Re: efficient implementation of string-replace-substring / string-replace-all

[Mark H Weaver <mhw@netris.org>]

Hi Arne,

Arne Babenhauserheide <arne_bab@web.de> writes:

> For wisp I created an efficient implementation of substring replacement and thought it might be useful for guile in general.
>
> I optimized it a bit to get rid of (likely) quadratic behaviour:
>
>
> ; ,time (string-replace-substring (xsubstring "abcdefghijkl" 0 99999) "def" "abc")
> ; 1.140127s real time, 1.139714s run time.  0.958733s spent in GC.
> ; 0.885618s real time, 0.885350s run time.  0.742805s spent in GC.
> ; second number after multiple runs

Here, you're including (xsubstring "abcdefghijkl" 0 99999) in the
benchmark.  Better to (define big (xsubstring "abcdefghijkl" 0 99999))
first, and then: ,time (string-replace-substring big "def" "abc")

> (define (string-replace-substring s substring replacement)
>        "Replace every instance of substring in s by replacement."
>        (let ((sublen (string-length substring)))
>            (let replacer
>                ((newstring s)
>                  (index (string-contains s substring)))
>                (if (not (equal? index #f))
>                   (let ((replaced (string-replace newstring replacement index (+ index sublen))))
>                     (replacer replaced (string-contains replaced substring index)))
>                   newstring))))

Here's an implementation that does this benchmark about 80 times faster
on my machine: (20 milliseconds vs 1.69 seconds)

--8<---------------cut here---------------start------------->8---
(define* (string-replace-substring s substr replacement
                                   #:optional
                                   (start 0)
                                   (end (string-length s)))
  (let ((substr-length (string-length substr)))
    (if (zero? substr-length)
        (error "string-replace-substring: empty substr")
        (let loop ((start start)
                   (pieces (list (substring s 0 start))))
          (let ((idx (string-contains s substr start end)))
            (if idx
                (loop (+ idx substr-length)
                      (cons* replacement
                             (substring s start idx)
                             pieces))
                (string-concatenate-reverse (cons (substring s start)
                                                  pieces))))))))
--8<---------------cut here---------------end--------------->8---

The reason this is so much faster is because it avoids needless
generation of intermediate strings.

     Regards,
       Mark