/* 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
* 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>
/* single digit addition */
int
mp_add_d (mp_int * a, mp_digit b, mp_int * c)
{
int res, ix, oldused;
mp_digit *tmpa, *tmpc, mu;
/* grow c as required */
if (c->alloc < a->used + 1) {
if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
return res;
}
}
/* if a is negative and |a| >= b, call c = |a| - b */
if (a->sign == MP_NEG && (a->used > 1 || a->dp[0] >= b)) {
/* temporarily fix sign of a */
a->sign = MP_ZPOS;
/* c = |a| - b */
res = mp_sub_d(a, b, c);
/* fix sign */
a->sign = c->sign = MP_NEG;
return res;
}
/* old number of used digits in c */
oldused = c->used;
/* sign always positive */
c->sign = MP_ZPOS;
/* source alias */
tmpa = a->dp;
/* destination alias */
tmpc = c->dp;
/* if a is positive */
if (a->sign == MP_ZPOS) {
/* setup size */
c->used = a->used + 1;
/* add digit, after this we're propagating
* the carry.
*/
*tmpc = *tmpa++ + b;
mu = *tmpc >> DIGIT_BIT;
*tmpc++ &= MP_MASK;
/* now handle rest of the digits */
for (ix = 1; ix < a->used; ix++) {
*tmpc = *tmpa++ + mu;
mu = *tmpc >> DIGIT_BIT;
*tmpc++ &= MP_MASK;
}
/* set final carry */
ix++;
*tmpc++ = mu;
} else {
/* a was negative and |a| < b */
c->used = 1;
/* the result is a single digit */
if (a->used == 1) {
*tmpc++ = b - a->dp[0];
} else {
*tmpc++ = b;
}
/* setup count so the clearing of oldused
* can fall through correctly
*/
ix = 1;
}
/* now zero to oldused */
while (ix++ < oldused) {
*tmpc++ = 0;
}
mp_clamp(c);
return MP_OKAY;
}
