(define encode-replacements
  (list (list " " "%20")
        (list " " "+")
        (list "!" "%21")
        (list "#" "%23")
        (list "$" "%24")
        (list "%" "%25")
        (list "&" "%26")
        (list "'" "%27")
        (list "(" "%28")
        (list ")" "%29")
        (list "*" "%2A")
        (list "+" "%2B")
        (list "," "%2C")
        (list "/" "%2F")
        (list ":" "%3A")
        (list ";" "%3B")
        (list "=" "%3D")
        (list "?" "%3F")
        (list "@" "%40")
        (list "[" "%5B")
        (list "]" "%5D")
        (list "<" "%3C")
        (list ">" "%3E")
        (list "\\" "%5C")
        (list "\"" "%22")
        (list "\n" "%0A")
        (list "\r" "%0D")))

(define decode-replacements (map reverse encode-replacements))

(define (get-replacement key mode)
  (let ((r (if (string=? mode "encode")
             (assoc key encode-replacements)
             (assoc key decode-replacements))))
    (if r (car (cdr r)) key)))

(define (endecode mode s)
  (if (not s)
    ""
    (letrec ((s-length (string-length s))
             (looper
               (lambda (i result)
                 (if (< i s-length)
                   (let ((key-length (if (and (string=? mode "decode")
                                              (string=? (string-copy s i (+ i 1)) "%")
                                              (> s-length (+ i 2)))
                                       3
                                       1)))
                     (looper (+ i key-length)
                             (string-append result
                                            (get-replacement
                                              (string-copy s i (+ i key-length))
                                              mode))))
                   result))))
      (looper 0 ""))))

(define (url-encode str) (cond ((string? str) (endecode "encode" str)) (else str)))
(define (url-decode str) (cond ((string? str) (endecode "decode" str)) (else str)))