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

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