1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. See the AUTHORS file for names of contributors. 4 5 #ifndef STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_ 6 #define STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_ 7 8 #include <string> 9 10 namespace leveldb { 11 12 class Slice; 13 14 // A Comparator object provides a total order across slices that are 15 // used as keys in an sstable or a database. A Comparator implementation 16 // must be thread-safe since leveldb may invoke its methods concurrently 17 // from multiple threads. 18 class Comparator { 19 public: 20 virtual ~Comparator(); 21 22 // Three-way comparison. Returns value: 23 // < 0 iff "a" < "b", 24 // == 0 iff "a" == "b", 25 // > 0 iff "a" > "b" 26 virtual int Compare(const Slice& a, const Slice& b) const = 0; 27 28 // The name of the comparator. Used to check for comparator 29 // mismatches (i.e., a DB created with one comparator is 30 // accessed using a different comparator. 31 // 32 // The client of this package should switch to a new name whenever 33 // the comparator implementation changes in a way that will cause 34 // the relative ordering of any two keys to change. 35 // 36 // Names starting with "leveldb." are reserved and should not be used 37 // by any clients of this package. 38 virtual const char* Name() const = 0; 39 40 // Advanced functions: these are used to reduce the space requirements 41 // for internal data structures like index blocks. 42 43 // If *start < limit, changes *start to a short string in [start,limit). 44 // Simple comparator implementations may return with *start unchanged, 45 // i.e., an implementation of this method that does nothing is correct. 46 virtual void FindShortestSeparator( 47 std::string* start, 48 const Slice& limit) const = 0; 49 50 // Changes *key to a short string >= *key. 51 // Simple comparator implementations may return with *key unchanged, 52 // i.e., an implementation of this method that does nothing is correct. 53 virtual void FindShortSuccessor(std::string* key) const = 0; 54 }; 55 56 // Return a builtin comparator that uses lexicographic byte-wise 57 // ordering. The result remains the property of this module and 58 // must not be deleted. 59 extern const Comparator* BytewiseComparator(); 60 61 } // namespace leveldb 62 63 #endif // STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_ 64
