(define in-biwa #f)

(define idiv (if in-biwa div quotient))
(define imod (if in-biwa mod remainder))
(define ls (lambda (n i) (* n (expt 2 i))))
(define rs (lambda (n i) (if (> i 0) (rs (idiv n 2) (- i 1)) n)))
(define b64map (map (lambda (p) (map char->integer p)) (list '(#\A #\Z) '(#\a #\z) '(#\0 #\9) '(#\+ #\+) '(#\/ #\/))))

;; (char->sixtet a-char)
;; =>
;; (a-six-bit-integer)
(define char->sixtet (lambda (c)
											 (let ([n (char->integer c)])
												 (let check-range ([rest b64map] [base 0])
													 (let* ([trial (car rest)] [nmin (car trial)] [nmax (cadr trial)])
														 (if (and (>= n nmin) (<= n nmax))
															 (+ base (- n (car trial)))
															 (check-range (cdr rest) (+ base 1 (- nmax nmin)))))))))

;; (base64->char (string->list a-base64-string))
;; =>
;; (string->list the-original)
(define base64->char
	(lambda (source)
		(call-with-values
			(lambda () 
				(let parse-next ([rest source] [offset 0])
					(let parse-sixtet ([rest rest])
						(if (null? rest)
							(values (list) 0)
							(if (memv (car rest) '(#\space #\newline #\tab #\=))
								(parse-sixtet (cdr rest))
								(let ([current (char->sixtet (car rest))])
									(call-with-values (lambda () (parse-next (cdr rest) (imod (+ offset 2) 8)))
																		(lambda (result next)
																			(values
																				(if (= offset 6)
																					result
																					(append	(list (integer->char (+ (imod (ls current (+ offset 2)) (ls 1 8)) (rs next (- 4 offset))))) result))
																				current)))))))))
			(lambda (result ignore) result))))

(display (base64->char (string->list "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=")))