tommath/bn_s_mp_add.c

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#include <tommath.h>
#ifdef BN_S_MP_ADD_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@iahu.ca, http://math.libtomcrypt.org
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*/
/* low level addition, based on HAC pp.594, Algorithm 14.7 */
int
s_mp_add (mp_int * a, mp_int * b, mp_int * c)
{
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mp_int *x;
int olduse, res, min, max;
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/* find sizes, we let |a| <= |b| which means we have to sort
* them. "x" will point to the input with the most digits
*/
if (a->used > b->used) {
min = b->used;
max = a->used;
x = a;
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} else {
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min = a->used;
max = b->used;
x = b;
}
/* init result */
if (c->alloc < max + 1) {
if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
return res;
}
}
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/* get old used digit count and set new one */
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olduse = c->used;
c->used = max + 1;
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{
register mp_digit u, *tmpa, *tmpb, *tmpc;
register int i;
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/* alias for digit pointers */
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/* first input */
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tmpa = a->dp;
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/* second input */
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tmpb = b->dp;
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/* destination */
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tmpc = c->dp;
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/* zero the carry */
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u = 0;
for (i = 0; i < min; i++) {
/* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
*tmpc = *tmpa++ + *tmpb++ + u;
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/* U = carry bit of T[i] */
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u = *tmpc >> ((mp_digit)DIGIT_BIT);
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/* take away carry bit from T[i] */
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*tmpc++ &= MP_MASK;
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}
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/* now copy higher words if any, that is in A+B
* if A or B has more digits add those in
*/
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if (min != max) {
for (; i < max; i++) {
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/* T[i] = X[i] + U */
*tmpc = x->dp[i] + u;
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/* U = carry bit of T[i] */
u = *tmpc >> ((mp_digit)DIGIT_BIT);
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/* take away carry bit from T[i] */
*tmpc++ &= MP_MASK;
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}
}
/* add carry */
*tmpc++ = u;
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/* clear digits above oldused */
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for (i = c->used; i < olduse; i++) {
*tmpc++ = 0;
}
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}
mp_clamp (c);
return MP_OKAY;
}
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#endif
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/* $Source$ */
/* $Revision$ */
/* $Date$ */