GNU bug report logs - #20154
25.0.50; json-encode-string is too slow for large strings

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Message #44 received at 20154 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 20154 <at> debbugs.gnu.org
Subject: Re: bug#20154: 25.0.50; json-encode-string is too slow for
 large strings
Date: Sat, 21 Mar 2015 00:26:03 +0200

As per your comment, this seems to be the best we can do for long 
strings, without diving into C:

(defun json-encode-string-1 (string)
  "Return a JSON representation of STRING."
  (with-temp-buffer
    (insert string)
    (goto-char (point-min))
    ;; Skip over ASCIIish printable characters.
    (while (re-search-forward "\\([\"\\/\b\f\n\r\t]\\)\\|[^ -~]" nil t)
      (replace-match
       (if (match-beginning 1)
           ;; Special JSON character (\n, \r, etc.).
           (format "\\%c" (char-after (match-beginning 0)))
         ;; Fallback: UCS code point in \uNNNN form.
         (format "\\u%04x" (char-after (match-beginning 0))))
       t t))
    (format "\"%s\"" (buffer-string))))

It brings the execution time down to ~0.14s here, on the same example.

And there'll need to be a fallback for short strings, because 
`with-temp-buffer' overhead is non-trivial.

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