### view libtommath/bn_mp_gcd.c @ 350:01e4180895ba

merge of '9a4e042fd565f46141e81e0c1ab90260303348fe' and 'bea3887a5875cf3ab8a1331e15e698b37b61fe37'
author Matt Johnston Mon, 07 Aug 2006 13:41:23 +0000 eed26cff980b 5ff8218bcee9
line wrap: on
line source
```#include <tommath.h>
#ifdef BN_MP_GCD_C
/* 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
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.org
*/

/* Greatest Common Divisor using the binary method */
int mp_gcd (mp_int * a, mp_int * b, mp_int * c)
{
mp_int  u, v;
int     k, u_lsb, v_lsb, res;

/* either zero than gcd is the largest */
if (mp_iszero (a) == 1 && mp_iszero (b) == 0) {
return mp_abs (b, c);
}
if (mp_iszero (a) == 0 && mp_iszero (b) == 1) {
return mp_abs (a, c);
}

/* optimized.  At this point if a == 0 then
* b must equal zero too
*/
if (mp_iszero (a) == 1) {
mp_zero(c);
return MP_OKAY;
}

/* get copies of a and b we can modify */
if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
return res;
}

if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
goto LBL_U;
}

/* must be positive for the remainder of the algorithm */
u.sign = v.sign = MP_ZPOS;

/* 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);

if (k > 0) {
/* divide the power of two out */
if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
goto LBL_V;
}

if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
goto LBL_V;
}
}

/* 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 LBL_V;
}
}

if (v_lsb != k) {
if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
goto LBL_V;
}
}

while (mp_iszero(&v) == 0) {
/* make sure v is the largest */
if (mp_cmp_mag(&u, &v) == MP_GT) {
/* swap u and v to make sure v is >= u */
mp_exch(&u, &v);
}

/* subtract smallest from largest */
if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
goto LBL_V;
}

/* Divide out all factors of two */
if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
goto LBL_V;
}
}

/* multiply by 2**k which we divided out at the beginning */
if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
goto LBL_V;
}
c->sign = MP_ZPOS;
res = MP_OKAY;
LBL_V:mp_clear (&u);
LBL_U:mp_clear (&v);
return res;
}
#endif
```