54 lines
1.2 KiB
C
54 lines
1.2 KiB
C
#include "tommath_private.h"
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#ifdef MP_MUL_2_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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/* b = a*2 */
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mp_err mp_mul_2(const mp_int *a, mp_int *b)
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{
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mp_err err;
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int x, oldused;
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mp_digit r;
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/* grow to accommodate result */
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if ((err = mp_grow(b, a->used + 1)) != MP_OKAY) {
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return err;
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}
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oldused = b->used;
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b->used = a->used;
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/* carry */
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r = 0;
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for (x = 0; x < a->used; x++) {
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/* get what will be the *next* carry bit from the
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* MSB of the current digit
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*/
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mp_digit rr = a->dp[x] >> (mp_digit)(MP_DIGIT_BIT - 1);
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/* now shift up this digit, add in the carry [from the previous] */
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b->dp[x] = ((a->dp[x] << 1uL) | r) & MP_MASK;
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/* copy the carry that would be from the source
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* digit into the next iteration
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*/
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r = rr;
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}
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/* new leading digit? */
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if (r != 0u) {
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/* add a MSB which is always 1 at this point */
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b->dp[b->used++] = 1;
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}
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/* now zero any excess digits on the destination
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* that we didn't write to
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*/
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s_mp_zero_digs(b->dp + b->used, oldused - b->used);
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b->sign = a->sign;
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return MP_OKAY;
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}
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#endif
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