rocksdb/include/rocksdb/comparator.h

// Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#ifndef STORAGE_ROCKSDB_INCLUDE_COMPARATOR_H_
#define STORAGE_ROCKSDB_INCLUDE_COMPARATOR_H_

#include <string>

namespace rocksdb {

class Slice;

// A Comparator object provides a total order across slices that are
// used as keys in an sstable or a database.  A Comparator implementation
// must be thread-safe since rocksdb may invoke its methods concurrently
// from multiple threads.
class Comparator {
 public:
  virtual ~Comparator();

  // Three-way comparison.  Returns value:
  //   < 0 iff "a" < "b",
  //   == 0 iff "a" == "b",
  //   > 0 iff "a" > "b"
  virtual int Compare(const Slice& a, const Slice& b) const = 0;

  // Compares two slices for equality. The following invariant should always
  // hold (and is the default implementation):
  //   Equal(a, b) iff Compare(a, b) == 0
  // Overwrite only if equality comparisons can be done more efficiently than
  // three-way comparisons.
  virtual bool Equal(const Slice& a, const Slice& b) const {
    return Compare(a, b) == 0;
  }

  // The name of the comparator.  Used to check for comparator
  // mismatches (i.e., a DB created with one comparator is
  // accessed using a different comparator.
  //
  // The client of this package should switch to a new name whenever
  // the comparator implementation changes in a way that will cause
  // the relative ordering of any two keys to change.
  //
  // Names starting with "rocksdb." are reserved and should not be used
  // by any clients of this package.
  virtual const char* Name() const = 0;

  // Advanced functions: these are used to reduce the space requirements
  // for internal data structures like index blocks.

  // If *start < limit, changes *start to a short string in [start,limit).
  // Simple comparator implementations may return with *start unchanged,
  // i.e., an implementation of this method that does nothing is correct.
  virtual void FindShortestSeparator(
      std::string* start,
      const Slice& limit) const = 0;

  // Changes *key to a short string >= *key.
  // Simple comparator implementations may return with *key unchanged,
  // i.e., an implementation of this method that does nothing is correct.
  virtual void FindShortSuccessor(std::string* key) const = 0;
};

// Return a builtin comparator that uses lexicographic byte-wise
// ordering.  The result remains the property of this module and
// must not be deleted.
extern const Comparator* BytewiseComparator();

// Return a builtin comparator that uses reverse lexicographic byte-wise
// ordering.
extern const Comparator* ReverseBytewiseComparator();

}  // namespace rocksdb

#endif  // STORAGE_ROCKSDB_INCLUDE_COMPARATOR_H_