105 lines
2.5 KiB
C
105 lines
2.5 KiB
C
#include "tommath_private.h"
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#ifdef BN_MP_GCD_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* SPDX-License-Identifier: Unlicense
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*/
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/* Greatest Common Divisor using the binary method */
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int 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, res;
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/* either zero than gcd is the largest */
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if (mp_iszero(a) == MP_YES) {
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return mp_abs(b, c);
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}
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if (mp_iszero(b) == MP_YES) {
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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 ((res = mp_init_copy(&u, a)) != MP_OKAY) {
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return res;
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}
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if ((res = 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 = 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 ((res = 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 ((res = 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 ((res = 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 ((res = 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) == MP_NO) {
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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 ((res = 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 ((res = 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 ((res = 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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res = 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 res;
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}
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#endif
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/* ref: $Format:%D$ */
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/* git commit: $Format:%H$ */
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/* commit time: $Format:%ai$ */
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