Add a new function 'line-number-at-position'

* doc/lispref/positions.texi (Text Lines): Document it.

* lisp/simple.el (count-lines): Use it.
(line-number-at-pos): Ditto.

* src/fns.c (Fline_number_at_position): New function (bug#22763).

doc/lispref/positions.texi

@@ -437,16 +437,18 @@ prints a message reporting the number of lines, words, and characters
 in the buffer, or in the region if the region is active.
 @end deffn
 
+@defun line-number-at-position pos
+This function returns the line number in the current buffer
+corresponding to the buffer position @var{pos}.  If narrowing is in
+effect, this is the line number in the visible part of the buffer.
+@end defun
+
 @defun line-number-at-pos &optional pos absolute
 @cindex line number
-This function returns the line number in the current buffer
-corresponding to the buffer position @var{pos}.  If @var{pos} is
-@code{nil} or omitted, the current buffer position is used. If
-@var{absolute} is @code{nil}, the default, counting starts at
-@code{(point-min)}, so the value refers to the contents of the
-accessible portion of the (potentially narrowed) buffer.  If
-@var{absolute} is non-@code{nil}, ignore any narrowing and return
-the absolute line number.
+This function is like @code{line-number-at-position}, but if @var{pos}
+is @code{nil} or omitted, the current buffer position is used.  In
+addition, if @var{absolute} is non-@code{nil}, ignore any narrowing
+and return the absolute line number.
 @end defun
 
 @ignore

etc/NEWS

@@ -2192,6 +2192,10 @@ back in Emacs 23.1.  The affected functions are: 'make-obsolete',
 
 * Lisp Changes in Emacs 28.1
 
++++
+** New function 'line-number-at-position'.
+This returns the line number in the visible portion of the buffer.
+
 ---
 ** New variable 'indent-line-ignored-functions'.
 This allows modes to cycle through a set of indentation functions

lisp/simple.el

@@ -1472,7 +1472,7 @@ included in the count."
 			        (assq prop buffer-invisibility-spec)))
 			  (setq invisible-count (1+ invisible-count))))
 		    invisible-count))))
-	    (t (- (buffer-size) (forward-line (buffer-size))))))))
+	    (t (1- (line-number-at-position (point-max))))))))
 
 (defun line-number-at-pos (&optional pos absolute)
   "Return buffer line number at position POS.
@@ -1483,16 +1483,11 @@ at (point-min), so the value refers to the contents of the
 accessible portion of the (potentially narrowed) buffer.  If
 ABSOLUTE is non-nil, ignore any narrowing and return the
 absolute line number."
-  (save-restriction
-    (when absolute
-      (widen))
-    (let ((opoint (or pos (point))) start)
-      (save-excursion
-        (goto-char (point-min))
-        (setq start (point))
-        (goto-char opoint)
-        (forward-line 0)
-        (1+ (count-lines start (point)))))))
+  (if absolute
+      (save-restriction
+        (widen)
+        (line-number-at-position (or pos (point))))
+    (line-number-at-position (or pos (point)))))
 
 (defcustom what-cursor-show-names nil
   "Whether to show character names in `what-cursor-position'."

src/fns.c

@@ -5758,6 +5758,23 @@ in OBJECT.  */)
   traverse_intervals (intervals, 0, collect_interval, collector);
   return CDR (collector);
 }
+
+DEFUN ("line-number-at-position", Fline_number_at_position,
+       Sline_number_at_position, 1, 1, 0,
+       doc: /* Return the line number at POSITION.
+If the buffer is narrowed, the position returned is the position in the
+visible part of the buffer.  */)
+  (register Lisp_Object position)
+{
+  CHECK_FIXNUM (position);
+  ptrdiff_t pos = XFIXNUM (position);
+
+  /* Check that POSITION is n the visible range of the buffer. */
+  if (pos < BEGV || pos > ZV)
+    args_out_of_range (make_int (BEGV), make_int (ZV));
+
+  return make_int (count_lines (BEGV_BYTE, CHAR_TO_BYTE (pos)) + 1);
+}
 
 
 void
@@ -5800,6 +5817,7 @@ syms_of_fns (void)
   defsubr (&Sdefine_hash_table_test);
   defsubr (&Sstring_search);
   defsubr (&Sobject_intervals);
+  defsubr (&Sline_number_at_position);
 
   /* Crypto and hashing stuff.  */
   DEFSYM (Qiv_auto, "iv-auto");

test/src/fns-tests.el

@@ -1098,3 +1098,11 @@
     (goto-char (point-max))
     (insert "fóo")
     (should (approx-equal (buffer-line-statistics) '(1002 50 49.9)))))
+
+(ert-deftest test-line-number-at-position ()
+  (with-temp-buffer
+    (insert (make-string 10 ?\n))
+    (should (= (line-number-at-position (point)) 11))
+    (should-error (line-number-at-position nil))
+    (should-error (line-number-at-position -1))
+    (should-error (line-number-at-position 100))))