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(define-module (aws cloudformation utils base32)
#:use-module (rnrs bytevectors)
#:use-module (srfi srfi-60)
#:export (base32-encode))
;; Credit to Ludovic Courtes
(define bytevector-quintet-ref
(let* ((ref bytevector-u8-ref)
(ref+ (lambda (bv offset)
(let ((o (+ 1 offset)))
(if (>= o (bytevector-length bv))
0
(bytevector-u8-ref bv o)))))
(ref0 (lambda (bv offset)
(bit-field (ref bv offset) 3 8)))
(ref1 (lambda (bv offset)
(logior (ash (bit-field (ref bv offset) 0 3) 2)
(bit-field (ref+ bv offset) 6 8))))
(ref2 (lambda (bv offset)
(bit-field (ref bv offset) 1 6)))
(ref3 (lambda (bv offset)
(logior (ash (bit-field (ref bv offset) 0 1) 4)
(bit-field (ref+ bv offset) 4 8))))
(ref4 (lambda (bv offset)
(logior (ash (bit-field (ref bv offset) 0 4) 1)
(bit-field (ref+ bv offset) 7 8))))
(ref5 (lambda (bv offset)
(bit-field (ref bv offset) 2 7)))
(ref6 (lambda (bv offset)
(logior (ash (bit-field (ref bv offset) 0 2) 3)
(bit-field (ref+ bv offset) 5 8))))
(ref7 (lambda (bv offset)
(bit-field (ref bv offset) 0 5)))
(refs (vector ref0 ref1 ref2 ref3 ref4 ref5 ref6 ref7)))
(lambda (bv index)
"Return the INDEXth quintet of BV."
(let ((p (vector-ref refs (modulo index 8))))
(p bv (quotient (* index 5) 8))))))
(define (base32-encode bv)
;; We are assuming that the bytevector length is divisible by 5,
;; because that is the case for SHA-1 hashes. A general-purpose
;; base32 encoder would need to pad the bytevector when this is not
;; the case.
(let* ((alphabet "0123456789ABCDEFGHJKMNPQRSTVWXYZ")
(n (/ (* (bytevector-length bv) 8) 5))
(s (make-string n)))
(let loop ((i 0))
(when (< i n)
(let ((x (bytevector-quintet-ref bv i)))
(string-set! s i (string-ref alphabet x))
(loop (+ i 1)))))
s))
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