119 lines
2.7 KiB
C
119 lines
2.7 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is 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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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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#include <tommath.h>
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/* Greatest Common Divisor using the binary method [Algorithm B, page 338, vol2 of TAOCP]
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*/
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int
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mp_gcd (mp_int * a, mp_int * b, mp_int * c)
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{
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mp_int u, v, t;
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int k, res, neg;
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/* either zero than gcd is the largest */
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if (mp_iszero (a) == 1 && mp_iszero (b) == 0) {
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return mp_copy (b, c);
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}
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if (mp_iszero (a) == 0 && mp_iszero (b) == 1) {
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return mp_copy (a, c);
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}
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if (mp_iszero (a) == 1 && mp_iszero (b) == 1) {
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mp_set (c, 1);
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return MP_OKAY;
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}
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/* if both are negative they share (-1) as a common divisor */
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neg = (a->sign == b->sign) ? a->sign : MP_ZPOS;
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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 __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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if ((res = mp_init (&t)) != MP_OKAY) {
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goto __V;
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}
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/* B1. Find power of two */
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k = 0;
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while ((u.dp[0] & 1) == 0 && (v.dp[0] & 1) == 0) {
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++k;
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if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
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goto __T;
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}
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if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
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goto __T;
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}
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}
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/* B2. Initialize */
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if ((u.dp[0] & 1) == 1) {
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if ((res = mp_copy (&v, &t)) != MP_OKAY) {
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goto __T;
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}
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t.sign = MP_NEG;
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} else {
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if ((res = mp_copy (&u, &t)) != MP_OKAY) {
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goto __T;
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}
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}
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do {
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/* B3 (and B4). Halve t, if even */
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while (t.used != 0 && (t.dp[0] & 1) == 0) {
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if ((res = mp_div_2 (&t, &t)) != MP_OKAY) {
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goto __T;
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}
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}
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/* B5. if t>0 then u=t otherwise v=-t */
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if (t.used != 0 && t.sign != MP_NEG) {
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if ((res = mp_copy (&t, &u)) != MP_OKAY) {
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goto __T;
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}
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} else {
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if ((res = mp_copy (&t, &v)) != MP_OKAY) {
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goto __T;
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}
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v.sign = (v.sign == MP_ZPOS) ? MP_NEG : MP_ZPOS;
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}
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/* B6. t = u - v, if t != 0 loop otherwise terminate */
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if ((res = mp_sub (&u, &v, &t)) != MP_OKAY) {
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goto __T;
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}
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}
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while (t.used != 0);
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if ((res = mp_mul_2d (&u, k, &u)) != MP_OKAY) {
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goto __T;
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}
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mp_exch (&u, c);
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c->sign = neg;
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res = MP_OKAY;
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__T:mp_clear (&t);
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__V:mp_clear (&u);
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__U:mp_clear (&v);
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return res;
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}
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