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	base64.c - ics2txt - convert icalendar .ics file to plain text
	git clone git://bitreich.org/ics2txt git://enlrupgkhuxnvlhsf6lc3fziv5h2hhfrinws…
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	---
	base64.c (3039B)
	---
	1 #include "base64.h"
	2 #include <assert.h>
	3 #include <stddef.h>
	4 #include <stdint.h>
	5 #include <string.h>
	6
	7 static char encode_map[64] =
	8 "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
	9
	10 void
	11 base64_encode(char const s, size_t slen, char d, size_t *dlen)
	12 {
	13 char const sbeg = s, send = s + slen, *dbeg = d;
	14 unsigned char x;
	15
	16 while (s < send) {
	17 switch ((s - sbeg) % 3) {
	18 case 0: /* AAAAAABB bbbbcccc ccdddddd */
	19 assert((size_t)(d - dbeg) + 1 < *dlen);
	20 d++ = encode_map[s >> 2];
	21 x = *s << 4 & 0x3f;
	22 break;
	23 case 1: /* aaaaaabb BBBBCCCC ccdddddd */
	24 assert((size_t)(d - dbeg) + 1 < *dlen);
	25 d++ = encode_map[x \| (s >> 4)];
	26 x = (*s << 2) & 0x3f;
	27 break;
	28 case 2: /* aaaaaabb bbbbcccc CCDDDDDD */
	29 assert((size_t)(d - dbeg) + 2 < *dlen);
	30 d++ = encode_map[x \| (s >> 6)];
	31 d++ = encode_map[s & 0x3f];
	32 break;
	33 }
	34 s++;
	35 }
	36
	37 /* flush extra content in 'x' */
	38 assert((size_t)(d - dbeg) + 1 < *dlen);
	39 if ((s - sbeg) % 3 != 2)
	40 *d++ = encode_map[x];
	41
	42 /* pad the end with '=' */
	43 while ((d - dbeg) % 4 != 0) {
	44 assert((size_t)(d - dbeg) + 1 < *dlen);
	45 *d++ = '=';
	46 }
	47
	48 *dlen = d - dbeg;
	49 }
	50
	51 static int8_t decode_map[256] = {
	52 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	53 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	54 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
	55 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, 0, -1, -1,
	56 -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
	57 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
	58 -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
	59 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,
	60 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	61 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	62 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	63 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	64 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	65 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	66 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	67 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	68 };
	69
	70 int
	71 base64_decode(char const s, size_t slen, char d, size_t dlen)
	72 {
	73 char const sbeg = s, send = sbeg + slen, dbeg = d;
	74
	75 for (; s + 3 < send; s += 4) {
	76 int8_t x0 = decode_map[(unsigned)s[0]];
	77 int8_t x1 = decode_map[(unsigned)s[1]];
	78 int8_t x2 = decode_map[(unsigned)s[2]];
	79 int8_t x3 = decode_map[(unsigned)s[3]];
	80 uint32_t x = (x0 << 18) \| (x1 << 12) \| (x2 << 6) \| (x3 <…
	81
	82 assert((size_t)(d - dbeg) + 3 < *dlen);
	83 *d++ = x >> 16;
	84 *d++ = x >> 8 & 0xff;
	85 *d++ = x & 0xff;
	86
	87 /* only "xxxx" or "xxx=" or "xx==" allowed */
	88 if (s[0] == '=' \|\| s[1] == '=' \|\| (s[2] == '=' && s[3] !…
	89 return -2;
	90 if (s[2] == '=')
	91 d--;
	92 if (s[3] == '=') {
	93 d--;
	94 break;
	95 }
	96
	97 if (x0 < 0 \|\| x1 < 0 \|\| x2 < 0 \|\| x3 < 0)
	98 return -1;
	99 }
	100
	101 *slen = s - sbeg;
	102 *dlen = d - dbeg;
	103 return 0;
	104 }