1 /*	$NetBSD: bn_s_mp_mul_high_digs.c,v 1.1.1.1 2011/04/13 18:14:55 elric Exp $	*/
2 
3 #include <tommath.h>
4 #ifdef BN_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 /* multiplies |a| * |b| and does not compute the lower digs digits
21  * [meant to get the higher part of the product]
22  */
23 int
24 s_mp_mul_high_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 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
34   if (((a->used + b->used + 1) < MP_WARRAY)
35       && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
36     return fast_s_mp_mul_high_digs (a, b, c, digs);
37   }
38 #endif
39 
40   if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
41     return res;
42   }
43   t.used = a->used + b->used + 1;
44 
45   pa = a->used;
46   pb = b->used;
47   for (ix = 0; ix < pa; ix++) {
48     /* clear the carry */
49     u = 0;
50 
51     /* left hand side of A[ix] * B[iy] */
52     tmpx = a->dp[ix];
53 
54     /* alias to the address of where the digits will be stored */
55     tmpt = &(t.dp[digs]);
56 
57     /* alias for where to read the right hand side from */
58     tmpy = b->dp + (digs - ix);
59 
60     for (iy = digs - ix; iy < pb; iy++) {
61       /* calculate the double precision result */
62       r       = ((mp_word)*tmpt) +
63                 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
64                 ((mp_word) u);
65 
66       /* get the lower part */
67       *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
68 
69       /* carry the carry */
70       u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
71     }
72     *tmpt = u;
73   }
74   mp_clamp (&t);
75   mp_exch (&t, c);
76   mp_clear (&t);
77   return MP_OKAY;
78 }
79 #endif
80 
81 /* Source: /cvs/libtom/libtommath/bn_s_mp_mul_high_digs.c,v */
82 /* Revision: 1.4 */
83 /* Date: 2006/12/28 01:25:13 */
84