src/storage/existence_filter.h

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#ifndef MOZC_STORAGE_EXISTENCE_FILTER_H_
#define MOZC_STORAGE_EXISTENCE_FILTER_H_

#include "base/port.h"
#include "base/scoped_ptr.h"

namespace mozc {
namespace storage {

// Bloom filter
class ExistenceFilter {
 public:
  struct Header {
    uint32 m;
    uint32 n;
    int k;
  };

  // 'm' is the number of bits in the bit vector
  // 'n' is the number of values that will be stored
  // 'k' is the number of hash values to use per insert/lookup
  // k must be less than 8
  ExistenceFilter(uint32 m, uint32 n, int k);
  ~ExistenceFilter();

  static ExistenceFilter* CreateOptimal(size_t size_in_bytes,
                                        uint32 estimated_insertions);

  void Clear();

  // Inserts a hash value into the filter
  // We generate 'k' separate internal hash values
  void Insert(uint64 hash);

  // Checks if the given 'hash' was previously inserted int the filter
  // It may return some false positives
  bool Exists(uint64 hash) const;

  // Returns the size (in bytes) of the bloom filter
  size_t Size() const;

  // Returns the minimum required size of the filter in bytes
  // under the given error rate and number of elements
  static size_t MinFilterSizeInBytesForErrorRate(float error_rate,
                                                 size_t num_elements);

  void Write(char **buf, size_t *size);

  static bool ReadHeader(const char *buf, Header* header);

  // Read Existence filter from buf[]
  // Note that the returned ExsitenceFilter is immutable filter.
  // Any mutable operations will destroy buf[].
  static ExistenceFilter* Read(const char *buf, size_t size);

 private:
  class BlockBitmap;

  // private constructor for ExistenceFilter::Read();
  ExistenceFilter(uint32 m, uint32 n, int k, bool is_mutable);

  static ExistenceFilter *CreateImmutableExietenceFilter(uint32 m,
                                                         uint32 n,
                                                         int k);

  scoped_ptr<BlockBitmap> rep_;  // points to bitmap
  const uint32 vec_size_;  // size of bitmap (in bits)
  const uint32 expected_nelts_;  // expected number of inserts
  const int32 num_hashes_;  // number of hashes per lookup

  DISALLOW_COPY_AND_ASSIGN(ExistenceFilter);
};

}  // namespace storage
}  // namespace mozc

#endif  // MOZC_STORAGE_EXISTENCE_FILTER_H_