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comparison bn_s_mp_mul_high_digs.c @ 1:22d5cf7d4b1a libtommath

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Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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-1:000000000000 1:22d5cf7d4b1a
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is a library that provides multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library was designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
14 */
15 #include <tommath.h>
16
17 /* multiplies |a| * |b| and does not compute the lower digs digits
18 * [meant to get the higher part of the product]
19 */
20 int
21 s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
22 {
23 mp_int t;
24 int res, pa, pb, ix, iy;
25 mp_digit u;
26 mp_word r;
27 mp_digit tmpx, *tmpt, *tmpy;
28
29 /* can we use the fast multiplier? */
30 if (((a->used + b->used + 1) < MP_WARRAY)
31 && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
32 return fast_s_mp_mul_high_digs (a, b, c, digs);
33 }
34
35 if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
36 return res;
37 }
38 t.used = a->used + b->used + 1;
39
40 pa = a->used;
41 pb = b->used;
42 for (ix = 0; ix < pa; ix++) {
43 /* clear the carry */
44 u = 0;
45
46 /* left hand side of A[ix] * B[iy] */
47 tmpx = a->dp[ix];
48
49 /* alias to the address of where the digits will be stored */
50 tmpt = &(t.dp[digs]);
51
52 /* alias for where to read the right hand side from */
53 tmpy = b->dp + (digs - ix);
54
55 for (iy = digs - ix; iy < pb; iy++) {
56 /* calculate the double precision result */
57 r = ((mp_word)*tmpt) +
58 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
59 ((mp_word) u);
60
61 /* get the lower part */
62 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
63
64 /* carry the carry */
65 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
66 }
67 *tmpt = u;
68 }
69 mp_clamp (&t);
70 mp_exch (&t, c);
71 mp_clear (&t);
72 return MP_OKAY;
73 }