1/*	$NetBSD$	*/
2
3#include <tommath.h>
4#ifdef BN_FAST_S_MP_MUL_DIGS_C
5/* LibTomMath, multiple-precision integer library -- Tom St Denis
6 *
7 * LibTomMath is a library that provides multiple-precision
8 * integer arithmetic as well as number theoretic functionality.
9 *
10 * The library was designed directly after the MPI library by
11 * Michael Fromberger but has been written from scratch with
12 * additional optimizations in place.
13 *
14 * The library is free for all purposes without any express
15 * guarantee it works.
16 *
17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18 */
19
20/* Fast (comba) multiplier
21 *
22 * This is the fast column-array [comba] multiplier.  It is
23 * designed to compute the columns of the product first
24 * then handle the carries afterwards.  This has the effect
25 * of making the nested loops that compute the columns very
26 * simple and schedulable on super-scalar processors.
27 *
28 * This has been modified to produce a variable number of
29 * digits of output so if say only a half-product is required
30 * you don't have to compute the upper half (a feature
31 * required for fast Barrett reduction).
32 *
33 * Based on Algorithm 14.12 on pp.595 of HAC.
34 *
35 */
36int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
37{
38  int     olduse, res, pa, ix, iz;
39  mp_digit W[MP_WARRAY];
40  register mp_word  _W;
41
42  /* grow the destination as required */
43  if (c->alloc < digs) {
44    if ((res = mp_grow (c, digs)) != MP_OKAY) {
45      return res;
46    }
47  }
48
49  /* number of output digits to produce */
50  pa = MIN(digs, a->used + b->used);
51
52  /* clear the carry */
53  _W = 0;
54  for (ix = 0; ix < pa; ix++) {
55      int      tx, ty;
56      int      iy;
57      mp_digit *tmpx, *tmpy;
58
59      /* get offsets into the two bignums */
60      ty = MIN(b->used-1, ix);
61      tx = ix - ty;
62
63      /* setup temp aliases */
64      tmpx = a->dp + tx;
65      tmpy = b->dp + ty;
66
67      /* this is the number of times the loop will iterrate, essentially
68         while (tx++ < a->used && ty-- >= 0) { ... }
69       */
70      iy = MIN(a->used-tx, ty+1);
71
72      /* execute loop */
73      for (iz = 0; iz < iy; ++iz) {
74         _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
75
76      }
77
78      /* store term */
79      W[ix] = ((mp_digit)_W) & MP_MASK;
80
81      /* make next carry */
82      _W = _W >> ((mp_word)DIGIT_BIT);
83 }
84
85  /* setup dest */
86  olduse  = c->used;
87  c->used = pa;
88
89  {
90    register mp_digit *tmpc;
91    tmpc = c->dp;
92    for (ix = 0; ix < pa+1; ix++) {
93      /* now extract the previous digit [below the carry] */
94      *tmpc++ = W[ix];
95    }
96
97    /* clear unused digits [that existed in the old copy of c] */
98    for (; ix < olduse; ix++) {
99      *tmpc++ = 0;
100    }
101  }
102  mp_clamp (c);
103  return MP_OKAY;
104}
105#endif
106
107/* Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_digs.c,v */
108/* Revision: 1.8 */
109/* Date: 2006/12/28 01:25:13 */
110