tommath/bn_mp_mul_2.c

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#include <tommath.h>
#ifdef BN_MP_MUL_2_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
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* Tom St Denis, tomstdenis@gmail.com, http://libtom.org
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*/
/* b = a*2 */
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int mp_mul_2(mp_int * a, mp_int * b)
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{
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int x, res, oldused;
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/* grow to accomodate result */
if (b->alloc < a->used + 1) {
if ((res = mp_grow (b, a->used + 1)) != MP_OKAY) {
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return res;
}
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}
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oldused = b->used;
b->used = a->used;
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{
register mp_digit r, rr, *tmpa, *tmpb;
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/* alias for source */
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tmpa = a->dp;
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/* alias for dest */
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tmpb = b->dp;
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/* carry */
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
* MSB of the current digit
*/
rr = *tmpa >> ((mp_digit)(DIGIT_BIT - 1));
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/* now shift up this digit, add in the carry [from the previous] */
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*tmpb++ = ((*tmpa++ << ((mp_digit)1)) | r) & MP_MASK;
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/* copy the carry that would be from the source
* digit into the next iteration
*/
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r = rr;
}
/* new leading digit? */
if (r != 0) {
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/* add a MSB which is always 1 at this point */
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*tmpb = 1;
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++(b->used);
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}
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/* now zero any excess digits on the destination
* that we didn't write to
*/
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tmpb = b->dp + b->used;
for (x = b->used; x < oldused; x++) {
*tmpb++ = 0;
}
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}
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b->sign = a->sign;
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return MP_OKAY;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */