// 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_LEVELDB_INCLUDE_COMPARATOR_H_ #define STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_ #include <string> #include "leveldb/export.h" namespace leveldb { 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 leveldb may invoke its methods concurrently // from multiple threads. class LEVELDB_EXPORT 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; // 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 "leveldb." 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. LEVELDB_EXPORT const Comparator* BytewiseComparator(); } // namespace leveldb #endif // STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_