Added generic API for date range validation (with callbacks).
[BearSSL] / int / i31_decmod.c
1 /*
2 * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22 * SOFTWARE.
23 */
24
25 #include "inner.h"
26
27 /* see inner.h */
28 uint32_t
29 br_i31_decode_mod(uint32_t *x, const void *src, size_t len, const uint32_t *m)
30 {
31 /*
32 * Two-pass algorithm: in the first pass, we determine whether the
33 * value fits; in the second pass, we do the actual write.
34 *
35 * During the first pass, 'r' contains the comparison result so
36 * far:
37 * 0x00000000 value is equal to the modulus
38 * 0x00000001 value is greater than the modulus
39 * 0xFFFFFFFF value is lower than the modulus
40 *
41 * Since we iterate starting with the least significant bytes (at
42 * the end of src[]), each new comparison overrides the previous
43 * except when the comparison yields 0 (equal).
44 *
45 * During the second pass, 'r' is either 0xFFFFFFFF (value fits)
46 * or 0x00000000 (value does not fit).
47 *
48 * We must iterate over all bytes of the source, _and_ possibly
49 * some extra virutal bytes (with value 0) so as to cover the
50 * complete modulus as well. We also add 4 such extra bytes beyond
51 * the modulus length because it then guarantees that no accumulated
52 * partial word remains to be processed.
53 */
54 const unsigned char *buf;
55 size_t mlen, tlen;
56 int pass;
57 uint32_t r;
58
59 buf = src;
60 mlen = (m[0] + 31) >> 5;
61 tlen = (mlen << 2);
62 if (tlen < len) {
63 tlen = len;
64 }
65 tlen += 4;
66 r = 0;
67 for (pass = 0; pass < 2; pass ++) {
68 size_t u, v;
69 uint32_t acc;
70 int acc_len;
71
72 v = 1;
73 acc = 0;
74 acc_len = 0;
75 for (u = 0; u < tlen; u ++) {
76 uint32_t b;
77
78 if (u < len) {
79 b = buf[len - 1 - u];
80 } else {
81 b = 0;
82 }
83 acc |= (b << acc_len);
84 acc_len += 8;
85 if (acc_len >= 31) {
86 uint32_t xw;
87
88 xw = acc & (uint32_t)0x7FFFFFFF;
89 acc_len -= 31;
90 acc = b >> (8 - acc_len);
91 if (v <= mlen) {
92 if (pass) {
93 x[v] = r & xw;
94 } else {
95 uint32_t cc;
96
97 cc = (uint32_t)CMP(xw, m[v]);
98 r = MUX(EQ(cc, 0), r, cc);
99 }
100 } else {
101 if (!pass) {
102 r = MUX(EQ(xw, 0), r, 1);
103 }
104 }
105 v ++;
106 }
107 }
108
109 /*
110 * When we reach this point at the end of the first pass:
111 * r is either 0, 1 or -1; we want to set r to 0 if it
112 * is equal to 0 or 1, and leave it to -1 otherwise.
113 *
114 * When we reach this point at the end of the second pass:
115 * r is either 0 or -1; we want to leave that value
116 * untouched. This is a subcase of the previous.
117 */
118 r >>= 1;
119 r |= (r << 1);
120 }
121
122 x[0] = m[0];
123 return r & (uint32_t)1;
124 }