blob: 74fd0c51859cc14c5d8787ab58bcedace64ede55 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
|
;;; Chickadee Game Toolkit
;;; Copyright © 2023 David Thompson <dthompson2@worcester.edu>
;;;
;;; Licensed under the Apache License, Version 2.0 (the "License");
;;; you may not use this file except in compliance with the License.
;;; You may obtain a copy of the License at
;;;
;;; http://www.apache.org/licenses/LICENSE-2.0
;;;
;;; Unless required by applicable law or agreed to in writing, software
;;; distributed under the License is distributed on an "AS IS" BASIS,
;;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;;; See the License for the specific language governing permissions and
;;; limitations under the License.
(define-module (chickadee base64)
#:use-module (rnrs bytevectors)
#:export (base64-decode))
(define %base64-chars
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
(define %decode-table
(let ((table (make-hash-table)))
(let loop ((i 0))
(when (< i 64)
(hashv-set! table (string-ref %base64-chars i) i)
(loop (+ i 1))))
(hashv-set! table #\= 0)
table))
(define (base64-decode str)
"Decode the base64 encoded string STR and return a bytevector
containing the decoded data."
(let* ((n (string-length str))
(groups (/ n 4)))
(define (padding? i)
(eqv? (string-ref str i) #\=))
(define (decode i)
(let ((c (string-ref str i)))
(or (hashv-ref %decode-table c)
(error 'base64-error "invalid base64 character" c))))
(define (decode4 group)
(let ((start (* group 4)))
(logior (ash (decode start) 18)
(ash (decode (+ start 1)) 12)
(ash (decode (+ start 2)) 6)
(decode (+ start 3)))))
(unless (exact-integer? groups)
(error 'base64-error "invalid character count"))
(let* ((padding (cond
((and (padding? (- n 2)) (padding? (- n 1))) 2)
((padding? (- n 1)) 1)
(else 0)))
(bv (make-bytevector (- (* groups 3) padding))))
(let loop ((i 0))
(cond
((= i (- groups 1))
(let ((x (ash (decode4 i) (* padding -8))))
(bytevector-sint-set! bv (* i 3) x (endianness big) (- 3 padding))
bv))
((< i groups)
(bytevector-sint-set! bv (* i 3) (decode4 i) (endianness big) 3)
(loop (+ i 1)))))
bv)))
|
