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#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? */
if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)
&& ((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);
}
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
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