annotate bn_mp_gcd.c @ 386:97db060d0ef5 libtommath-orig libtommath-0.40

Update to LibTomMath 0.40
author Matt Johnston <matt@ucc.asn.au>
date Thu, 11 Jan 2007 03:11:15 +0000
parents 91fbc376f010
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1 #include <tommath.h>
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2 #ifdef BN_MP_GCD_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.com
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16 */
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17
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18 /* Greatest Common Divisor using the binary method */
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19 int mp_gcd (mp_int * a, mp_int * b, mp_int * c)
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20 {
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21 mp_int u, v;
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22 int k, u_lsb, v_lsb, res;
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23
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24 /* either zero than gcd is the largest */
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25 if (mp_iszero (a) == MP_YES) {
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26 return mp_abs (b, c);
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27 }
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28 if (mp_iszero (b) == MP_YES) {
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29 return mp_abs (a, c);
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30 }
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31
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32 /* get copies of a and b we can modify */
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33 if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
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34 return res;
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35 }
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36
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37 if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
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38 goto LBL_U;
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39 }
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40
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41 /* must be positive for the remainder of the algorithm */
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42 u.sign = v.sign = MP_ZPOS;
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43
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44 /* B1. Find the common power of two for u and v */
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45 u_lsb = mp_cnt_lsb(&u);
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46 v_lsb = mp_cnt_lsb(&v);
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47 k = MIN(u_lsb, v_lsb);
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48
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49 if (k > 0) {
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50 /* divide the power of two out */
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51 if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
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52 goto LBL_V;
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53 }
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54
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55 if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
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56 goto LBL_V;
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57 }
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58 }
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59
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60 /* divide any remaining factors of two out */
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61 if (u_lsb != k) {
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62 if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
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63 goto LBL_V;
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64 }
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65 }
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66
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67 if (v_lsb != k) {
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68 if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
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69 goto LBL_V;
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70 }
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71 }
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72
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73 while (mp_iszero(&v) == 0) {
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74 /* make sure v is the largest */
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75 if (mp_cmp_mag(&u, &v) == MP_GT) {
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76 /* swap u and v to make sure v is >= u */
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77 mp_exch(&u, &v);
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78 }
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79
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80 /* subtract smallest from largest */
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81 if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
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82 goto LBL_V;
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83 }
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84
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85 /* Divide out all factors of two */
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86 if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
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87 goto LBL_V;
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88 }
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89 }
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90
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91 /* multiply by 2**k which we divided out at the beginning */
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92 if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
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93 goto LBL_V;
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94 }
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95 c->sign = MP_ZPOS;
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96 res = MP_OKAY;
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97 LBL_V:mp_clear (&u);
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98 LBL_U:mp_clear (&v);
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99 return res;
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100 }
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101 #endif
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102
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103 /* $Source: /cvs/libtom/libtommath/bn_mp_gcd.c,v $ */
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104 /* $Revision: 1.4 $ */
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105 /* $Date: 2006/03/31 14:18:44 $ */