### view libtommath/bn_mp_mul_2.c @ 1853:2be3115a8762defaulttipmainmaster

debugging test runner authorized_keys perms
author Matt Johnston Tue, 19 Oct 2021 13:45:59 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_MP_MUL_2_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* b = a*2 */
mp_err mp_mul_2(const mp_int *a, mp_int *b)
{
int     x, oldused;
mp_err err;

/* grow to accomodate result */
if (b->alloc < (a->used + 1)) {
if ((err = mp_grow(b, a->used + 1)) != MP_OKAY) {
return err;
}
}

oldused = b->used;
b->used = a->used;

{
mp_digit r, rr, *tmpa, *tmpb;

/* alias for source */
tmpa = a->dp;

/* alias for dest */
tmpb = b->dp;

/* 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
*/
rr = *tmpa >> (mp_digit)(MP_DIGIT_BIT - 1);

/* now shift up this digit, add in the carry [from the previous] */
*tmpb++ = ((*tmpa++ << 1uL) | r) & MP_MASK;

/* copy the carry that would be from the source
* digit into the next iteration
*/
r = rr;
}

if (r != 0u) {
/* add a MSB which is always 1 at this point */
*tmpb = 1;
++(b->used);
}

/* now zero any excess digits on the destination
* that we didn't write to
*/
MP_ZERO_DIGITS(b->dp + b->used, oldused - b->used);
}
b->sign = a->sign;
return MP_OKAY;
}
#endif
```