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/*
* 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
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