1 #include "tommath_private.h"
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
6 /* Greatest Common Divisor using the binary method */
7 mp_err mp_gcd(const mp_int *a, const mp_int *b, mp_int *c)
13 /* either zero than gcd is the largest */
21 /* get copies of a and b we can modify */
22 if ((err = mp_init_copy(&u, a)) != MP_OKAY) {
26 if ((err = mp_init_copy(&v, b)) != MP_OKAY) {
30 /* must be positive for the remainder of the algorithm */
31 u.sign = v.sign = MP_ZPOS;
33 /* B1. Find the common power of two for u and v */
34 u_lsb = mp_cnt_lsb(&u);
35 v_lsb = mp_cnt_lsb(&v);
36 k = MP_MIN(u_lsb, v_lsb);
39 /* divide the power of two out */
40 if ((err = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
44 if ((err = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
49 /* divide any remaining factors of two out */
51 if ((err = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
57 if ((err = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
62 while (!MP_IS_ZERO(&v)) {
63 /* make sure v is the largest */
64 if (mp_cmp_mag(&u, &v) == MP_GT) {
65 /* swap u and v to make sure v is >= u */
69 /* subtract smallest from largest */
70 if ((err = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
74 /* Divide out all factors of two */
75 if ((err = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
80 /* multiply by 2**k which we divided out at the beginning */
81 if ((err = mp_mul_2d(&u, k, c)) != MP_OKAY) {