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_from_monty(uint32_t *x, const uint32_t *m, uint32_t m0i)
30  {
31  	size_t len, u, v;
32  
33  	len = (m[0] + 31) >> 5;
34  	for (u = 0; u < len; u ++) {
35  		uint32_t f;
36  		uint64_t cc;
37  
38  		f = MUL31_lo(x[1], m0i);
39  		cc = 0;
40  		for (v = 0; v < len; v ++) {
41  			uint64_t z;
42  
43  			z = (uint64_t)x[v + 1] + MUL31(f, m[v + 1]) + cc;
44  			cc = z >> 31;
45  			if (v != 0) {
46  				x[v] = (uint32_t)z & 0x7FFFFFFF;
47  			}
48  		}
49  		x[len] = (uint32_t)cc;
50  	}
51  
52  	/*
53  	 * We may have to do an extra subtraction, but only if the
54  	 * value in x[] is indeed greater than or equal to that of m[],
55  	 * which is why we must do two calls (first call computes the
56  	 * carry, second call performs the subtraction only if the carry
57  	 * is 0).
58  	 */
59  	br_i31_sub(x, m, NOT(br_i31_sub(x, m, 0)));
60  }