/* 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://libtommath.iahu.ca
*/
#include <tommath.h>
/* high level subtraction (handles signs) */
int
mp_sub (mp_int * a, mp_int * b, mp_int * c)
{
int sa, sb, res;
sa = a->sign;
sb = b->sign;
/* handle four cases */
if (sa == MP_ZPOS && sb == MP_ZPOS) {
/* both positive, a - b, but if b>a then we do -(b - a) */
if (mp_cmp_mag (a, b) == MP_LT) {
/* b>a */
res = s_mp_sub (b, a, c);
c->sign = MP_NEG;
} else {
res = s_mp_sub (a, b, c);
c->sign = MP_ZPOS;
}
} else if (sa == MP_ZPOS && sb == MP_NEG) {
/* a - -b == a + b */
res = s_mp_add (a, b, c);
c->sign = MP_ZPOS;
} else if (sa == MP_NEG && sb == MP_ZPOS) {
/* -a - b == -(a + b) */
res = s_mp_add (a, b, c);
c->sign = MP_NEG;
} else {
/* -a - -b == b - a, but if a>b == -(a - b) */
if (mp_cmp_mag (a, b) == MP_GT) {
res = s_mp_sub (a, b, c);
c->sign = MP_NEG;
} else {
res = s_mp_sub (b, a, c);
c->sign = MP_ZPOS;
}
}
return res;
}
