1 /* $NetBSD: bn_mp_mod_2d.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $ */
2
3 #include <tommath.h>
4 #ifdef BN_MP_MOD_2D_C
5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
6 *
7 * LibTomMath is a library that provides multiple-precision
8 * integer arithmetic as well as number theoretic functionality.
9 *
10 * The library was designed directly after the MPI library by
11 * Michael Fromberger but has been written from scratch with
12 * additional optimizations in place.
13 *
14 * The library is free for all purposes without any express
15 * guarantee it works.
16 *
17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18 */
19
20 /* calc a value mod 2**b */
21 int
mp_mod_2d(mp_int * a,int b,mp_int * c)22 mp_mod_2d (mp_int * a, int b, mp_int * c)
23 {
24 int x, res;
25
26 /* if b is <= 0 then zero the int */
27 if (b <= 0) {
28 mp_zero (c);
29 return MP_OKAY;
30 }
31
32 /* if the modulus is larger than the value than return */
33 if (b >= (int) (a->used * DIGIT_BIT)) {
34 res = mp_copy (a, c);
35 return res;
36 }
37
38 /* copy */
39 if ((res = mp_copy (a, c)) != MP_OKAY) {
40 return res;
41 }
42
43 /* zero digits above the last digit of the modulus */
44 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
45 c->dp[x] = 0;
46 }
47 /* clear the digit that is not completely outside/inside the modulus */
48 c->dp[b / DIGIT_BIT] &=
49 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
50 mp_clamp (c);
51 return MP_OKAY;
52 }
53 #endif
54
55 /* Source: /cvs/libtom/libtommath/bn_mp_mod_2d.c,v */
56 /* Revision: 1.4 */
57 /* Date: 2006/12/28 01:25:13 */
58