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comparison libtommath/bn_mp_gcd.c @ 399:a707e6148060

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merge of '5fdf69ca60d1683cdd9f4c2595134bed26394834' and '6b61c50f4cf888bea302ac8fcf5dbb573b443251'
author Matt Johnston <matt@ucc.asn.au>
date Sat, 03 Feb 2007 08:20:34 +0000
parents 5ff8218bcee9
children 60fc6476e044
comparison
equal deleted inserted replaced
394:17d097fc111c 399:a707e6148060
1 #include <tommath.h>
2 #ifdef BN_MP_GCD_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, [email protected], http://math.libtomcrypt.com
16 */
17
18 /* Greatest Common Divisor using the binary method */
19 int mp_gcd (mp_int * a, mp_int * b, mp_int * c)
20 {
21 mp_int u, v;
22 int k, u_lsb, v_lsb, res;
23
24 /* either zero than gcd is the largest */
25 if (mp_iszero (a) == MP_YES) {
26 return mp_abs (b, c);
27 }
28 if (mp_iszero (b) == MP_YES) {
29 return mp_abs (a, c);
30 }
31
32 /* get copies of a and b we can modify */
33 if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
34 return res;
35 }
36
37 if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
38 goto LBL_U;
39 }
40
41 /* must be positive for the remainder of the algorithm */
42 u.sign = v.sign = MP_ZPOS;
43
44 /* B1. Find the common power of two for u and v */
45 u_lsb = mp_cnt_lsb(&u);
46 v_lsb = mp_cnt_lsb(&v);
47 k = MIN(u_lsb, v_lsb);
48
49 if (k > 0) {
50 /* divide the power of two out */
51 if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
52 goto LBL_V;
53 }
54
55 if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
56 goto LBL_V;
57 }
58 }
59
60 /* divide any remaining factors of two out */
61 if (u_lsb != k) {
62 if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
63 goto LBL_V;
64 }
65 }
66
67 if (v_lsb != k) {
68 if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
69 goto LBL_V;
70 }
71 }
72
73 while (mp_iszero(&v) == 0) {
74 /* make sure v is the largest */
75 if (mp_cmp_mag(&u, &v) == MP_GT) {
76 /* swap u and v to make sure v is >= u */
77 mp_exch(&u, &v);
78 }
79
80 /* subtract smallest from largest */
81 if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
82 goto LBL_V;
83 }
84
85 /* Divide out all factors of two */
86 if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
87 goto LBL_V;
88 }
89 }
90
91 /* multiply by 2**k which we divided out at the beginning */
92 if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
93 goto LBL_V;
94 }
95 c->sign = MP_ZPOS;
96 res = MP_OKAY;
97 LBL_V:mp_clear (&u);
98 LBL_U:mp_clear (&v);
99 return res;
100 }
101 #endif
102
103 /* $Source: /cvs/libtom/libtommath/bn_mp_gcd.c,v $ */
104 /* $Revision: 1.4 $ */
105 /* $Date: 2006/03/31 14:18:44 $ */