1/* mpz_tstbit -- test a specified bit.
2
3Copyright 2000, 2002 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 3 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
19
20#include "gmp.h"
21#include "gmp-impl.h"
22
23
24/* For negatives the effective twos complement is achieved by negating the
25   limb tested, either with a ones or twos complement.  Twos complement
26   ("-") is used if there's only zero limbs below the one being tested.
27   Ones complement ("~") is used if there's a non-zero below.  Note that "-"
28   is correct even if the limb examined is 0 (and the true beginning of twos
29   complement is further up).
30
31   Testing the limbs below p is unavoidable on negatives, but will usually
32   need to examine only *(p-1).  The search is done from *(p-1) down to
33   *u_ptr, since that might give better cache locality, and because a
34   non-zero limb is perhaps a touch more likely in the middle of a number
35   than at the low end.
36
37   Bits past the end of available data simply follow sign of u.  Notice that
38   the limb_index >= abs_size test covers u=0 too.  */
39
40int
41mpz_tstbit (mpz_srcptr u, mp_bitcnt_t bit_index) __GMP_NOTHROW
42{
43  mp_srcptr      u_ptr      = PTR(u);
44  mp_size_t      size       = SIZ(u);
45  unsigned       abs_size   = ABS(size);
46  mp_size_t      limb_index = bit_index / GMP_NUMB_BITS;
47  mp_srcptr      p          = u_ptr + limb_index;
48  mp_limb_t      limb;
49
50  if (limb_index >= abs_size)
51    return (size < 0);
52
53  limb = *p;
54  if (size < 0)
55    {
56      limb = -limb;     /* twos complement */
57
58      while (p != u_ptr)
59        {
60          p--;
61          if (*p != 0)
62            {
63              limb--;   /* make it a ones complement instead */
64              break;
65            }
66        }
67    }
68
69  return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
70}
71