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  void
29  br_i31_reduce(uint32_t *x, const uint32_t *a, const uint32_t *m)
30  {
31  	uint32_t m_bitlen, a_bitlen;
32  	size_t mlen, alen, u;
33  
34  	m_bitlen = m[0];
35  	mlen = (m_bitlen + 31) >> 5;
36  
37  	x[0] = m_bitlen;
38  	if (m_bitlen == 0) {
39  		return;
40  	}
41  
42  	/*
43  	 * If the source is shorter, then simply copy all words from a[]
44  	 * and zero out the upper words.
45  	 */
46  	a_bitlen = a[0];
47  	alen = (a_bitlen + 31) >> 5;
48  	if (a_bitlen < m_bitlen) {
49  		memcpy(x + 1, a + 1, alen * sizeof *a);
50  		for (u = alen; u < mlen; u ++) {
51  			x[u + 1] = 0;
52  		}
53  		return;
54  	}
55  
56  	/*
57  	 * The source length is at least equal to that of the modulus.
58  	 * We must thus copy N-1 words, and input the remaining words
59  	 * one by one.
60  	 */
61  	memcpy(x + 1, a + 2 + (alen - mlen), (mlen - 1) * sizeof *a);
62  	x[mlen] = 0;
63  	for (u = 1 + alen - mlen; u > 0; u --) {
64  		br_i31_muladd_small(x, a[u], m);
65  	}
66  }