1/*	$NetBSD: bn_s_mp_mul_digs.c,v 1.2 2017/01/28 21:31:47 christos Exp $	*/
2
3#include <tommath.h>
4#ifdef BN_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/* multiplies |a| * |b| and only computes upto digs digits of result
21 * HAC pp. 595, Algorithm 14.12  Modified so you can control how
22 * many digits of output are created.
23 */
24int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
25{
26  mp_int  t;
27  int     res, pa, pb, ix, iy;
28  mp_digit u;
29  mp_word r;
30  mp_digit tmpx, *tmpt, *tmpy;
31
32  /* can we use the fast multiplier? */
33  if (((digs) < MP_WARRAY) &&
34      MIN (a->used, b->used) <
35          (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
36    return fast_s_mp_mul_digs (a, b, c, digs);
37  }
38
39  if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
40    return res;
41  }
42  t.used = digs;
43
44  /* compute the digits of the product directly */
45  pa = a->used;
46  for (ix = 0; ix < pa; ix++) {
47    /* set the carry to zero */
48    u = 0;
49
50    /* limit ourselves to making digs digits of output */
51    pb = MIN (b->used, digs - ix);
52
53    /* setup some aliases */
54    /* copy of the digit from a used within the nested loop */
55    tmpx = a->dp[ix];
56
57    /* an alias for the destination shifted ix places */
58    tmpt = t.dp + ix;
59
60    /* an alias for the digits of b */
61    tmpy = b->dp;
62
63    /* compute the columns of the output and propagate the carry */
64    for (iy = 0; iy < pb; iy++) {
65      /* compute the column as a mp_word */
66      r       = ((mp_word)*tmpt) +
67                ((mp_word)tmpx) * ((mp_word)*tmpy++) +
68                ((mp_word) u);
69
70      /* the new column is the lower part of the result */
71      *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
72
73      /* get the carry word from the result */
74      u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
75    }
76    /* set carry if it is placed below digs */
77    if (ix + iy < digs) {
78      *tmpt = u;
79    }
80  }
81
82  mp_clamp (&t);
83  mp_exch (&t, c);
84
85  mp_clear (&t);
86  return MP_OKAY;
87}
88#endif
89
90/* Source: /cvs/libtom/libtommath/bn_s_mp_mul_digs.c,v  */
91/* Revision: 1.4  */
92/* Date: 2006/12/28 01:25:13  */
93