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65 lines
1.7 KiB
65 lines
1.7 KiB
#include "tommath_private.h"
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#ifdef BN_S_MP_MUL_HIGH_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 does not compute the lower digs digits
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* [meant to get the higher part of the product]
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*/
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mp_err 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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mp_int t;
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int pa, pb, ix, iy;
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mp_err err;
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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 (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)
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&& ((a->used + b->used + 1) < MP_WARRAY)
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&& (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
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return s_mp_mul_high_digs_fast(a, b, c, digs);
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}
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if ((err = mp_init_size(&t, a->used + b->used + 1)) != MP_OKAY) {
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return err;
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}
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t.used = a->used + b->used + 1;
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pa = a->used;
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pb = b->used;
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for (ix = 0; ix < pa; ix++) {
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/* clear the carry */
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u = 0;
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/* left hand side of A[ix] * B[iy] */
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tmpx = a->dp[ix];
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/* alias to the address of where the digits will be stored */
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tmpt = &(t.dp[digs]);
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/* alias for where to read the right hand side from */
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tmpy = b->dp + (digs - ix);
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for (iy = digs - ix; iy < pb; iy++) {
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/* calculate the double precision result */
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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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/* get the lower part */
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*tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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/* carry the carry */
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u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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}
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*tmpt = u;
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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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