73 lines
2.5 KiB
C
73 lines
2.5 KiB
C
/* mpz_tstbit -- test a specified bit.
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Copyright 2000, 2002 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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/* For negatives the effective twos complement is achieved by negating the
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limb tested, either with a ones or twos complement. Twos complement
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("-") is used if there's only zero limbs below the one being tested.
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Ones complement ("~") is used if there's a non-zero below. Note that "-"
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is correct even if the limb examined is 0 (and the true beginning of twos
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complement is further up).
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Testing the limbs below p is unavoidable on negatives, but will usually
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need to examine only *(p-1). The search is done from *(p-1) down to
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*u_ptr, since that might give better cache locality, and because a
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non-zero limb is perhaps a touch more likely in the middle of a number
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than at the low end.
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Bits past the end of available data simply follow sign of u. Notice that
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the limb_index >= abs_size test covers u=0 too. */
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int
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mpz_tstbit (mpz_srcptr u, unsigned long bit_index)
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{
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mp_srcptr u_ptr = PTR(u);
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mp_size_t size = SIZ(u);
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unsigned abs_size = ABS(size);
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unsigned long limb_index = bit_index / GMP_NUMB_BITS;
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mp_srcptr p = u_ptr + limb_index;
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mp_limb_t limb;
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if (limb_index >= abs_size)
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return (size < 0);
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limb = *p;
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if (size < 0)
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{
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limb = -limb; /* twos complement */
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while (p != u_ptr)
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{
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p--;
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if (*p != 0)
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{
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limb--; /* make it a ones complement instead */
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break;
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}
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}
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}
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return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
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}
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