s1-mod/deps/libtommath/mp_mul_2.c
2024-02-27 03:09:30 -05:00

54 lines
1.2 KiB
C

#include "tommath_private.h"
#ifdef MP_MUL_2_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* b = a*2 */
mp_err mp_mul_2(const mp_int *a, mp_int *b)
{
mp_err err;
int x, oldused;
mp_digit r;
/* grow to accommodate result */
if ((err = mp_grow(b, a->used + 1)) != MP_OKAY) {
return err;
}
oldused = b->used;
b->used = a->used;
/* carry */
r = 0;
for (x = 0; x < a->used; x++) {
/* get what will be the *next* carry bit from the
* MSB of the current digit
*/
mp_digit rr = a->dp[x] >> (mp_digit)(MP_DIGIT_BIT - 1);
/* now shift up this digit, add in the carry [from the previous] */
b->dp[x] = ((a->dp[x] << 1uL) | r) & MP_MASK;
/* copy the carry that would be from the source
* digit into the next iteration
*/
r = rr;
}
/* new leading digit? */
if (r != 0u) {
/* add a MSB which is always 1 at this point */
b->dp[b->used++] = 1;
}
/* now zero any excess digits on the destination
* that we didn't write to
*/
s_mp_zero_digs(b->dp + b->used, oldused - b->used);
b->sign = a->sign;
return MP_OKAY;
}
#endif