1/*
2 * Copyright (c) 2017 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 */
28void
29br_i15_decode_reduce(uint16_t *x,
30	const void *src, size_t len, const uint16_t *m)
31{
32	uint32_t m_ebitlen, m_rbitlen;
33	size_t mblen, k;
34	const unsigned char *buf;
35	uint32_t acc;
36	int acc_len;
37
38	/*
39	 * Get the encoded bit length.
40	 */
41	m_ebitlen = m[0];
42
43	/*
44	 * Special case for an invalid (null) modulus.
45	 */
46	if (m_ebitlen == 0) {
47		x[0] = 0;
48		return;
49	}
50
51	/*
52	 * Clear the destination.
53	 */
54	br_i15_zero(x, m_ebitlen);
55
56	/*
57	 * First decode directly as many bytes as possible. This requires
58	 * computing the actual bit length.
59	 */
60	m_rbitlen = m_ebitlen >> 4;
61	m_rbitlen = (m_ebitlen & 15) + (m_rbitlen << 4) - m_rbitlen;
62	mblen = (m_rbitlen + 7) >> 3;
63	k = mblen - 1;
64	if (k >= len) {
65		br_i15_decode(x, src, len);
66		x[0] = m_ebitlen;
67		return;
68	}
69	buf = src;
70	br_i15_decode(x, buf, k);
71	x[0] = m_ebitlen;
72
73	/*
74	 * Input remaining bytes, using 15-bit words.
75	 */
76	acc = 0;
77	acc_len = 0;
78	while (k < len) {
79		uint32_t v;
80
81		v = buf[k ++];
82		acc = (acc << 8) | v;
83		acc_len += 8;
84		if (acc_len >= 15) {
85			br_i15_muladd_small(x, acc >> (acc_len - 15), m);
86			acc_len -= 15;
87			acc &= ~((uint32_t)-1 << acc_len);
88		}
89	}
90
91	/*
92	 * We may have some bits accumulated. We then perform a shift to
93	 * be able to inject these bits as a full 15-bit word.
94	 */
95	if (acc_len != 0) {
96		acc = (acc | (x[1] << acc_len)) & 0x7FFF;
97		br_i15_rshift(x, 15 - acc_len);
98		br_i15_muladd_small(x, acc, m);
99	}
100}
101