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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) 2012 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.
// Class for specifying user-defined functions which perform a
// transformation on a slice. It is not required that every slice
// belong to the domain and/or range of a function. Subclasses should
// define InDomain and InRange to determine which slices are in either
// of these sets respectively.
#include <string>
namespace rocksdb {
class Slice;
* A SliceTranform is a generic pluggable way of transforming one string
* to another. Its primary use-case is in configuring rocksdb
* to store prefix blooms by setting prefix_extractor in
* ColumnFamilyOptions.
class SliceTransform {
virtual ~SliceTransform() {};
// Return the name of this transformation.
virtual const char* Name() const = 0;
// Extract a prefix from a specified key. This method is called when
// a key is inserted into the db, and the returned slice is used to
// create a bloom filter.
virtual Slice Transform(const Slice& key) const = 0;
// Determine whether the specified key is compatible with the logic
// specified in the Transform method. This method is invoked for every
// key that is inserted into the db. If this method returns true,
// then Transform is called to translate the key to its prefix and
// that returned prefix is inserted into the bloom filter. If this
// method returns false, then the call to Transform is skipped and
// no prefix is inserted into the bloom filters.
// For example, if the Transform method operates on a fixed length
// prefix of size 4, then an invocation to InDomain("abc") returns
// false because the specified key length(3) is shorter than the
// prefix size of 4.
// Wiki documentation here:
virtual bool InDomain(const Slice& key) const = 0;
// This is currently not used and remains here for backward compatibility.
virtual bool InRange(const Slice& dst) const { return false; }
// Transform(s)=Transform(`prefix`) for any s with `prefix` as a prefix.
// This function is not used by RocksDB, but for users. If users pass
// Options by string to RocksDB, they might not know what prefix extractor
// they are using. This function is to help users can determine:
// if they want to iterate all keys prefixing `prefix`, whether it is
// safe to use prefix bloom filter and seek to key `prefix`.
// If this function returns true, this means a user can Seek() to a prefix
// using the bloom filter. Otherwise, user needs to skip the bloom filter
// by setting ReadOptions.total_order_seek = true.
// Here is an example: Suppose we implement a slice transform that returns
// the first part of the string after spliting it using delimiter ",":
// 1. SameResultWhenAppended("abc,") should return true. If applying prefix
// bloom filter using it, all slices matching "abc:.*" will be extracted
// to "abc,", so any SST file or memtable containing any of those key
// will not be filtered out.
// 2. SameResultWhenAppended("abc") should return false. A user will not
// guaranteed to see all the keys matching "abc.*" if a user seek to "abc"
// against a DB with the same setting. If one SST file only contains
// "abcd,e", the file can be filtered out and the key will be invisible.
// i.e., an implementation always returning false is safe.
virtual bool SameResultWhenAppended(const Slice& prefix) const {
return false;
extern const SliceTransform* NewFixedPrefixTransform(size_t prefix_len);
extern const SliceTransform* NewCappedPrefixTransform(size_t cap_len);
extern const SliceTransform* NewNoopTransform();