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64 righe
2.4 KiB
64 righe
2.4 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_
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#define STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_
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#include <string>
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#include "leveldb/export.h"
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namespace leveldb {
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class Slice;
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// A Comparator object provides a total order across slices that are
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// used as keys in an sstable or a database. A Comparator implementation
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// must be thread-safe since leveldb may invoke its methods concurrently
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// from multiple threads.
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class LEVELDB_EXPORT Comparator {
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public:
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virtual ~Comparator();
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// Three-way comparison. Returns value:
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// < 0 iff "a" < "b",
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// == 0 iff "a" == "b",
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// > 0 iff "a" > "b"
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virtual int Compare(const Slice& a, const Slice& b) const = 0;
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// The name of the comparator. Used to check for comparator
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// mismatches (i.e., a DB created with one comparator is
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// accessed using a different comparator.
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//
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// The client of this package should switch to a new name whenever
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// the comparator implementation changes in a way that will cause
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// the relative ordering of any two keys to change.
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//
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// Names starting with "leveldb." are reserved and should not be used
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// by any clients of this package.
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virtual const char* Name() const = 0;
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// Advanced functions: these are used to reduce the space requirements
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// for internal data structures like index blocks.
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// If *start < limit, changes *start to a short string in [start,limit).
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// Simple comparator implementations may return with *start unchanged,
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// i.e., an implementation of this method that does nothing is correct.
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virtual void FindShortestSeparator(std::string* start,
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const Slice& limit) const = 0;
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// Changes *key to a short string >= *key.
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// Simple comparator implementations may return with *key unchanged,
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// i.e., an implementation of this method that does nothing is correct.
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virtual void FindShortSuccessor(std::string* key) const = 0;
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};
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// Return a builtin comparator that uses lexicographic byte-wise
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// ordering. The result remains the property of this module and
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// must not be deleted.
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LEVELDB_EXPORT const Comparator* BytewiseComparator();
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_INCLUDE_COMPARATOR_H_
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