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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root 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.
#pragma once
#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;
// if it is a wrapped comparator, may return the root one.
// return itself it is not wrapped.
virtual const Comparator* GetRootComparator() const { return this; }
// given two keys, determine if t is the successor of s
virtual bool IsSameLengthImmediateSuccessor(const Slice& /*s*/,
const Slice& /*t*/) const {
return false;
}
// return true if two keys with different byte sequences can be regarded
// as equal by this comparator.
// The major use case is to determine if DataBlockHashIndex is compatible
// with the customized comparator.
virtual bool CanKeysWithDifferentByteContentsBeEqual() const { return true; }
};
// 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
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