diff options
Diffstat (limited to 'Source/JavaScriptCore/wtf/BloomFilter.h')
-rw-r--r-- | Source/JavaScriptCore/wtf/BloomFilter.h | 139 |
1 files changed, 139 insertions, 0 deletions
diff --git a/Source/JavaScriptCore/wtf/BloomFilter.h b/Source/JavaScriptCore/wtf/BloomFilter.h new file mode 100644 index 0000000..f81d83e --- /dev/null +++ b/Source/JavaScriptCore/wtf/BloomFilter.h @@ -0,0 +1,139 @@ +/* + * Copyright (C) 2011 Apple Inc. All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions + * are met: + * 1. Redistributions of source code must retain the above copyright + * notice, this list of conditions and the following disclaimer. + * 2. Redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in the + * documentation and/or other materials provided with the distribution. + * + * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, + * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR + * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS + * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF + * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS + * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN + * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) + * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF + * THE POSSIBILITY OF SUCH DAMAGE. + */ + +#ifndef BloomFilter_h +#define BloomFilter_h + +#include <wtf/AlwaysInline.h> +#include <wtf/text/AtomicString.h> + +namespace WTF { + +// Counting bloom filter with k=2 and 8 bit counters. Uses 2^keyBits bytes of memory. +// False positive rate is approximately (1-e^(-2n/m))^2, where n is the number of unique +// keys and m is the table size (==2^keyBits). +template <unsigned keyBits> +class BloomFilter { +public: + COMPILE_ASSERT(keyBits <= 16, bloom_filter_key_size); + + static const size_t tableSize = 1 << keyBits; + static const unsigned keyMask = (1 << keyBits) - 1; + static uint8_t maximumCount() { return std::numeric_limits<uint8_t>::max(); } + + BloomFilter() { clear(); } + + void add(unsigned hash); + void remove(unsigned hash); + + // The filter may give false positives (claim it may contain a key it doesn't) + // but never false negatives (claim it doesn't contain a key it does). + bool mayContain(unsigned hash) const { return firstSlot(hash) && secondSlot(hash); } + + // The filter must be cleared before reuse even if all keys are removed. + // Otherwise overflowed keys will stick around. + void clear(); + + void add(const AtomicString& string) { add(string.impl()->existingHash()); } + void add(const String& string) { add(string.impl()->hash()); } + void remove(const AtomicString& string) { remove(string.impl()->existingHash()); } + void remove(const String& string) { remove(string.impl()->hash()); } + + bool mayContain(const AtomicString& string) const { return mayContain(string.impl()->existingHash()); } + bool mayContain(const String& string) const { return mayContain(string.impl()->hash()); } + +#if !ASSERT_DISABLED + // Slow. + bool likelyEmpty() const; + bool isClear() const; +#endif + +private: + uint8_t& firstSlot(unsigned hash) { return m_table[hash & keyMask]; } + uint8_t& secondSlot(unsigned hash) { return m_table[(hash >> 16) & keyMask]; } + const uint8_t& firstSlot(unsigned hash) const { return m_table[hash & keyMask]; } + const uint8_t& secondSlot(unsigned hash) const { return m_table[(hash >> 16) & keyMask]; } + + uint8_t m_table[tableSize]; +}; + +template <unsigned keyBits> +inline void BloomFilter<keyBits>::add(unsigned hash) +{ + uint8_t& first = firstSlot(hash); + uint8_t& second = secondSlot(hash); + if (LIKELY(first < maximumCount())) + ++first; + if (LIKELY(second < maximumCount())) + ++second; +} + +template <unsigned keyBits> +inline void BloomFilter<keyBits>::remove(unsigned hash) +{ + uint8_t& first = firstSlot(hash); + uint8_t& second = secondSlot(hash); + ASSERT(first); + ASSERT(second); + // In case of an overflow, the slot sticks in the table until clear(). + if (LIKELY(first < maximumCount())) + --first; + if (LIKELY(second < maximumCount())) + --second; +} + +template <unsigned keyBits> +inline void BloomFilter<keyBits>::clear() +{ + memset(m_table, 0, tableSize); +} + +#if !ASSERT_DISABLED +template <unsigned keyBits> +bool BloomFilter<keyBits>::likelyEmpty() const +{ + for (size_t n = 0; n < tableSize; ++n) { + if (m_table[n] && m_table[n] != maximumCount()) + return false; + } + return true; +} + +template <unsigned keyBits> +bool BloomFilter<keyBits>::isClear() const +{ + for (size_t n = 0; n < tableSize; ++n) { + if (m_table[n]) + return false; + } + return true; +} +#endif + +} + +using WTF::BloomFilter; + +#endif |