bug#20154: 25.0.50; json-encode-string is too slow for large strings

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 20 Mar 2015 17:20:25 +0200
> From: Dmitry Gutov <dgutov@yandex.ru>
> CC: 20154@debbugs.gnu.org
> 
> On 03/20/2015 05:03 PM, Eli Zaretskii wrote:
> 
> > Yes, I could.  What's your point, though?
> 
> That if asking the question takes the same time as doing the profiling 
> yourself, the latter would be more efficient. I don't really mind, just 
> puzzled.

I have other things on my plate while I read email.  If my advice
bothers you, I can shut up in the future.

>  > json-encode-char and mapconcat take most of the time, so it seems.
> 
> So it does. But here's an alternative implementation I tried:
> 
> (defun json-encode-big-string (str)
>    (with-temp-buffer
>      (insert str)
>      (goto-char (point-min))
>      (while (re-search-forward "[\"\\/\b\f\b\r\t]\\|[^ -~]" nil t)
>        (replace-match (json-encode-char (char-after (match-beginning 0)))
>                       t t))
>      (format "\"%s\"" (buffer-string))))
> 
> It takes 0.15s here, which is still too long.

I suggest to rewrite json-encode-char, it does a lot of unnecessary
stuff, starting with the call to encode-char (which was needed in
Emacs 22 and before, but no more).  The call to rassoc is also
redundant, since you already have that covered in your regexp.