/* 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>
/* Greatest Common Divisor using the binary method [Algorithm B, page 338, vol2 of TAOCP]
*/
int
mp_gcd (mp_int * a, mp_int * b, mp_int * c)
{
mp_int u, v, t;
int k, res, neg;
/* either zero than gcd is the largest */
if (mp_iszero (a) == 1 && mp_iszero (b) == 0) {
return mp_copy (b, c);
}
if (mp_iszero (a) == 0 && mp_iszero (b) == 1) {
return mp_copy (a, c);
}
if (mp_iszero (a) == 1 && mp_iszero (b) == 1) {
mp_set (c, 1);
return MP_OKAY;
}
/* if both are negative they share (-1) as a common divisor */
neg = (a->sign == b->sign) ? a->sign : MP_ZPOS;
if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
return res;
}
if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
goto __U;
}
/* must be positive for the remainder of the algorithm */
u.sign = v.sign = MP_ZPOS;
if ((res = mp_init (&t)) != MP_OKAY) {
goto __V;
}
/* B1. Find power of two */
k = 0;
while (mp_iseven(&u) == 1 && mp_iseven(&v) == 1) {
++k;
if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
goto __T;
}
if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
goto __T;
}
}
/* B2. Initialize */
if (mp_isodd(&u) == 1) {
/* t = -v */
if ((res = mp_copy (&v, &t)) != MP_OKAY) {
goto __T;
}
t.sign = MP_NEG;
} else {
/* t = u */
if ((res = mp_copy (&u, &t)) != MP_OKAY) {
goto __T;
}
}
do {
/* B3 (and B4). Halve t, if even */
while (t.used != 0 && mp_iseven(&t) == 1) {
if ((res = mp_div_2 (&t, &t)) != MP_OKAY) {
goto __T;
}
}
/* B5. if t>0 then u=t otherwise v=-t */
if (t.used != 0 && t.sign != MP_NEG) {
if ((res = mp_copy (&t, &u)) != MP_OKAY) {
goto __T;
}
} else {
if ((res = mp_copy (&t, &v)) != MP_OKAY) {
goto __T;
}
v.sign = (v.sign == MP_ZPOS) ? MP_NEG : MP_ZPOS;
}
/* B6. t = u - v, if t != 0 loop otherwise terminate */
if ((res = mp_sub (&u, &v, &t)) != MP_OKAY) {
goto __T;
}
} while (mp_iszero(&t) == 0);
/* multiply by 2^k which we divided out at the beginning */
if ((res = mp_mul_2d (&u, k, &u)) != MP_OKAY) {
goto __T;
}
mp_exch (&u, c);
c->sign = neg;
res = MP_OKAY;
__T:mp_clear (&t);
__V:mp_clear (&u);
__U:mp_clear (&v);
return res;
}
