/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is library that provides for multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library is designed directly after the MPI library by
* 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, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
/* low level addition, based on HAC pp.594, Algorithm 14.7 */
int
s_mp_add (mp_int * a, mp_int * b, mp_int * c)
{
mp_int *x;
int olduse, res, min, max;
/* 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;
} else if (a->used < b->used) {
min = a->used;
max = b->used;
x = b;
} else {
min = max = a->used;
x = NULL;
}
/* init result */
if (c->alloc < max + 1) {
if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
return res;
}
}
olduse = c->used;
c->used = max + 1;
/* add digits from lower part */
/* set the carry to zero */
{
register mp_digit u, *tmpa, *tmpb, *tmpc;
register int i;
/* alias for digit pointers */
/* first input */
tmpa = a->dp;
/* second input */
tmpb = b->dp;
/* destination */
tmpc = c->dp;
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;
/* U = carry bit of T[i] */
u = *tmpc >> DIGIT_BIT;
/* take away carry bit from T[i] */
*tmpc++ &= MP_MASK;
}
/* now copy higher words if any, that is in A+B if A or B has more digits add those in */
if (min != max) {
for (; i < max; i++) {
/* T[i] = X[i] + U */
*tmpc = x->dp[i] + u;
/* U = carry bit of T[i] */
u = *tmpc >> DIGIT_BIT;
/* take away carry bit from T[i] */
*tmpc++ &= MP_MASK;
}
}
/* add carry */
*tmpc++ = u;
/* clear digits above used (since we may not have grown result above) */
for (i = c->used; i < olduse; i++) {
*tmpc++ = 0;
}
}
mp_clamp (c);
return MP_OKAY;
}
