Re: Temporarily select-window, without updating mode-line face and cursor fill?

[JD Smith]

For very large file, line-number-at-pos, which uses count-lines, if far too 
slow to be used in a modeline :eval.  I suspect this is why the “%l” mode line 
formatter does not itself use count-lines.  Here’s my analysis of that (on top 
of some 7yr old complaints about the speed of line-number-at-pos): 
https://emacs.stackexchange.com/questions/3821/a-faster-method-to-obtain-line-number-at-pos-in-large-buffers/64656#64656

Summary: format-mode-line is >10x faster on “random” lines, and thousands of 
times faster for “nearby lines” (presumably due to caching).  The latter is 
particular important for rapid scrolling updates. 

I managed to get this working quite well on large files (like /usr/dict/words); 
see https://github.com/jdtsmith/mlscroll.  I indeed use count-lines for 
incrementing line count from window-start to window-end, since they are 
“close”, and have simplified the fast line count to:

(defun mlscroll-fast-line-number-at-pos (pos &optional win)
  (let ((old (window-point win)))
    (set-window-point win pos)
    (prog1
        (string-to-number (format-mode-line "%l" 0 win))
      (set-window-point win old))))

If you had any concrete suggestions for calculating window positions (like 
window-start and window-end, not just buffer positions) in a non-active window 
without selecting it, which is superior to my approach, I’d be happy to hear. 

> On May 2, 2021, at 2:55 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: JD Smith <jdtsmith@gmail.com>
>> Date: Sat, 1 May 2021 18:17:46 -0400
>> Cc: emacs-devel@gnu.org
>> 
>> (defun mlscroll-fast-line-number-at-pos (pos &optional win)
>>  "Line number at position.
>> Compute line number at position POS. Uses fast mode-line
>> formatting.  If WIN is non-nil, find line number at position within
>> that window."
>>  (string-to-number
>>   (if win
>>       (let ((old (window-point win)))
>>       (set-window-point win pos)
>>       (prog1
>>           (format-mode-line "%l" 0 win)
>>         (set-window-point win old)))
>>       (save-excursion
>>         (goto-char pos)
>>         (format-mode-line "%l" 0)))))
> 
> I don't recommend using format-mode-line for counting lines.  Why not
> use count-lines instead?