1/*	$NetBSD$	*/
2
3#include <tommath.h>
4#ifdef BN_FAST_S_MP_MUL_HIGH_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/* this is a modified version of fast_s_mul_digs that only produces
21 * output digits *above* digs.  See the comments for fast_s_mul_digs
22 * to see how it works.
23 *
24 * This is used in the Barrett reduction since for one of the multiplications
25 * only the higher digits were needed.  This essentially halves the work.
26 *
27 * Based on Algorithm 14.12 on pp.595 of HAC.
28 */
29int fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
30{
31  int     olduse, res, pa, ix, iz;
32  mp_digit W[MP_WARRAY];
33  mp_word  _W;
34
35  /* grow the destination as required */
36  pa = a->used + b->used;
37  if (c->alloc < pa) {
38    if ((res = mp_grow (c, pa)) != MP_OKAY) {
39      return res;
40    }
41  }
42
43  /* number of output digits to produce */
44  pa = a->used + b->used;
45  _W = 0;
46  for (ix = digs; ix < pa; ix++) {
47      int      tx, ty, iy;
48      mp_digit *tmpx, *tmpy;
49
50      /* get offsets into the two bignums */
51      ty = MIN(b->used-1, ix);
52      tx = ix - ty;
53
54      /* setup temp aliases */
55      tmpx = a->dp + tx;
56      tmpy = b->dp + ty;
57
58      /* this is the number of times the loop will iterrate, essentially its
59         while (tx++ < a->used && ty-- >= 0) { ... }
60       */
61      iy = MIN(a->used-tx, ty+1);
62
63      /* execute loop */
64      for (iz = 0; iz < iy; iz++) {
65         _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
66      }
67
68      /* store term */
69      W[ix] = ((mp_digit)_W) & MP_MASK;
70
71      /* make next carry */
72      _W = _W >> ((mp_word)DIGIT_BIT);
73  }
74
75  /* setup dest */
76  olduse  = c->used;
77  c->used = pa;
78
79  {
80    register mp_digit *tmpc;
81
82    tmpc = c->dp + digs;
83    for (ix = digs; ix < pa; ix++) {
84      /* now extract the previous digit [below the carry] */
85      *tmpc++ = W[ix];
86    }
87
88    /* clear unused digits [that existed in the old copy of c] */
89    for (; ix < olduse; ix++) {
90      *tmpc++ = 0;
91    }
92  }
93  mp_clamp (c);
94  return MP_OKAY;
95}
96#endif
97
98/* Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_high_digs.c,v */
99/* Revision: 1.6 */
100/* Date: 2006/12/28 01:25:13 */
101