c3-lang/libtommath/bn_mp_invmod_slow.c

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• Hash : 99c84acc
  Author :
  Date : 2015-11-13T10:28:23
  

  add parentheses for explicit operator precedence

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• bn_mp_invmod_slow.c

• #include <tommath_private.h>
  #ifdef BN_MP_INVMOD_SLOW_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
   */
  
  /* hac 14.61, pp608 */
  int mp_invmod_slow (mp_int * a, mp_int * b, mp_int * c)
  {
    mp_int  x, y, u, v, A, B, C, D;
    int     res;
  
    /* b cannot be negative */
    if ((b->sign == MP_NEG) || (mp_iszero(b) == MP_YES)) {
      return MP_VAL;
    }
  
    /* init temps */
    if ((res = mp_init_multi(&x, &y, &u, &v, 
                             &A, &B, &C, &D, NULL)) != MP_OKAY) {
       return res;
    }
  
    /* x = a, y = b */
    if ((res = mp_mod(a, b, &x)) != MP_OKAY) {
        goto LBL_ERR;
    }
    if ((res = mp_copy (b, &y)) != MP_OKAY) {
      goto LBL_ERR;
    }
  
    /* 2. [modified] if x,y are both even then return an error! */
    if ((mp_iseven (&x) == MP_YES) && (mp_iseven (&y) == MP_YES)) {
      res = MP_VAL;
      goto LBL_ERR;
    }
  
    /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
    if ((res = mp_copy (&x, &u)) != MP_OKAY) {
      goto LBL_ERR;
    }
    if ((res = mp_copy (&y, &v)) != MP_OKAY) {
      goto LBL_ERR;
    }
    mp_set (&A, 1);
    mp_set (&D, 1);
  
  top:
    /* 4.  while u is even do */
    while (mp_iseven (&u) == MP_YES) {
      /* 4.1 u = u/2 */
      if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
        goto LBL_ERR;
      }
      /* 4.2 if A or B is odd then */
      if ((mp_isodd (&A) == MP_YES) || (mp_isodd (&B) == MP_YES)) {
        /* A = (A+y)/2, B = (B-x)/2 */
        if ((res = mp_add (&A, &y, &A)) != MP_OKAY) {
           goto LBL_ERR;
        }
        if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
           goto LBL_ERR;
        }
      }
      /* A = A/2, B = B/2 */
      if ((res = mp_div_2 (&A, &A)) != MP_OKAY) {
        goto LBL_ERR;
      }
      if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
        goto LBL_ERR;
      }
    }
  
    /* 5.  while v is even do */
    while (mp_iseven (&v) == MP_YES) {
      /* 5.1 v = v/2 */
      if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
        goto LBL_ERR;
      }
      /* 5.2 if C or D is odd then */
      if ((mp_isodd (&C) == MP_YES) || (mp_isodd (&D) == MP_YES)) {
        /* C = (C+y)/2, D = (D-x)/2 */
        if ((res = mp_add (&C, &y, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
        if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
           goto LBL_ERR;
        }
      }
      /* C = C/2, D = D/2 */
      if ((res = mp_div_2 (&C, &C)) != MP_OKAY) {
        goto LBL_ERR;
      }
      if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
        goto LBL_ERR;
      }
    }
  
    /* 6.  if u >= v then */
    if (mp_cmp (&u, &v) != MP_LT) {
      /* u = u - v, A = A - C, B = B - D */
      if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
        goto LBL_ERR;
      }
  
      if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) {
        goto LBL_ERR;
      }
  
      if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
        goto LBL_ERR;
      }
    } else {
      /* v - v - u, C = C - A, D = D - B */
      if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
        goto LBL_ERR;
      }
  
      if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) {
        goto LBL_ERR;
      }
  
      if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
        goto LBL_ERR;
      }
    }
  
    /* if not zero goto step 4 */
    if (mp_iszero (&u) == MP_NO)
      goto top;
  
    /* now a = C, b = D, gcd == g*v */
  
    /* if v != 1 then there is no inverse */
    if (mp_cmp_d (&v, 1) != MP_EQ) {
      res = MP_VAL;
      goto LBL_ERR;
    }
  
    /* if its too low */
    while (mp_cmp_d(&C, 0) == MP_LT) {
        if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
    }
    
    /* too big */
    while (mp_cmp_mag(&C, b) != MP_LT) {
        if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
    }
    
    /* C is now the inverse */
    mp_exch (&C, c);
    res = MP_OKAY;
  LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL);
    return res;
  }
  #endif
  
  /* $Source$ */
  /* $Revision$ */
  /* $Date$ */
  

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