1 #include <tommath.h>
2 #ifdef BN_FAST_S_MP_MUL_DIGS_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
16 */
17
18 /* Fast (comba) multiplier
19 *
20 * This is the fast column-array [comba] multiplier. It is
21 * designed to compute the columns of the product first
22 * then handle the carries afterwards. This has the effect
23 * of making the nested loops that compute the columns very
24 * simple and schedulable on super-scalar processors.
25 *
26 * This has been modified to produce a variable number of
27 * digits of output so if say only a half-product is required
28 * you don't have to compute the upper half (a feature
29 * required for fast Barrett reduction).
30 *
31 * Based on Algorithm 14.12 on pp.595 of HAC.
32 *
33 */
fast_s_mp_mul_digs(mp_int * a,mp_int * b,mp_int * c,int digs)34 int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
35 {
36 int olduse, res, pa, ix, iz;
37 mp_digit W[MP_WARRAY];
38 register mp_word _W;
39
40 /* grow the destination as required */
41 if (c->alloc < digs) {
42 if ((res = mp_grow (c, digs)) != MP_OKAY) {
43 return res;
44 }
45 }
46
47 /* number of output digits to produce */
48 pa = MIN(digs, a->used + b->used);
49
50 /* clear the carry */
51 _W = 0;
52 for (ix = 0; ix < pa; ix++) {
53 int tx, ty;
54 int iy;
55 mp_digit *tmpx, *tmpy;
56
57 /* get offsets into the two bignums */
58 ty = MIN(b->used-1, ix);
59 tx = ix - ty;
60
61 /* setup temp aliases */
62 tmpx = a->dp + tx;
63 tmpy = b->dp + ty;
64
65 /* this is the number of times the loop will iterrate, essentially
66 while (tx++ < a->used && ty-- >= 0) { ... }
67 */
68 iy = MIN(a->used-tx, ty+1);
69
70 /* execute loop */
71 for (iz = 0; iz < iy; ++iz) {
72 _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
73
74 }
75
76 /* store term */
77 W[ix] = ((mp_digit)_W) & MP_MASK;
78
79 /* make next carry */
80 _W = _W >> ((mp_word)DIGIT_BIT);
81 }
82
83 /* setup dest */
84 olduse = c->used;
85 c->used = pa;
86
87 {
88 register mp_digit *tmpc;
89 tmpc = c->dp;
90 for (ix = 0; ix < pa+1; ix++) {
91 /* now extract the previous digit [below the carry] */
92 *tmpc++ = W[ix];
93 }
94
95 /* clear unused digits [that existed in the old copy of c] */
96 for (; ix < olduse; ix++) {
97 *tmpc++ = 0;
98 }
99 }
100 mp_clamp (c);
101 return MP_OKAY;
102 }
103 #endif
104
105 /* $Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_digs.c,v $ */
106 /* $Revision: 1.7 $ */
107 /* $Date: 2006/03/31 14:18:44 $ */
108