tommath/bn_mp_gcd.c

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C
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/* 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.
*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
#include <tommath.h>
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/* Greatest Common Divisor using the binary method */
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int
mp_gcd (mp_int * a, mp_int * b, mp_int * c)
{
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mp_int u, v;
int k, u_lsb, v_lsb, res;
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/* 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 ((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;
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/* B1. Find the common power of two for u and v */
u_lsb = mp_cnt_lsb(&u);
v_lsb = mp_cnt_lsb(&v);
k = MIN(u_lsb, v_lsb);
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if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
goto __V;
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}
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if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
goto __V;
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}
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/* divide any remaining factors of two out */
if (u_lsb != k) {
if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
goto __V;
}
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}
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if (v_lsb != k) {
if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
goto __V;
}
}
while (mp_iszero(&v) == 0) {
/* make sure v is the largest */
if (mp_cmp_mag(&u, &v) == MP_GT) {
mp_exch(&u, &v);
}
/* subtract smallest from largest */
if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
goto __V;
}
/* Divide out all factors of two */
if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
goto __V;
}
}
/* multiply by 2**k which we divided out at the beginning */
if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
goto __V;
}
c->sign = MP_ZPOS;
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res = MP_OKAY;
__V:mp_clear (&u);
__U:mp_clear (&v);
return res;
}