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