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comparison libtommath/bn_s_mp_mul_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_DIGS_FAST_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
5
6 /* Fast (comba) multiplier
7 *
8 * This is the fast column-array [comba] multiplier. It is
9 * designed to compute the columns of the product first
10 * then handle the carries afterwards. This has the effect
11 * of making the nested loops that compute the columns very
12 * simple and schedulable on super-scalar processors.
13 *
14 * This has been modified to produce a variable number of
15 * digits of output so if say only a half-product is required
16 * you don't have to compute the upper half (a feature
17 * required for fast Barrett reduction).
18 *
19 * Based on Algorithm 14.12 on pp.595 of HAC.
20 *
21 */
22 mp_err s_mp_mul_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
23 {
24 int olduse, pa, ix, iz;
25 mp_err err;
26 mp_digit W[MP_WARRAY];
27 mp_word _W;
28
29 /* grow the destination as required */
30 if (c->alloc < digs) {
31 if ((err = mp_grow(c, digs)) != MP_OKAY) {
32 return err;
33 }
34 }
35
36 /* number of output digits to produce */
37 pa = MP_MIN(digs, a->used + b->used);
38
39 /* clear the carry */
40 _W = 0;
41 for (ix = 0; ix < pa; ix++) {
42 int tx, ty;
43 int iy;
44 mp_digit *tmpx, *tmpy;
45
46 /* get offsets into the two bignums */
47 ty = MP_MIN(b->used-1, ix);
48 tx = ix - ty;
49
50 /* setup temp aliases */
51 tmpx = a->dp + tx;
52 tmpy = b->dp + ty;
53
54 /* this is the number of times the loop will iterrate, essentially
55 while (tx++ < a->used && ty-- >= 0) { ... }
56 */
57 iy = MP_MIN(a->used-tx, ty+1);
58
59 /* execute loop */
60 for (iz = 0; iz < iy; ++iz) {
61 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
62
63 }
64
65 /* store term */
66 W[ix] = (mp_digit)_W & MP_MASK;
67
68 /* make next carry */
69 _W = _W >> (mp_word)MP_DIGIT_BIT;
70 }
71
72 /* setup dest */
73 olduse = c->used;
74 c->used = pa;
75
76 {
77 mp_digit *tmpc;
78 tmpc = c->dp;
79 for (ix = 0; ix < pa; ix++) {
80 /* now extract the previous digit [below the carry] */
81 *tmpc++ = W[ix];
82 }
83
84 /* clear unused digits [that existed in the old copy of c] */
85 MP_ZERO_DIGITS(tmpc, olduse - ix);
86 }
87 mp_clamp(c);
88 return MP_OKAY;
89 }
90 #endif