*new procedure UNESCAPE (unescape URI-components using RegExps)

*move HEXCHAR->INT from uri.scm to here
*use UNESCAPE in PARSED-URI->HTTP-URL

scheme/lib/url.scm

@@ -204,12 +204,11 @@
 (define (parsed-uri->http-url host port path query)
 
   (let 	((portnumber (and port (string->number port)))
-	(decoded-path (map unescape-uri path))
-	(decoded-query (and query (unescape-uri query))))
+	(decoded-path (map unescape path))
+	(decoded-query (and query (unescape query))))
 
   (make-http-url host portnumber decoded-path decoded-query)))
 
-;;TODO: separate procedures for decoding path and query
 ;;really decode query here??
 
 ;;; parse a HTTP 1.1. Request_URI into a http-url record
@@ -251,3 +250,50 @@
     (string-append scheme-and-host-string port-string path-string query-string)))
 
 ;;TODO: separate procedures for encoding path and query
+
+
+
+;;; decode a URI
+;;; walk over string s and unescape all occurrences of RegExp 'escaped' (see above).
+
+;;; Remark: 
+;;; we assume no non-ASCII characters occur in the URI; therefore the
+;;; ascii table is used for conversion of the octet the hexnumber
+;;; represents to a char.
+
+;;; Caution:
+;;; a URI must be separated into its components (i.e. parsed by
+;;; PARSE-URI) before the escaped characters within those components
+;;; can be safely decoded. Don't use UNESCAPE on an unparsed URI.
+
+(define (unescape s)
+  (regexp-fold 
+   escaped
+   (lambda (start-search match res)
+     (let* ((start-match (match:start match))
+	    (hexchar-low (string-ref s (+ start-match 2)))
+	    (hexchar-high (string-ref s (+ start-match 1)))
+	    (hex-low (hexchar->int hexchar-low))
+	    (hex-high (hexchar->int hexchar-high))
+	    (ascii (+ (* 16 hex-high) hex-low)))
+       (string-append 
+	res 
+	(substring s start-search start-match)
+	(string (ascii->char ascii)))))
+   ""
+   s
+   (lambda (start-search res)
+    (string-append res (substring s start-search (string-length s))))))
+
+
+; make use of the fact that numbers and characters are in order in the ascii table
+(define (hexchar->int c)      
+  (- (char->ascii c) 
+     (if (char-numeric? c)
+	 (char->ascii #\0)         
+	 (- (if (char-upper-case? c)
+		(char->ascii #\A)
+		(char->ascii #\a))
+	    10))))
+
+