[racket-dev] strange performance on regexp-match: proposed patch

I've worked on those fns in the past but have no idea if that is a
good change. I'd say that the thing to do is to run the tests and then
think of a few more tests based on your new code (maybe use random
testing to compare te two versions of the fns) and then go ahead and
commit.


Robby

On Wednesday, April 27, 2011, Danny Yoo <dyoo at cs.wpi.edu> wrote:
> I've isolated a performance issue on form-urlencoded->alist.  On
> strings with very long key/values, the code appears to consume an
> unusual amount of memory.  Does the following look ok?
>
>
> diff --git a/collects/net/uri-codec-unit.rkt b/collects/net/uri-codec-unit.rkt
> index c992d62..01d4c05 100644
> --- a/collects/net/uri-codec-unit.rkt
> +++ b/collects/net/uri-codec-unit.rkt
> @@ -269,7 +269,17 @@ See more in PR8831.
>  ;; string -> listof (cons string string)
>  ;; http://www.w3.org/TR/html401/appendix/notes.html#ampersands-in-uris
>  (define (form-urlencoded->alist str)
> -  (define keyval-regexp #rx"^([^=]*)(?:=(.*))?$")
> +  (define (split-key-value s)
> +    (let ([n (string-length s)])
> +      (let loop ([i 0])
> +        (cond [(= i n)
> +               (values s #f)]
> +              [(char=? (string-ref s i) #\=)
> +               (values (substring s 0 i)
> +                       (substring s (add1 i)))]
> +              [else
> +               (loop (add1 i))]))))
> +
>    (define value-regexp
>      (case (current-alist-separator-mode)
>        [(semi) #rx"[;]"]
> @@ -278,10 +288,10 @@ See more in PR8831.
>    (if (equal? "" str)
>      '()
>      (map (lambda (keyval)
> -           (let ([m (regexp-match keyval-regexp keyval)]) ; cannot fail
> -             (cons (string->symbol (form-urlencoded-decode (cadr m)))
> +           (let-values ([(key val) (split-key-value keyval)])
> +             (cons (string->symbol (form-urlencoded-decode key))
>                     ;; can be #f for no "=..." part
> -                   (and (caddr m) (form-urlencoded-decode (caddr m))))))
> +                   (and val (form-urlencoded-decode val)))))
>           (regexp-split value-regexp str))))
>
>  (define current-alist-separator-mode
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