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 either:
9
10  * the GNU Lesser General Public License as published by the Free
11    Software Foundation; either version 3 of the License, or (at your
12    option) any later version.
13
14or
15
16  * the GNU General Public License as published by the Free Software
17    Foundation; either version 2 of the License, or (at your option) any
18    later version.
19
20or both in parallel, as here.
21
22The GNU MP Library is distributed in the hope that it will be useful, but
23WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
25for more details.
26
27You should have received copies of the GNU General Public License and the
28GNU Lesser General Public License along with the GNU MP Library.  If not,
29see https://www.gnu.org/licenses/.  */
30
31#include "gmp-impl.h"
32
33
34/* For negatives the effective twos complement is achieved by negating the
35   limb tested, either with a ones or twos complement.  Twos complement
36   ("-") is used if there's only zero limbs below the one being tested.
37   Ones complement ("~") is used if there's a non-zero below.  Note that "-"
38   is correct even if the limb examined is 0 (and the true beginning of twos
39   complement is further up).
40
41   Testing the limbs below p is unavoidable on negatives, but will usually
42   need to examine only *(p-1).  The search is done from *(p-1) down to
43   *u_ptr, since that might give better cache locality, and because a
44   non-zero limb is perhaps a touch more likely in the middle of a number
45   than at the low end.
46
47   Bits past the end of available data simply follow sign of u.  Notice that
48   the limb_index >= abs_size test covers u=0 too.  */
49
50int
51mpz_tstbit (mpz_srcptr u, mp_bitcnt_t bit_index) __GMP_NOTHROW
52{
53  mp_srcptr      u_ptr      = PTR(u);
54  mp_size_t      size       = SIZ(u);
55  unsigned       abs_size   = ABS(size);
56  mp_size_t      limb_index = bit_index / GMP_NUMB_BITS;
57  mp_srcptr      p          = u_ptr + limb_index;
58  mp_limb_t      limb;
59
60  if (limb_index >= abs_size)
61    return (size < 0);
62
63  limb = *p;
64  if (size < 0)
65    {
66      limb = -limb;     /* twos complement */
67
68      while (p != u_ptr)
69	{
70	  p--;
71	  if (*p != 0)
72	    {
73	      limb--;	/* make it a ones complement instead */
74	      break;
75	    }
76	}
77    }
78
79  return (limb >> (bit_index % GMP_NUMB_BITS)) & 1;
80}
81