bug#44983: Truncate long lines of grep output

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: dgutov@yandex.ru, 44983@debbugs.gnu.org
> Date: Wed, 02 Dec 2020 22:53:18 +0200
> 
> > Does it help to set bidi-inhibit-bpa non-nil?
> 
> This helped to open the file with a lot of '{'.
> But on minified files grep.el is still very slow.

What are "minified files"?

And when you say "slow" do you mean slow in receiving Grep output,
slow in displaying the received output, or slow in moving though the
*grep* buffer after everything was displayed?

> Then instead of hiding long lines using font-lock,
> I tried to do the same using the process filter:
> 
> (defun grep-filter ()
>   (save-excursion
>     (let ((end (point-marker)))
>       (goto-char compilation-filter-start)
>       (forward-line 0)
>       (while (< (point) end)
>         (let ((eol (line-end-position)))
>           (when (> (- eol (point)) 64)
>             (put-text-property (+ 64 (point)) (line-end-position)
>                                'display "[…]"))
>           (forward-line 1))))))
> 
> Still very slow.

Same question as above.

> Then tried to delete the excessive parts of long lines:
> 
> (defun grep-filter-try ()
>   (save-excursion
>     (let ((end (point-marker)))
>       (goto-char compilation-filter-start)
>       (forward-line 0)
>       (while (< (point) end)
>         (let ((eol (line-end-position)))
>           (when (> (- eol (point)) 64)
>             (delete-region (min (+ 64 (point)) (point-max)) (line-end-position)))
>           (forward-line 1))))))
> 
> Now Emacs becomes more responsive.  But still output processing
> takes too much time.

What is "output processing", and how did you measure the time it
takes?

> Finally, the last thing to try was the same solution that Richard
> showed in bug#44941:
> 
>   grep -a "$@" | cut -c -200
> 
> that gives the best possible result.
> 
> I doubt that it would be possible to invent something better.
> 
> So the question is should this be customizable for adding
> `cut -c` automatically to the end of the grep command line,
> possibly with `stdbuf -oL` suggested by Andreas.

I suggested to request the equivalent of "cut -c" to be a feature
added to Grep.

Failing that, I don't think Emacs should do something like that,
especially since 'cut' is not guaranteed to be available.  Users who
have such problems can, of course, modify the Grep command to do that.