BloomFilter.h

/*
 * 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/Allocator.h"
#include "wtf/Compiler.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 {
  USING_FAST_MALLOC(BloomFilter);

 public:
  static_assert(keyBits <= 16, "bloom filter key size check");

  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 DCHECK_IS_ON()
  // 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);
  DCHECK(first);
  DCHECK(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 DCHECK_IS_ON()
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

}  // namespace WTF

using WTF::BloomFilter;

#endif