mpir/mpz/tstbit.c

/* mpz_tstbit -- test a specified bit.

Copyright 2000, 2002 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 Lesser General Public License as published by
the Free Software Foundation; either version 2.1 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 Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA. */

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


/* For negatives the effective twos complement is achieved by negating the
   limb tested, either with a ones or twos complement.  Twos complement
   ("-") is used if there's only zero limbs below the one being tested.
   Ones complement ("~") is used if there's a non-zero below.  Note that "-"
   is correct even if the limb examined is 0 (and the true beginning of twos
   complement is further up).

   Testing the limbs below p is unavoidable on negatives, but will usually
   need to examine only *(p-1).  The search is done from *(p-1) down to
   *u_ptr, since that might give better cache locality, and because a
   non-zero limb is perhaps a touch more likely in the middle of a number
   than at the low end.

   Bits past the end of available data simply follow sign of u.  Notice that
   the limb_index >= abs_size test covers u=0 too.  */

int
mpz_tstbit (mpz_srcptr u, unsigned long bit_index)
{
  mp_srcptr      u_ptr      = PTR(u);
  mp_size_t      size       = SIZ(u);
  unsigned       abs_size   = ABS(size);
  unsigned long  limb_index = bit_index / GMP_NUMB_BITS;
  mp_srcptr      p          = u_ptr + limb_index;
  mp_limb_t      limb;

  if (limb_index >= abs_size)
    return (size < 0);

  limb = *p;
  if (size < 0)
    {
      limb = -limb;     /* twos complement */

      while (p != u_ptr)
        {
          p--;
          if (*p != 0)
            {
              limb--;   /* make it a ones complement instead */
              break;
            }
        }
    }

  return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
}