File: [local] / OpenXM_contrib / gmp / mpz / Attic / tstbit.c (download)
Revision 1.1.1.2 (vendor branch), Mon Aug 25 16:06:34 2003 UTC (21 years ago) by ohara
Branch: GMP
CVS Tags: VERSION_4_1_2, RELEASE_1_2_3, RELEASE_1_2_2_KNOPPIX_b, RELEASE_1_2_2_KNOPPIX Changes since 1.1.1.1: +43 -41
lines
Import gmp 4.1.2
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/* 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., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, 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;
}