/* mpz_tstbit -- test a specified bit.  Simulate 2's complement representation.

Copyright (C) 1997 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Library General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
License for more details.

You should have received a copy of the GNU Library General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"

int
#if __STDC__
mpz_tstbit (mpz_srcptr d, unsigned long int bit_index)
#else
mpz_tstbit (d, bit_index)
     mpz_srcptr d;
     unsigned long int bit_index;
#endif
{
  mp_size_t dsize = d->_mp_size;
  mp_ptr dp = d->_mp_d;
  mp_size_t limb_index;

  limb_index = bit_index / BITS_PER_MP_LIMB;
  if (dsize >= 0)
    {
      if (limb_index < dsize)
	return (dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
      else
	/* Testing a bit outside of a positive number.  */
	return 0;
    }
  else
    {
      mp_size_t zero_bound;

      dsize = -dsize;

      /* Locate the least significant non-zero limb.  */
      for (zero_bound = 0; dp[zero_bound] == 0; zero_bound++)
	;

      if (limb_index > zero_bound)
	{
	  if (limb_index < dsize)
	    return (~dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
	  else
	    /* Testing a bit outside of a negative number.  */
	    return 1;
	}
      else if (limb_index == zero_bound)
	return (-dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
      else
	return 0;
    }
}