93 lines
2.1 KiB
C
93 lines
2.1 KiB
C
#include "tommath_private.h"
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#ifdef MP_GCD_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* Greatest Common Divisor using the binary method */
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mp_err mp_gcd(const mp_int *a, const mp_int *b, mp_int *c)
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{
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mp_int u, v;
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int k, u_lsb, v_lsb;
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mp_err err;
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/* either zero than gcd is the largest */
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if (mp_iszero(a)) {
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return mp_abs(b, c);
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}
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if (mp_iszero(b)) {
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return mp_abs(a, c);
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}
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/* get copies of a and b we can modify */
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if ((err = mp_init_copy(&u, a)) != MP_OKAY) {
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return err;
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}
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if ((err = mp_init_copy(&v, b)) != MP_OKAY) {
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goto LBL_U;
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}
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/* must be positive for the remainder of the algorithm */
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u.sign = v.sign = MP_ZPOS;
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/* B1. Find the common power of two for u and v */
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u_lsb = mp_cnt_lsb(&u);
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v_lsb = mp_cnt_lsb(&v);
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k = MP_MIN(u_lsb, v_lsb);
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if (k > 0) {
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/* divide the power of two out */
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if ((err = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
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goto LBL_V;
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}
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if ((err = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
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goto LBL_V;
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}
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}
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/* divide any remaining factors of two out */
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if (u_lsb != k) {
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if ((err = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
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goto LBL_V;
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}
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}
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if (v_lsb != k) {
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if ((err = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
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goto LBL_V;
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}
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}
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while (!mp_iszero(&v)) {
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/* make sure v is the largest */
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if (mp_cmp_mag(&u, &v) == MP_GT) {
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/* swap u and v to make sure v is >= u */
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mp_exch(&u, &v);
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}
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/* subtract smallest from largest */
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if ((err = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
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goto LBL_V;
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}
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/* Divide out all factors of two */
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if ((err = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
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goto LBL_V;
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}
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}
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/* multiply by 2**k which we divided out at the beginning */
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if ((err = mp_mul_2d(&u, k, c)) != MP_OKAY) {
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goto LBL_V;
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}
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c->sign = MP_ZPOS;
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err = MP_OKAY;
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LBL_V:
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mp_clear(&u);
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LBL_U:
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mp_clear(&v);
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return err;
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}
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#endif
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