#include "tommath_private.h"
#ifdef BN_S_MP_MUL_DIGS_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* multiplies |a| * |b| and only computes upto digs digits of result
* HAC pp. 595, Algorithm 14.12 Modified so you can control how
* many digits of output are created.
*/
mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
{
mp_int t;
mp_err err;
int pa, pb, ix, iy;
mp_digit u;
mp_word r;
mp_digit tmpx, *tmpt, *tmpy;
/* can we use the fast multiplier? */
if ((digs < MP_WARRAY) &&
(MP_MIN(a->used, b->used) < MP_MAXFAST)) {
return s_mp_mul_digs_fast(a, b, c, digs);
}
if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
return err;
}
t.used = digs;
/* compute the digits of the product directly */
pa = a->used;
for (ix = 0; ix < pa; ix++) {
/* set the carry to zero */
u = 0;
/* limit ourselves to making digs digits of output */
pb = MP_MIN(b->used, digs - ix);
/* setup some aliases */
/* copy of the digit from a used within the nested loop */
tmpx = a->dp[ix];
/* an alias for the destination shifted ix places */
tmpt = t.dp + ix;
/* an alias for the digits of b */
tmpy = b->dp;
/* compute the columns of the output and propagate the carry */
for (iy = 0; iy < pb; iy++) {
/* compute the column as a mp_word */
r = (mp_word)*tmpt +
((mp_word)tmpx * (mp_word)*tmpy++) +
(mp_word)u;
/* the new column is the lower part of the result */
*tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
/* get the carry word from the result */
u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
}
/* set carry if it is placed below digs */
if ((ix + iy) < digs) {
*tmpt = u;
}
}
mp_clamp(&t);
mp_exch(&t, c);
mp_clear(&t);
return MP_OKAY;
}
#endif
