1 /*
2 * Copyright (C) 2011 Apple Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
14 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
15 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
17 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
18 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
19 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
20 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
21 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
22 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
23 * THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26 #ifndef BloomFilter_h
27 #define BloomFilter_h
28
29 #include "wtf/Compiler.h"
30 #include "wtf/text/AtomicString.h"
31
32 namespace WTF {
33
34 // Counting bloom filter with k=2 and 8 bit counters. Uses 2^keyBits bytes of memory.
35 // False positive rate is approximately (1-e^(-2n/m))^2, where n is the number of unique
36 // keys and m is the table size (==2^keyBits).
37 template <unsigned keyBits>
38 class BloomFilter {
39 public:
40 COMPILE_ASSERT(keyBits <= 16, bloom_filter_key_size);
41
42 static const size_t tableSize = 1 << keyBits;
43 static const unsigned keyMask = (1 << keyBits) - 1;
maximumCount()44 static uint8_t maximumCount() { return std::numeric_limits<uint8_t>::max(); }
45
BloomFilter()46 BloomFilter() { clear(); }
47
48 void add(unsigned hash);
49 void remove(unsigned hash);
50
51 // The filter may give false positives (claim it may contain a key it doesn't)
52 // but never false negatives (claim it doesn't contain a key it does).
mayContain(unsigned hash)53 bool mayContain(unsigned hash) const { return firstSlot(hash) && secondSlot(hash); }
54
55 // The filter must be cleared before reuse even if all keys are removed.
56 // Otherwise overflowed keys will stick around.
57 void clear();
58
add(const AtomicString & string)59 void add(const AtomicString& string) { add(string.impl()->existingHash()); }
add(const String & string)60 void add(const String& string) { add(string.impl()->hash()); }
remove(const AtomicString & string)61 void remove(const AtomicString& string) { remove(string.impl()->existingHash()); }
remove(const String & string)62 void remove(const String& string) { remove(string.impl()->hash()); }
63
mayContain(const AtomicString & string)64 bool mayContain(const AtomicString& string) const { return mayContain(string.impl()->existingHash()); }
mayContain(const String & string)65 bool mayContain(const String& string) const { return mayContain(string.impl()->hash()); }
66
67 #if ASSERT_ENABLED
68 // Slow.
69 bool likelyEmpty() const;
70 bool isClear() const;
71 #endif
72
73 private:
firstSlot(unsigned hash)74 uint8_t& firstSlot(unsigned hash) { return m_table[hash & keyMask]; }
secondSlot(unsigned hash)75 uint8_t& secondSlot(unsigned hash) { return m_table[(hash >> 16) & keyMask]; }
firstSlot(unsigned hash)76 const uint8_t& firstSlot(unsigned hash) const { return m_table[hash & keyMask]; }
secondSlot(unsigned hash)77 const uint8_t& secondSlot(unsigned hash) const { return m_table[(hash >> 16) & keyMask]; }
78
79 uint8_t m_table[tableSize];
80 };
81
82 template <unsigned keyBits>
add(unsigned hash)83 inline void BloomFilter<keyBits>::add(unsigned hash)
84 {
85 uint8_t& first = firstSlot(hash);
86 uint8_t& second = secondSlot(hash);
87 if (LIKELY(first < maximumCount()))
88 ++first;
89 if (LIKELY(second < maximumCount()))
90 ++second;
91 }
92
93 template <unsigned keyBits>
remove(unsigned hash)94 inline void BloomFilter<keyBits>::remove(unsigned hash)
95 {
96 uint8_t& first = firstSlot(hash);
97 uint8_t& second = secondSlot(hash);
98 ASSERT(first);
99 ASSERT(second);
100 // In case of an overflow, the slot sticks in the table until clear().
101 if (LIKELY(first < maximumCount()))
102 --first;
103 if (LIKELY(second < maximumCount()))
104 --second;
105 }
106
107 template <unsigned keyBits>
clear()108 inline void BloomFilter<keyBits>::clear()
109 {
110 memset(m_table, 0, tableSize);
111 }
112
113 #if ASSERT_ENABLED
114 template <unsigned keyBits>
likelyEmpty()115 bool BloomFilter<keyBits>::likelyEmpty() const
116 {
117 for (size_t n = 0; n < tableSize; ++n) {
118 if (m_table[n] && m_table[n] != maximumCount())
119 return false;
120 }
121 return true;
122 }
123
124 template <unsigned keyBits>
isClear()125 bool BloomFilter<keyBits>::isClear() const
126 {
127 for (size_t n = 0; n < tableSize; ++n) {
128 if (m_table[n])
129 return false;
130 }
131 return true;
132 }
133 #endif
134
135 }
136
137 using WTF::BloomFilter;
138
139 #endif
140