third_party/gmp/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 either:

  * the GNU Lesser General Public License as published by the Free
    Software Foundation; either version 3 of the License, or (at your
    option) any later version.

or

  * the GNU General Public License as published by the Free Software
    Foundation; either version 2 of the License, or (at your option) any
    later version.

or both in parallel, as here.

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 General Public License
for more details.

You should have received copies of the GNU General Public License and the
GNU Lesser General Public License along with the GNU MP Library.  If not,
see https://www.gnu.org/licenses/.  */

#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, mp_bitcnt_t bit_index) __GMP_NOTHROW
{
  mp_srcptr      u_ptr      = PTR(u);
  mp_size_t      size       = SIZ(u);
  unsigned       abs_size   = ABS(size);
  mp_size_t      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;
}