96 lines
2.4 KiB
C
96 lines
2.4 KiB
C
#include "tommath_private.h"
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#ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* SPDX-License-Identifier: Unlicense
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*/
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/* this is a modified version of fast_s_mul_digs that only produces
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* output digits *above* digs. See the comments for fast_s_mul_digs
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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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int fast_s_mp_mul_high_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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{
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int olduse, res, pa, ix, iz;
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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 (c->alloc < pa) {
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if ((res = mp_grow(c, pa)) != MP_OKAY) {
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return res;
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}
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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;
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mp_digit *tmpx, *tmpy;
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/* get offsets into the two bignums */
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ty = MIN(b->used-1, ix);
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tx = ix - ty;
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/* setup temp aliases */
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tmpx = a->dp + tx;
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tmpy = b->dp + 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 = 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)*tmpx++ * (mp_word)*tmpy--;
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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)DIGIT_BIT;
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}
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/* setup dest */
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olduse = c->used;
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c->used = pa;
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{
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mp_digit *tmpc;
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tmpc = c->dp + digs;
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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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*tmpc++ = 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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for (; ix < olduse; ix++) {
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*tmpc++ = 0;
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}
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}
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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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/* ref: $Format:%D$ */
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/* git commit: $Format:%H$ */
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/* commit time: $Format:%ai$ */
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