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

[Ivan Shmakov]

>>>>> Dmitry Gutov <dgutov@yandex.ru> writes:

[…]

 > Here's the updated definition (by the way, `json-special-chars' is
 > still needed, to convert ?\n to ?n, and so on, and the performance
 > hit is negligible).

        Perhaps a plain vector may fit there?

 > (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)
 >       (let ((c (char-before)))
 >         (delete-region (1- (point)) (point))
 >         (if (match-beginning 1)
 >             ;; Special JSON character (\n, \r, etc.).
 >             (insert "\\" (car (rassoc c json-special-chars)))
 >           ;; Fallback: UCS code point in \uNNNN form.
 >           (insert (format "\\u%04x" c)))))
 >     (concat "\"" (buffer-string) "\"")))

        FWIW, using replace-match in the loop seem to speed up the
        routine by another few percents.

    (while (re-search-forward "\\([\"\\/\b\f\n\r\t]\\)\\|[^ -~]" nil t)
      (let ((c (char-before)))
        (replace-match
         (if (match-beginning 1)
             ;; Special JSON character (\n, \r, etc.).
             (string ?\\ (car (rassq c json-special-chars)))
           ;; Fallback: UCS code point in \uNNNN form.
           (format "\\u%04x" c))
         t t)))

[…]

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