# publiclibtom/libtommath

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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 /* LibTomMath, multiple-precision integer library -- Tom St Denis * * LibTomMath is library that provides for multiple-precision * integer arithmetic as well as number theoretic functionality. * * The library is designed directly after the MPI library by * Michael Fromberger but has been written from scratch with * additional optimizations in place. * * The library is free for all purposes without any express * guarantee it works. * * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org */#include /* Greatest Common Divisor using the binary method [Algorithm B, page 338, vol2 of TAOCP] */intmp_gcd (mp_int * a, mp_int * b, mp_int * c){  mp_int u, v, t;  int k, res, neg;  /* either zero than gcd is the largest */  if (mp_iszero (a) == 1 && mp_iszero (b) == 0) {    return mp_copy (b, c);  }  if (mp_iszero (a) == 0 && mp_iszero (b) == 1) {    return mp_copy (a, c);  }  if (mp_iszero (a) == 1 && mp_iszero (b) == 1) {    mp_set (c, 1);    return MP_OKAY;  }  /* if both are negative they share (-1) as a common divisor */  neg = (a->sign == b->sign) ? a->sign : MP_ZPOS;  if ((res = mp_init_copy (&u, a)) != MP_OKAY) {    return res;  }  if ((res = mp_init_copy (&v, b)) != MP_OKAY) {    goto __U;  }  /* must be positive for the remainder of the algorithm */  u.sign = v.sign = MP_ZPOS;  if ((res = mp_init (&t)) != MP_OKAY) {    goto __V;  }  /* B1. Find power of two */  k = 0;  while (mp_iseven(&u) == 1 && mp_iseven(&v) == 1) {    ++k;    if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {      goto __T;    }    if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {      goto __T;    }  }  /* B2. Initialize */  if (mp_isodd(&u) == 1) {    /* t = -v */    if ((res = mp_copy (&v, &t)) != MP_OKAY) {      goto __T;    }    t.sign = MP_NEG;  } else {    /* t = u */    if ((res = mp_copy (&u, &t)) != MP_OKAY) {      goto __T;    }  }  do {    /* B3 (and B4). Halve t, if even */    while (t.used != 0 && mp_iseven(&t) == 1) {      if ((res = mp_div_2 (&t, &t)) != MP_OKAY) {        goto __T;      }    }    /* B5. if t>0 then u=t otherwise v=-t */    if (t.used != 0 && t.sign != MP_NEG) {      if ((res = mp_copy (&t, &u)) != MP_OKAY) {        goto __T;      }    } else {      if ((res = mp_copy (&t, &v)) != MP_OKAY) {        goto __T;      }      v.sign = (v.sign == MP_ZPOS) ? MP_NEG : MP_ZPOS;    }    /* B6. t = u - v, if t != 0 loop otherwise terminate */    if ((res = mp_sub (&u, &v, &t)) != MP_OKAY) {      goto __T;    }  } while (mp_iszero(&t) == 0);  /* multiply by 2^k which we divided out at the beginning */   if ((res = mp_mul_2d (&u, k, &u)) != MP_OKAY) {    goto __T;  }  mp_exch (&u, c);  c->sign = neg;  res = MP_OKAY;__T:mp_clear (&t);__V:mp_clear (&u);__U:mp_clear (&v);  return res;}
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