75 lines
2.0 KiB
C
75 lines
2.0 KiB
C
#include "tommath_private.h"
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#ifdef BN_S_MP_MUL_DIGS_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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/* multiplies |a| * |b| and only computes upto digs digits of result
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* HAC pp. 595, Algorithm 14.12 Modified so you can control how
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* many digits of output are created.
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*/
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int s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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{
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mp_int t;
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int res, pa, pb, ix, iy;
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mp_digit u;
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mp_word r;
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mp_digit tmpx, *tmpt, *tmpy;
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/* can we use the fast multiplier? */
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if ((digs < (int)MP_WARRAY) &&
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(MP_MIN(a->used, b->used) <
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(int)(1u << ((CHAR_BIT * sizeof(mp_word)) - (2u * (size_t)DIGIT_BIT))))) {
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return fast_s_mp_mul_digs(a, b, c, digs);
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}
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if ((res = mp_init_size(&t, digs)) != MP_OKAY) {
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return res;
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}
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t.used = digs;
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/* compute the digits of the product directly */
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pa = a->used;
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for (ix = 0; ix < pa; ix++) {
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/* set the carry to zero */
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u = 0;
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/* limit ourselves to making digs digits of output */
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pb = MP_MIN(b->used, digs - ix);
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/* setup some aliases */
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/* copy of the digit from a used within the nested loop */
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tmpx = a->dp[ix];
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/* an alias for the destination shifted ix places */
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tmpt = t.dp + ix;
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/* an alias for the digits of b */
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tmpy = b->dp;
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/* compute the columns of the output and propagate the carry */
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for (iy = 0; iy < pb; iy++) {
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/* compute the column as a mp_word */
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r = (mp_word)*tmpt +
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((mp_word)tmpx * (mp_word)*tmpy++) +
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(mp_word)u;
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/* the new column is the lower part of the result */
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*tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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/* get the carry word from the result */
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u = (mp_digit)(r >> (mp_word)DIGIT_BIT);
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}
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/* set carry if it is placed below digs */
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if ((ix + iy) < digs) {
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*tmpt = u;
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}
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}
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mp_clamp(&t);
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mp_exch(&t, c);
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mp_clear(&t);
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return MP_OKAY;
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}
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#endif
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