sunet/scheme/lib/uri.scm

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2002-06-08 11:07:01 -04:00
;;; -*- Scheme -*-
;;; Copyright (c) 1995 by Olin Shivers.
;;; URI syntax -- [scheme] : path [? search ] [# fragmentid]
;;; Imports and non-R4RS'isms
;;; let-optionals
;;; receive values (MV return)
;;; ascii->char char->ascii
;;; index rindex
;;; char-set-index char-set-rindex
;;; string-reduce
;;; char-set package
;;; bitwise logical funs and arithmetic-shift
;;; join-strings (scsh field-reader code.)
;;; References:
;;; - ftp://ftp.internic.net/rfc/rfc1630.txt
;;; Original RFC
;;; - http://www.w3.org/hypertext/WWW/Addressing/URL/URI_Overview.html
;;; General Web page of URI pointers.
;;; I wrote a URI parser that slavishly obeyed Tim Berners-Lee's
;;; spec (rfc 1630). This was a waste of time, as most URL's do not
;;; obey his spec, which is incomplete and inconsistent with the URL spec
;;; in any event. This parser is much simpler. It parses a URI into four
;;; fields:
;;; [ <scheme> ] : <path> [ ? <search> ] [ # fragid ]
;;; The returned fields are *not* unescaped, as the rules for parsing the
;;; <path> component in particular need unescaped text, and are dependent
;;; on <scheme>. The URL parser is responsible for doing this.
;;; If the <scheme>, <search> or <fragid> portions are not specified,
;;; they are #f. Otherwise, <scheme>, <search>, and <fragid> are strings;
;;; <path> is a non-empty string list.
;;; The parsing technique is inwards from both ends.
;;; - First we search forwards for the first reserved char (= ; / # ? : space)
;;; If it's a colon, then that's the <scheme> part, otw no <scheme> part.
;;; Remove it.
;;; - Then we search backwards from the end for the last reserved char.
;;; If it's a sharp, then that's the <fragment-id> part -- remove it.
;;; - Then we search backwards from the end for the last reserved char.
;;; If it's a question-mark, then that's the <search> part -- remove it.
;;; - What's left is the path. Split at slashes. "" -> ("")
;;;
;;; This scheme is tolerant of the various ways people build broken
;;; URI's out there on the Net , p.e. \#= is a reserved character, but
;;; used unescaped in the search-part. It was given to me by Dan
;;; Connolly of the W3C and slightly modified.
;;; Returns four values: scheme, path, search, frag-id. Each value is
;;; either #f or a string except of the path, which is a nonempty list
;;; of string (as mentioned above).
(define uri-reserved (string->char-set ";/#?: ="))
(define (parse-uri s)
(let* ((slen (string-length s))
;; Search forwards for colon (or intervening reserved char).
(rs1 (string-index s uri-reserved)) ; 1st reserved char
(colon (and rs1 (char=? (string-ref s rs1) #\:) rs1))
(path-start (if colon (+ colon 1) 0))
;; Search backwards for # (or intervening reserved char).
(rs-last (string-index-right s uri-reserved))
(sharp (and rs-last (char=? (string-ref s rs-last) #\#) rs-last))
;; Search backwards for ? (or intervening reserved char).
;; (NB: #\= may be after #\? and before #\#)
(rs-penult (string-index-right
s
(char-set-delete uri-reserved #\=)
(or sharp slen)))
(ques (and rs-penult (char=? (string-ref s rs-penult) #\?) rs-penult))
(path-end (or ques sharp slen)))
(values (and colon (substring s 0 colon))
(split-uri-path s path-start path-end)
(and ques (substring s (+ ques 1) (or sharp slen)))
(and sharp (substring s (+ sharp 1) slen)))))
;;; Caution:
;;; Don't use this proc until *after* you've parsed the URL -- unescaping
;;; might introduce reserved chars (like slashes and colons) that could
;;; blow your parse.
(define (unescape-uri s . maybe-start/end)
(let-optionals maybe-start/end ((start 0)
(end (string-length s)))
(let* ((esc-seq? (lambda (i) (and (< (+ i 2) end)
(char=? (string-ref s i) #\%)
(hex-digit? (string-ref s (+ i 1)))
(hex-digit? (string-ref s (+ i 2))))))
(hits (let lp ((i start) (hits 0)) ; count # of esc seqs.
(if (< i end)
(if (esc-seq? i)
(lp (+ i 3) (+ hits 1))
(lp (+ i 1) hits))
hits))))
(if (and (zero? hits) (zero? start) (= end (string-length s))) s
(let* ((nlen (- (- end start) (* hits 2))) ; the new
; length of the
; unescaped
; string
(ns (make-string nlen))) ; stores the result
(let lp ((i start) (j 0)) ; sweap over the string
(if (< j nlen)
(lp (cond
((esc-seq? i) ; unescape
; escape-sequence
(string-set! ns j
(let ((d1 (string-ref s (+ i 1)))
(d2 (string-ref s (+ i 2))))
(ascii->char (+ (* 16 (hexchar->int d1))
(hexchar->int d2)))))
(+ i 3))
(else (string-set! ns j (string-ref s i))
(+ i 1)))
(+ j 1))))
ns)))))
(define hex-digit?
(let ((hex-digits (string->char-set "0123456789abcdefABCDEF")))
(lambda (c) (char-set-contains? hex-digits c))))
; 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))))
(define int->hexchar
(let ((table '#(#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9
#\A #\B #\C #\D #\E #\F)))
(lambda (i) (vector-ref table i))))
;;; Caution:
;;; All reserved chars (e.g., slash, sharp, colon) get escaped: "=;/#?: "
;;; So don't apply this proc to chunks of text with syntactically meaningful
;;; reserved chars (e.g., paths with URI slashes or colons) -- they'll be
;;; escaped, and lose their special meaning. E.g. it would be a mistake
;;; to apply ESCAPE-URI to "//lcs.mit.edu:8001/foo/bar.html" because the
;;; slashes and colons would be escaped.
(define uri-escaped-chars
(char-set-complement (char-set-union char-set:letter+digit
(string->char-set "$-_@.&!*\"'(),+"))))
;;; Takes a set of chars to escape. This is because we sometimes need to
;;; escape larger sets of chars for different parts of a URI.
(define (escape-uri s . maybe-escaped-chars)
(let-optionals maybe-escaped-chars ((escaped-chars uri-escaped-chars))
(let ((nlen (string-fold
(lambda (c i)
(+ i
(if (char-set-contains? escaped-chars c)
3 1)))
0
s))) ; new length of escaped string
(if (= nlen (string-length s)) s
(let ((ns (make-string nlen)))
(string-fold
(lambda (c i) ; replace each occurance of an
; character to escape with %ff where ff
; is the ascii-code in hexadecimal
; notation
(+ i (cond
((char-set-contains? escaped-chars c)
(string-set! ns i #\%)
(let* ((d (char->ascii c))
(dhi (bitwise-and (arithmetic-shift d -4) #xF))
(dlo (bitwise-and d #xF)))
(string-set! ns (+ i 1)
(int->hexchar dhi))
(string-set! ns (+ i 2)
(int->hexchar dlo)))
3)
(else (string-set! ns i c)
1))))
0
s)
ns)))))
;;; Four args: context URI's <scheme> : <path> values, and
;;; main URI's <scheme> : <path> values.
;;; If the path cannot be resolved, return #f #f (this occurs if <path>
;;; begins with n sequential slashes, and <context-path> doesn't
;;; have that many sequential slashes anywhere). All paths are
;;; represented as non-empty lists.
(define (resolve-uri cscheme cp scheme p)
(if scheme (values scheme p) ; If URI has own <scheme>, it is absolute.
(if (and (pair? p) (string=? (car p) "")) ; Path P begins with a slash.
(receive (numsl p) ; Count and strip off initial
(do ((i 1 (+ i 1)) ; slashes (i.e., initial ""'s)
(q (cdr p) (cdr q)))
((or (null? q) (not (string=? (car q) "")))
(values i q)))
;; Skip through CP until we find that many sequential /'s.
(let lp ((cp-tail cp)
(rhead '()) ; CP prefix, reversed.
(j 0)) ; J counts sequential /
(cond
((and (pair? cp-tail) (string=? (car cp-tail) "")) ; More ""'s
(lp (cdr cp-tail)
(cons (car cp-tail) rhead)
(+ j 0)))
((= j numsl) ; Win
(values cscheme (simplify-uri-path (rev-append rhead p))))
((pair? cp-tail) ; Keep looking.
(lp (cdr cp-tail)
(cons (car cp-tail) rhead)
1))
(else (values #f #f))))) ; Lose.
;; P doesn't begin with a slash.
(values cscheme (simplify-uri-path
(rev-append (cdr (reverse cp)) ; Drop non-dir part
p)))))) ; and append P.
(define (rev-append a b) ; (append (reverse a) b)
(let rev-app ((a a) (b b)) ; Should be defined in a list-proc
(if (pair? a) ; package, not here.
(rev-app (cdr a) (cons (car a) b))
b)))
;;; Cribbed from scsh's fname.scm
(define (split-uri-path uri start end) ; Split at /'s (infix grammar).
(let split ((i start)) ; "" -> ("")
(cond
((>= i end) '(""))
((string-index uri #\/ i) =>
(lambda (slash)
(cons (substring uri i slash)
(split (+ slash 1)))))
(else (list (substring uri i end))))))
;;; The elements of PLIST must be escaped in case they contain slashes.
;;; This procedure doesn't escape them for you; you must do that yourself:
;;; (uri-path-list->path (map escape-uri pathlist))
(define (uri-path-list->path plist)
(string-join plist "/")) ; Insert slashes between elts of PLIST.
;;; Remove . and <segment>/.. elements from path. The result is a
;;; (maybe empty) list representing a path that does not contain "."
;;; and ".." elements neither at the beginning nor somewhere else. I
;;; tried to follow RFC2396 here. The procedure returns #f if the path
;;; tries to back up past root (like "//.." or "/foo/../.."). "//" may
;;; occur somewhere in the path but not being backed up. Usually,
;;; relative paths are intended to be used with a base
;;; url. Accordingly to RFC2396 (as I hope) relative paths are
;;; considered not to start with "/". They are appended to a base
;;; URL-path and then simplified. So before you start to simplify a
;;; URL try to find out if it is a relative path (i.e. it does not
;;; start with a "/").
(define (simplify-uri-path p)
(if (null? p) #f ; P must be non-null
(let lp ((path-list (cdr p))
(stack (list (car p))))
(if (null? path-list) ; we're done
(reverse stack)
(cond
((string=? (car path-list) "..") ; back up
; neither the empty path nor root
(if (not (or (null? stack) (string=? (car stack) "")))
(lp (cdr path-list) (cdr stack))
#f))
((string=? (car path-list) ".") ; leave this
(lp (cdr path-list) stack))
((string=? (car path-list) "") ; back to root
(lp (cdr path-list) '("")))
(else ; usual segment
(lp (cdr path-list) (cons (car path-list) stack))))))))