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://libtom.org
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 */
34int 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: 1.8 $ */
107/* $Date: 2006/12/28 01:25:13 $ */
108