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#include "tommath_private.h"
#ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* this is a modified version of fast_s_mul_digs that only produces
* output digits *above* digs. See the comments for fast_s_mul_digs
* to see how it works.
*
* This is used in the Barrett reduction since for one of the multiplications
* only the higher digits were needed. This essentially halves the work.
*
* Based on Algorithm 14.12 on pp.595 of HAC.
*/
mp_err s_mp_mul_high_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
{
int olduse, pa, ix, iz;
mp_err err;
mp_digit W[MP_WARRAY];
mp_word _W;
/* grow the destination as required */
pa = a->used + b->used;
if (c->alloc < pa) {
if ((err = mp_grow(c, pa)) != MP_OKAY) {
return err;
}
}
/* number of output digits to produce */
pa = a->used + b->used;
_W = 0;
for (ix = digs; ix < pa; ix++) {
int tx, ty, iy;
mp_digit *tmpx, *tmpy;
/* get offsets into the two bignums */
ty = MP_MIN(b->used-1, ix);
tx = ix - ty;
/* setup temp aliases */
tmpx = a->dp + tx;
tmpy = b->dp + ty;
/* this is the number of times the loop will iterrate, essentially its
while (tx++ < a->used && ty-- >= 0) { ... }
*/
iy = MP_MIN(a->used-tx, ty+1);
/* execute loop */
for (iz = 0; iz < iy; iz++) {
_W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
}
/* store term */
W[ix] = (mp_digit)_W & MP_MASK;
/* make next carry */
_W = _W >> (mp_word)MP_DIGIT_BIT;
}
/* setup dest */
olduse = c->used;
c->used = pa;
{
mp_digit *tmpc;
tmpc = c->dp + digs;
for (ix = digs; ix < pa; ix++) {
/* now extract the previous digit [below the carry] */
*tmpc++ = W[ix];
}
/* clear unused digits [that existed in the old copy of c] */
MP_ZERO_DIGITS(tmpc, olduse - ix);
}
mp_clamp(c);
return MP_OKAY;
}
#endif
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