libtommath/bn_mp_div_2.c

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#include <tommath.h>
#ifdef BN_MP_DIV_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.
*
* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
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*/
/* b = a/2 */
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int mp_div_2(mp_int * a, mp_int * b)
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{
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int x, res, oldused;
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/* copy */
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if (b->alloc < a->used) {
if ((res = mp_grow (b, a->used)) != MP_OKAY) {
return res;
}
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}
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oldused = b->used;
b->used = a->used;
{
register mp_digit r, rr, *tmpa, *tmpb;
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/* source alias */
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tmpa = a->dp + b->used - 1;
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/* dest alias */
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tmpb = b->dp + b->used - 1;
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/* carry */
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r = 0;
for (x = b->used - 1; x >= 0; x--) {
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/* get the carry for the next iteration */
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rr = *tmpa & 1;
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/* shift the current digit, add in carry and store */
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*tmpb-- = (*tmpa-- >> 1) | (r << (DIGIT_BIT - 1));
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/* forward carry to next iteration */
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r = rr;
}
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/* zero excess digits */
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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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mp_clamp (b);
return MP_OKAY;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */