2 #ifdef BN_FAST_S_MP_MUL_DIGS_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
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.
12 * The library is free for all purposes without any express
15 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
18 /* Fast (comba) multiplier
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.
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).
31 * Based on Algorithm 14.12 on pp.595 of HAC.
34 int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
36 int olduse, res, pa, ix, iz;
37 mp_digit W[MP_WARRAY];
40 /* grow the destination as required */
41 if (c->alloc < digs) {
42 if ((res = mp_grow (c, digs)) != MP_OKAY) {
47 /* number of output digits to produce */
48 pa = MIN(digs, a->used + b->used);
52 for (ix = 0; ix < pa; ix++) {
55 mp_digit *tmpx, *tmpy;
57 /* get offsets into the two bignums */
58 ty = MIN(b->used-1, ix);
61 /* setup temp aliases */
65 /* this is the number of times the loop will iterrate, essentially
66 while (tx++ < a->used && ty-- >= 0) { ... }
68 iy = MIN(a->used-tx, ty+1);
71 for (iz = 0; iz < iy; ++iz) {
72 _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
77 W[ix] = ((mp_digit)_W) & MP_MASK;
80 _W = _W >> ((mp_word)DIGIT_BIT);
88 register mp_digit *tmpc;
90 for (ix = 0; ix < pa+1; ix++) {
91 /* now extract the previous digit [below the carry] */
95 /* clear unused digits [that existed in the old copy of c] */
96 for (; ix < olduse; ix++) {
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 $ */