1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
|
/* 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 3 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. If not, see http://www.gnu.org/licenses/. */
#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;
}
|
