MySQL  8.0.18
Source Code Documentation
my_bit.h
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1 /*
2  Copyright (c) 2007, 2017, Oracle and/or its affiliates. All rights reserved.
3 
4  This program is free software; you can redistribute it and/or modify
5  it under the terms of the GNU General Public License, version 2.0,
6  as published by the Free Software Foundation.
7 
8  This program is also distributed with certain software (including
9  but not limited to OpenSSL) that is licensed under separate terms,
10  as designated in a particular file or component or in included license
11  documentation. The authors of MySQL hereby grant you an additional
12  permission to link the program and your derivative works with the
13  separately licensed software that they have included with MySQL.
14 
15  This program is distributed in the hope that it will be useful,
16  but WITHOUT ANY WARRANTY; without even the implied warranty of
17  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
18  GNU General Public License, version 2.0, for more details.
19 
20  You should have received a copy of the GNU General Public License
21  along with this program; if not, write to the Free Software
22  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
23 
24 #ifndef MY_BIT_INCLUDED
25 #define MY_BIT_INCLUDED
26 
27 /**
28  @file include/my_bit.h
29  Some useful bit functions.
30 */
31 
32 #include <sys/types.h>
33 
34 #include "my_config.h"
35 #include "my_inttypes.h"
36 #include "my_macros.h"
37 
38 extern const char _my_bits_nbits[256];
39 extern const uchar _my_bits_reverse_table[256];
40 
41 /*
42  Find smallest X in 2^X >= value
43  This can be used to divide a number with value by doing a shift instead
44 */
45 
46 static inline uint my_bit_log2(ulong value) {
47  uint bit;
48  for (bit = 0; value > 1; value >>= 1, bit++)
49  ;
50  return bit;
51 }
52 
53 static inline uint my_count_bits(ulonglong v) {
54 #if SIZEOF_LONG_LONG > 4
55  /* The following code is a bit faster on 16 bit machines than if we would
56  only shift v */
57  ulong v2 = (ulong)(v >> 32);
58  return (uint)(uchar)(
59  _my_bits_nbits[(uchar)v] + _my_bits_nbits[(uchar)(v >> 8)] +
60  _my_bits_nbits[(uchar)(v >> 16)] + _my_bits_nbits[(uchar)(v >> 24)] +
61  _my_bits_nbits[(uchar)(v2)] + _my_bits_nbits[(uchar)(v2 >> 8)] +
62  _my_bits_nbits[(uchar)(v2 >> 16)] + _my_bits_nbits[(uchar)(v2 >> 24)]);
63 #else
64  return (uint)(uchar)(
65  _my_bits_nbits[(uchar)v] + _my_bits_nbits[(uchar)(v >> 8)] +
66  _my_bits_nbits[(uchar)(v >> 16)] + _my_bits_nbits[(uchar)(v >> 24)]);
67 #endif
68 }
69 
70 static inline uint my_count_bits_uint32(uint32 v) {
71  return (uint)(uchar)(
72  _my_bits_nbits[(uchar)v] + _my_bits_nbits[(uchar)(v >> 8)] +
73  _my_bits_nbits[(uchar)(v >> 16)] + _my_bits_nbits[(uchar)(v >> 24)]);
74 }
75 
76 /*
77  Next highest power of two
78 
79  SYNOPSIS
80  my_round_up_to_next_power()
81  v Value to check
82 
83  RETURN
84  Next or equal power of 2
85  Note: 0 will return 0
86 
87  NOTES
88  Algorithm by Sean Anderson, according to:
89  http://graphics.stanford.edu/~seander/bithacks.html
90  (Orignal code public domain)
91 
92  Comments shows how this works with 01100000000000000000000000001011
93 */
94 
96  v--; /* 01100000000000000000000000001010 */
97  v |= v >> 1; /* 01110000000000000000000000001111 */
98  v |= v >> 2; /* 01111100000000000000000000001111 */
99  v |= v >> 4; /* 01111111110000000000000000001111 */
100  v |= v >> 8; /* 01111111111111111100000000001111 */
101  v |= v >> 16; /* 01111111111111111111111111111111 */
102  return v + 1; /* 10000000000000000000000000000000 */
103 }
104 
106  uint32 w = v >> 1;
107  w |= w >> 1;
108  w |= w >> 2;
109  w |= w >> 4;
110  w |= w >> 8;
111  w |= w >> 16;
112  return v & w;
113 }
114 
116  return (_my_bits_reverse_table[key & 255] << 24) |
117  (_my_bits_reverse_table[(key >> 8) & 255] << 16) |
118  (_my_bits_reverse_table[(key >> 16) & 255] << 8) |
119  _my_bits_reverse_table[(key >> 24)];
120 }
121 
122 /**
123  Determine if a single bit is set among some bits.
124  @tparam IntType an integer type
125  @param bits the bits to examine
126  @retval true if bits equals to a power of 2
127  @retval false otherwise
128 */
129 
130 template <typename IntType>
131 constexpr bool is_single_bit(IntType bits) {
132  /*
133  Proof of correctness:
134  (1) is_single_bit(0)==false is left as an exercise to the reader.
135  (2) is_single_bit(1)==true is left as an exercise to the reader.
136  (3) is_single_bit(1<<(N+1))==true because the most significant set bit
137  in (bits - 1) would be 1<<N, and obviously (bits & (bits - 1)) == 0.
138  (4) In all other cases, is_single_bit(bits)==false.
139  In these cases, we must have multiple bits set, that is,
140  bits==m|1<<N such that N>=0 and m!=0 and the least significant
141  bit that is set in m is greater than 1<<N. In this case,
142  (bits-1)==m|((1<<N)-1), and (bits&(bits-1))==m, which we defined to be
143  nonzero. So, m==0 will not hold.
144 
145  Note: The above proof (3),(4) is applicable also to the case where
146  IntType is signed using two's complement arithmetics, and the most
147  significant bit is set, or in other words, bits<0.
148  */
149  return bits != 0 && (bits & (bits - 1)) == 0;
150 }
151 
152 #endif /* MY_BIT_INCLUDED */
Some common macros.
unsigned long long int ulonglong
Definition: my_inttypes.h:55
unsigned char uchar
Definition: my_inttypes.h:51
static uint my_count_bits_uint32(uint32 v)
Definition: my_bit.h:70
static uint my_count_bits(ulonglong v)
Definition: my_bit.h:53
Some integer typedefs for easier portability.
constexpr bool is_single_bit(IntType bits)
Determine if a single bit is set among some bits.
Definition: my_bit.h:131
const uchar _my_bits_reverse_table[256]
Definition: my_bit.cc:51
static uint32 my_round_up_to_next_power(uint32 v)
Definition: my_bit.h:95
static uint my_bit_log2(ulong value)
Definition: my_bit.h:46
unsigned int uint
Definition: uca-dump.cc:29
static uint32 my_clear_highest_bit(uint32 v)
Definition: my_bit.h:105
uint32_t uint32
Definition: my_inttypes.h:66
static const char * key
Definition: suite_stubs.c:14
static uint32 my_reverse_bits(uint32 key)
Definition: my_bit.h:115
const string value("\alue\)
unsigned long ulong
Definition: my_inttypes.h:48
const char _my_bits_nbits[256]
Definition: my_bit.cc:34