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
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
|
/* mpz_scan0 -- search for a 0 bit.
Copyright 2000, 2001, 2002, 2004 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"
#include "longlong.h"
/* mpn_scan0 can't be used for the u>0 search since there might not be a 0
bit before the end of the data. mpn_scan1 could be used for the inverted
search under u<0, but usually the search won't go very far so it seems
reasonable to inline that code. */
mp_bitcnt_t
mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW
{
mp_srcptr u_ptr = PTR(u);
mp_size_t size = SIZ(u);
mp_size_t abs_size = ABS(size);
mp_srcptr u_end = u_ptr + abs_size;
mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS;
mp_srcptr p = u_ptr + starting_limb;
mp_limb_t limb;
int cnt;
/* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
u<0. Notice this test picks up all cases of u==0 too. */
if (starting_limb >= abs_size)
return (size >= 0 ? starting_bit : ULONG_MAX);
limb = *p;
if (size >= 0)
{
/* Mask to 1 all bits before starting_bit, thus ignoring them. */
limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
/* Search for a limb which isn't all ones. If the end is reached then
the zero bit immediately past the end is returned. */
while (limb == GMP_NUMB_MAX)
{
p++;
if (p == u_end)
return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
limb = *p;
}
/* Now seek low 1 bit. */
limb = ~limb;
}
else
{
mp_srcptr q;
/* If there's a non-zero limb before ours then we're in the ones
complement region. Search from *(p-1) downwards since that might
give better cache locality, and since a non-zero in the middle of a
number is perhaps a touch more likely than at the end. */
q = p;
while (q != u_ptr)
{
q--;
if (*q != 0)
goto inverted;
}
/* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
If limb==0 here then this isn't the beginning of twos complement
inversion, but that doesn't matter because limb==0 is a zero bit
immediately (-1 is all ones for below). */
limb--;
inverted:
/* Now seeking a 1 bit. */
/* Mask to 0 all bits before starting_bit, thus ignoring them. */
limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
if (limb == 0)
{
/* If the high limb is zero after masking, then no 1 bits past
starting_bit. */
p++;
if (p == u_end)
return ULONG_MAX;
/* Search further for a non-zero limb. The high limb is non-zero,
if nothing else. */
for (;;)
{
limb = *p;
if (limb != 0)
break;
p++;
ASSERT (p < u_end);
}
}
}
ASSERT (limb != 0);
count_trailing_zeros (cnt, limb);
return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
}
|
