Re: what is wrong with this loop?

B. Pym wrote:

B. Pym wrote:
 

Kenny Tilton wrote:
 

here are some test cases:
 
(decode-http-encoded "aaaa") ;; this works
(decode-http-encoded "aaaa%62ccc" ) ;; this dies because start is 0
                                    ;; and end is -3 (0 based index)

 
....
 

(defun decode-http-encoded( x )
   (with-output-to-string(s)
     (loop for start = 0 then (+ pct 3)
         for pct = (position #\% x :start start)
         while pct
         do (princ (subseq x start pct) s )
           (let ((is (subseq x (1+ pct) (+ pct 3))))
             (princ (code-char (parse-integer is)) s))

 
The integer to be parsed is in hexadecimal, Kenny.
 

         finally (princ (subseq x start) s))
     s ))

 
Instead of CL (COBOL-Like), let's use a Lispy language.
 
Gauche Scheme:
 
The easy way.
 
(define (decode-http-encoded x)
  (regexp-replace-all
    #/%(..)/
    x
    (lambda (m) (integer->char (string->number (m 1) 16)))))
 
 
(decode-http-encoded "foo%20bar%20%41ct")
 ===>
"foo bar Act"
 
 
Harder.
 
(use gauche.sequence)  ;; subseq size-of
 
(define (decode-http-encoded x)
  (define (hex->c s) (integer->char (string->number s 16)))
  (with-output-to-string
    (lambda()
      (do ((i 0 (+ i 1)))
        ((= i (size-of x)))
        (let1 c (ref x i)
          (when (equal? c #\%)
            (set! c (hex->c (subseq x (+ i 1)(+ i 3))))
            (inc! i 2))
          (display c))))))

 
To make the processing easier, first convert the string
to a list of characters.
 
(define (decode-http-encoded x)
  (define (hex->c s) (integer->char (string->number s 16)))
  (let1 z (string->list x)
    (with-output-to-string
      (lambda()
        (while (pair? z)
          (let1 c (pop! z)
            (and (eqv? c #\%)
              (set! c (hex->c (string (pop! z) (pop! z)))))
            (display c)))))))
 

Let's define a safe version of pop! that doesn't
raise an error if the list is empty.

(define-syntax pop~
  (syntax-rules ()
    [ (pop~ List)
      (if (null? List)
        #f
        (pop! List)) ]))

(define (decode-http-encoded x)
  (define (hex->c s) (integer->char (string->number s 16)))
  (let1 z (string->list x)
    (with-output-to-string
      (lambda()
        (while (pop~ z) => c
          (and (eqv? c #\%)
            (set! c (hex->c (string (pop! z) (pop! z)))))
          (display c))))))