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// 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>
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 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.
extern const Comparator* BytewiseComparator();
} // namespace leveldb
#endif // STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_
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