c3-lang/libtommath/bn_mp_gcd.c

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• Hash : 4e3f1344
  Author :
  Date : 2015-11-12T01:49:07
  

  provide tommath_private.h tommath.h contains declarations for the public part of the library. tommath_private.h contains the functions which are private to ltm and should not be exposed to the public.

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• bn_mp_gcd.c

• #include <tommath_private.h>
  #ifdef BN_MP_GCD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis
   *
   * LibTomMath is a library that provides multiple-precision
   * integer arithmetic as well as number theoretic functionality.
   *
   * The library was 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, tstdenis82@gmail.com, http://libtom.org
   */
  
  /* Greatest Common Divisor using the binary method */
  int mp_gcd (mp_int * a, mp_int * b, mp_int * c)
  {
    mp_int  u, v;
    int     k, u_lsb, v_lsb, res;
  
    /* either zero than gcd is the largest */
    if (mp_iszero (a) == MP_YES) {
      return mp_abs (b, c);
    }
    if (mp_iszero (b) == MP_YES) {
      return mp_abs (a, c);
    }
  
    /* get copies of a and b we can modify */
    if ((res = mp_init_copy (&u, a)) != MP_OKAY) {
      return res;
    }
  
    if ((res = mp_init_copy (&v, b)) != MP_OKAY) {
      goto LBL_U;
    }
  
    /* must be positive for the remainder of the algorithm */
    u.sign = v.sign = MP_ZPOS;
  
    /* B1.  Find the common power of two for u and v */
    u_lsb = mp_cnt_lsb(&u);
    v_lsb = mp_cnt_lsb(&v);
    k     = MIN(u_lsb, v_lsb);
  
    if (k > 0) {
       /* divide the power of two out */
       if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) {
          goto LBL_V;
       }
  
       if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) {
          goto LBL_V;
       }
    }
  
    /* divide any remaining factors of two out */
    if (u_lsb != k) {
       if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) {
          goto LBL_V;
       }
    }
  
    if (v_lsb != k) {
       if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) {
          goto LBL_V;
       }
    }
  
    while (mp_iszero(&v) == MP_NO) {
       /* make sure v is the largest */
       if (mp_cmp_mag(&u, &v) == MP_GT) {
          /* swap u and v to make sure v is >= u */
          mp_exch(&u, &v);
       }
       
       /* subtract smallest from largest */
       if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) {
          goto LBL_V;
       }
       
       /* Divide out all factors of two */
       if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) {
          goto LBL_V;
       } 
    } 
  
    /* multiply by 2**k which we divided out at the beginning */
    if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) {
       goto LBL_V;
    }
    c->sign = MP_ZPOS;
    res = MP_OKAY;
  LBL_V:mp_clear (&u);
  LBL_U:mp_clear (&v);
    return res;
  }
  #endif
  
  /* $Source$ */
  /* $Revision$ */
  /* $Date$ */
  

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