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#include "tommath_private.h"
#ifdef BN_MP_GCD_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* Greatest Common Divisor using the binary method */
mp_err mp_gcd(const mp_int *a, const mp_int *b, mp_int *c)
{
mp_int u, v;
int k, u_lsb, v_lsb;
mp_err err;
/* either zero than gcd is the largest */
if (MP_IS_ZERO(a)) {
return mp_abs(b, c);
}
if (MP_IS_ZERO(b)) {
return mp_abs(a, c);
}
/* get copies of a and b we can modify */
if ((err = mp_init_copy(&u, a)) != MP_OKAY) {
return err;
}
if ((err = 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 = MP_MIN(u_lsb, v_lsb);
if (k > 0) {
/* divide the power of two out */
if ((err = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
goto LBL_V;
}
if ((err = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
goto LBL_V;
}
}
/* divide any remaining factors of two out */
if (u_lsb != k) {
if ((err = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
goto LBL_V;
}
}
if (v_lsb != k) {
if ((err = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
goto LBL_V;
}
}
while (!MP_IS_ZERO(&v)) {
/* 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 ((err = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
goto LBL_V;
}
/* Divide out all factors of two */
if ((err = 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 ((err = mp_mul_2d(&u, k, c)) != MP_OKAY) {
goto LBL_V;
}
c->sign = MP_ZPOS;
err = MP_OKAY;
LBL_V:
mp_clear(&u);
LBL_U:
mp_clear(&v);
return err;
}
#endif
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