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comparison libtommath/bn_s_mp_mul_high_digs_fast.c @ 1692:1051e4eea25a

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Update LibTomMath to 1.2.0 (#84) * update C files * update other files * update headers * update makefiles * remove mp_set/get_double() * use ltm 1.2.0 API * update ltm_desc * use bundled tommath if system-tommath is too old * XMALLOC etc. were changed to MP_MALLOC etc.
author Steffen Jaeckel <s@jaeckel.eu>
date Tue, 26 May 2020 17:36:47 +0200
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1691:2d3745d58843 1692:1051e4eea25a
1 #include "tommath_private.h"
2 #ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
5
6 /* this is a modified version of fast_s_mul_digs that only produces
7 * output digits *above* digs. See the comments for fast_s_mul_digs
8 * to see how it works.
9 *
10 * This is used in the Barrett reduction since for one of the multiplications
11 * only the higher digits were needed. This essentially halves the work.
12 *
13 * Based on Algorithm 14.12 on pp.595 of HAC.
14 */
15 mp_err s_mp_mul_high_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
16 {
17 int olduse, pa, ix, iz;
18 mp_err err;
19 mp_digit W[MP_WARRAY];
20 mp_word _W;
21
22 /* grow the destination as required */
23 pa = a->used + b->used;
24 if (c->alloc < pa) {
25 if ((err = mp_grow(c, pa)) != MP_OKAY) {
26 return err;
27 }
28 }
29
30 /* number of output digits to produce */
31 pa = a->used + b->used;
32 _W = 0;
33 for (ix = digs; ix < pa; ix++) {
34 int tx, ty, iy;
35 mp_digit *tmpx, *tmpy;
36
37 /* get offsets into the two bignums */
38 ty = MP_MIN(b->used-1, ix);
39 tx = ix - ty;
40
41 /* setup temp aliases */
42 tmpx = a->dp + tx;
43 tmpy = b->dp + ty;
44
45 /* this is the number of times the loop will iterrate, essentially its
46 while (tx++ < a->used && ty-- >= 0) { ... }
47 */
48 iy = MP_MIN(a->used-tx, ty+1);
49
50 /* execute loop */
51 for (iz = 0; iz < iy; iz++) {
52 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
53 }
54
55 /* store term */
56 W[ix] = (mp_digit)_W & MP_MASK;
57
58 /* make next carry */
59 _W = _W >> (mp_word)MP_DIGIT_BIT;
60 }
61
62 /* setup dest */
63 olduse = c->used;
64 c->used = pa;
65
66 {
67 mp_digit *tmpc;
68
69 tmpc = c->dp + digs;
70 for (ix = digs; ix < pa; ix++) {
71 /* now extract the previous digit [below the carry] */
72 *tmpc++ = W[ix];
73 }
74
75 /* clear unused digits [that existed in the old copy of c] */
76 MP_ZERO_DIGITS(tmpc, olduse - ix);
77 }
78 mp_clamp(c);
79 return MP_OKAY;
80 }
81 #endif