66 lines
1.7 KiB
C
66 lines
1.7 KiB
C
#include "tommath_private.h"
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#ifdef S_MP_MUL_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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mp_err s_mp_mul(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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mp_err err;
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int pa, ix;
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if (digs < 0) {
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return MP_VAL;
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}
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/* can we use the fast multiplier? */
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if ((digs < MP_WARRAY) &&
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(MP_MIN(a->used, b->used) < MP_MAX_COMBA)) {
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return s_mp_mul_comba(a, b, c, digs);
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}
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if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
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return err;
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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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int iy, pb;
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mp_digit 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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/* 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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mp_word r = (mp_word)t.dp[ix + iy] +
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((mp_word)a->dp[ix] * (mp_word)b->dp[iy]) +
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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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t.dp[ix + iy] = (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)MP_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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t.dp[ix + pb] = 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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