;; This file is part of scheme-GNUnet
;; Copyright (C) 2021 GNUnet e.V.
;;
;; scheme-GNUnet is free software: you can redistribute it and/or modify it
;; under the terms of the GNU Affero General Public License as published
;; by the Free Software Foundation, either version 3 of the License,
;; or (at your option) any later version.
;;
;; GNUnet is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;; Affero General Public License for more details.
;;
;; You should have received a copy of the GNU Affero General Public License
;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
;;
;; SPDX-License-Identifier: AGPL-3.0-or-later

;; TODO: look into integrating this into Guile proper.
(define-module (web form)
  #:use-module (rnrs bytevectors)
  #:use-module (ice-9 match)
  #:use-module (ice-9 binary-ports)
  #:use-module (ice-9 textual-ports)
  #:use-module (ice-9 control)
  #:use-module (ice-9 string-fun)
  #:export (urlencoded->alist))


;; application/x-www-form-urlencoded, documented in 8.2.1.
;; of RFC 1866

;; 8.2.1 ‘[...] space characters are replaced by #\+ [...]’
;;
;; Presumably only #\  is meant here and not the non-breaking space (NBSP),
;; otherwise NBSP could not be distinguished from the regular space character
;; #\ .
;;
;; 8.2.1 ‘[...] [non-alphanumeric] characters are replaced by %HH [...]’.
;;
;; Presumably with ‘non-alphanumeric’, ‘non-alphanumeric or non-ASCII’
;; is meant here, otherwise the validity of application/x-www-form-urlencoded
;; data could depend on the Unicode standard used.
;;
;; In practice, Firefox doesn't escape - and _, so include those as well
;; for compatibility.

;; TODO: isn't a-zA-Z0-9 problematic under some locales?
(define encoded-pat "^(\\+|[a-zA-Z0-9_-]|%[0-9A-F][0-9A-F])*$")
(define encoded-regex (make-regexp encoded-pat))

(define (try-utf8->string bv)
  "Like utf8->string, but return #false instead of raising an error if
@var{bv} is not valid UTF-8."
  (catch 'decoding-error
    ;; RFC 1866 doesn't specify the character encoding, so assume UTF-8.
    (lambda () (utf8->string bv))
    (lambda _ #false)))

(define (urlencoded-string->alist string)
  (let/ec return
    (let ()
      (define (oops)
	(return #false))
      (when (string-null? string)
	(return '()))
      (define fields (string-split string #\&))
      (define (unescape string)
	;; Validate the syntax of STRING ...
	(unless (regexp-exec encoded-regex string)
	  (oops))
	;; ... replace #\+ with #\  ...
	(define string-with-space (string-replace-substring string "+" " "))
	(define bv
	  (call-with-output-bytevector
	   (lambda (port)
	     ;; ... and undo % escapes.
	     (define (search remainder)
	       (define next-% (string-index remainder #\%))
	       (if next-%
		   (begin
		     (put-string port (substring remainder 0 next-%))
		     (undo-% (substring remainder next-%)))
		   (put-string port remainder)))
	     (define (undo-% remainder)
	       (define octet
		 (string->number (substring remainder 1 3) 16))
	       ;; 8.2.1 ‘[...] [non-alphanumeric] characters are replaced by
	       ;; %HH [...]’.
	       ;;
	       ;; The syntax of application/x-www-form-urlencoded is given in
	       ;; terms of how to encode the fields, and alphanumeric characters
	       ;; are not included there, thus alphanumeric characters are
	       ;; forbidden.
	       (when (or (<= (char->integer #\a) octet (char->integer #\z))
			 (<= (char->integer #\A) octet (char->integer #\Z))
			 (<= (char->integer #\0) octet (char->integer #\9)))
		 (oops))
	       (put-u8 port octet)
	       (search (substring remainder 3)))
	     (search string-with-space))))
	;; RFC 1866 doesn't specify the character encoding, so assume UTF-8.
	;; The resulting bytevector could be bogus UTF-8, so catch
	;; 'decoding-error'.
	(or (try-utf8->string bv)
	    (oops)))
      (define (decode-field field)
	(match (string-split field #\=)
	  ((escaped-field-name escaped-field-value)
	   (cons (unescape escaped-field-name) (unescape escaped-field-value)))
	  (_ (oops))))
      (map decode-field fields))))

(define (urlencoded->alist body)
  "Decode body, a bytevector holding a application/x-www-form-urlencoded,
to an association list of string-valued key-value pairs.  Return #false
if the bytevector could not be parsed."
  (and=> (try-utf8->string body) urlencoded-string->alist))