66 lines
1.7 KiB
C
66 lines
1.7 KiB
C
#include "tommath_private.h"
|
|
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
|
|
/* SPDX-License-Identifier: Unlicense */
|
|
|
|
/* multiplies |a| * |b| and does not compute the lower digs digits
|
|
* [meant to get the higher part of the product]
|
|
*/
|
|
mp_err s_mp_mul_high_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
|
|
{
|
|
mp_int t;
|
|
int pa, pb, ix, iy;
|
|
mp_err err;
|
|
mp_digit u;
|
|
mp_word r;
|
|
mp_digit tmpx, *tmpt, *tmpy;
|
|
|
|
/* can we use the fast multiplier? */
|
|
#ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_C
|
|
if (((a->used + b->used + 1) < MP_WARRAY)
|
|
&& (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
|
|
return s_mp_mul_high_digs_fast(a, b, c, digs);
|
|
}
|
|
#endif
|
|
|
|
if ((err = mp_init_size(&t, a->used + b->used + 1)) != MP_OKAY) {
|
|
return err;
|
|
}
|
|
t.used = a->used + b->used + 1;
|
|
|
|
pa = a->used;
|
|
pb = b->used;
|
|
for (ix = 0; ix < pa; ix++) {
|
|
/* clear the carry */
|
|
u = 0;
|
|
|
|
/* left hand side of A[ix] * B[iy] */
|
|
tmpx = a->dp[ix];
|
|
|
|
/* alias to the address of where the digits will be stored */
|
|
tmpt = &(t.dp[digs]);
|
|
|
|
/* alias for where to read the right hand side from */
|
|
tmpy = b->dp + (digs - ix);
|
|
|
|
for (iy = digs - ix; iy < pb; iy++) {
|
|
/* calculate the double precision result */
|
|
r = (mp_word)*tmpt +
|
|
((mp_word)tmpx * (mp_word)*tmpy++) +
|
|
(mp_word)u;
|
|
|
|
/* get the lower part */
|
|
*tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
|
|
|
|
/* carry the carry */
|
|
u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
|
|
}
|
|
*tmpt = u;
|
|
}
|
|
mp_clamp(&t);
|
|
mp_exch(&t, c);
|
|
mp_clear(&t);
|
|
return MP_OKAY;
|
|
}
|
|
#endif
|