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comparison bn_s_mp_mul_digs.c @ 1:22d5cf7d4b1a libtommath

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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 only computes upto digs digits of result
18 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
19 * many digits of output are created.
20 */
21 int
22 s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
23 {
24 mp_int t;
25 int res, pa, pb, ix, iy;
26 mp_digit u;
27 mp_word r;
28 mp_digit tmpx, *tmpt, *tmpy;
29
30 /* can we use the fast multiplier? */
31 if (((digs) < MP_WARRAY) &&
32 MIN (a->used, b->used) <
33 (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
34 return fast_s_mp_mul_digs (a, b, c, digs);
35 }
36
37 if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
38 return res;
39 }
40 t.used = digs;
41
42 /* compute the digits of the product directly */
43 pa = a->used;
44 for (ix = 0; ix < pa; ix++) {
45 /* set the carry to zero */
46 u = 0;
47
48 /* limit ourselves to making digs digits of output */
49 pb = MIN (b->used, digs - ix);
50
51 /* setup some aliases */
52 /* copy of the digit from a used within the nested loop */
53 tmpx = a->dp[ix];
54
55 /* an alias for the destination shifted ix places */
56 tmpt = t.dp + ix;
57
58 /* an alias for the digits of b */
59 tmpy = b->dp;
60
61 /* compute the columns of the output and propagate the carry */
62 for (iy = 0; iy < pb; iy++) {
63 /* compute the column as a mp_word */
64 r = ((mp_word)*tmpt) +
65 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
66 ((mp_word) u);
67
68 /* the new column is the lower part of the result */
69 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
70
71 /* get the carry word from the result */
72 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
73 }
74 /* set carry if it is placed below digs */
75 if (ix + iy < digs) {
76 *tmpt = u;
77 }
78 }
79
80 mp_clamp (&t);
81 mp_exch (&t, c);
82
83 mp_clear (&t);
84 return MP_OKAY;
85 }