### view libtommath/bn_mp_mul_2.c @ 284:eed26cff980b

author Matt Johnston Wed, 08 Mar 2006 13:23:49 +0000 5ff8218bcee9
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```#include <tommath.h>
#ifdef BN_MP_MUL_2_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.org
*/

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

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

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

{
register 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)(DIGIT_BIT - 1));

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

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

if (r != 0) {
/* 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
*/
tmpb = b->dp + b->used;
for (x = b->used; x < oldused; x++) {
*tmpb++ = 0;
}
}
b->sign = a->sign;
return MP_OKAY;
}
#endif
```