2 #ifdef BN_S_MP_MUL_HIGH_DIGS_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
12 * The library is free for all purposes without any express
15 * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
18 /* multiplies |a| * |b| and does not compute the lower digs digits
19 * [meant to get the higher part of the product]
22 s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
25 int res, pa, pb, ix, iy;
28 mp_digit tmpx, *tmpt, *tmpy;
30 /* can we use the fast multiplier? */
31 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
32 if (((a->used + b->used + 1) < MP_WARRAY)
33 && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
34 return fast_s_mp_mul_high_digs (a, b, c, digs);
38 if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
41 t.used = a->used + b->used + 1;
45 for (ix = 0; ix < pa; ix++) {
49 /* left hand side of A[ix] * B[iy] */
52 /* alias to the address of where the digits will be stored */
55 /* alias for where to read the right hand side from */
56 tmpy = b->dp + (digs - ix);
58 for (iy = digs - ix; iy < pb; iy++) {
59 /* calculate the double precision result */
60 r = ((mp_word)*tmpt) +
61 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
64 /* get the lower part */
65 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
68 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));