 9 deletions

diff --git a/lisp/progmodes/c-ts-mode.el b/lisp/progmodes/c-ts-mode.el
index 4b66824c44f..53f7839e4af 100644
--- a/lisp/progmodes/c-ts-mode.el
+++ b/lisp/progmodes/c-ts-mode.el
@@ -88,19 +88,23 @@
   :safe 'integerp
   :group 'c)
 
-(defun c-ts-mode-toggle-comment-style ()
+(defun c-ts-mode-toggle-comment-style (&optional arg)
   "Toggle the comment style between block and line comments.
 Optional numeric ARG, if supplied, switches to block comment
 style when positive, to line comment style when negative, and
 just toggles it when zero or left out."
-  (interactive)
-  (pcase-let ((`(,starter . ,ender)
-               (if (string= comment-start "// ")
-                   (cons "/* " " */")
-                 (cons "// " ""))))
-    (setq-local comment-start starter
-                comment-end ender))
-  (c-ts-mode-set-modeline))
+  (interactive "P")
+  (let ((prevstate-line (string= comment-start "// ")))
+    (when (or (not arg)
+              (zerop (setq arg (prefix-numeric-value arg)))
+              (xor (> 0 arg) prevstate-line))
+      (pcase-let ((`(,starter . ,ender)
+                   (if prevstate-line
+                       (cons "/* " " */")
+                     (cons "// " ""))))
+        (setq-local comment-start starter
+                    comment-end ender))
+      (c-ts-mode-set-modeline))))
 
 (defun c-ts-mode-set-modeline ()
   (setq mode-name