1 // Copyright (c) 2012 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.
5 // A database can be configured with a custom FilterPolicy object.
6 // This object is responsible for creating a small filter from a set
7 // of keys. These filters are stored in leveldb and are consulted
8 // automatically by leveldb to decide whether or not to read some
9 // information from disk. In many cases, a filter can cut down the
10 // number of disk seeks form a handful to a single disk seek per
13 // Most people will want to use the builtin bloom filter support (see
14 // NewBloomFilterPolicy() below).
16 #ifndef STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_
17 #define STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_
27 virtual ~FilterPolicy();
29 // Return the name of this policy. Note that if the filter encoding
30 // changes in an incompatible way, the name returned by this method
31 // must be changed. Otherwise, old incompatible filters may be
32 // passed to methods of this type.
33 virtual const char* Name() const = 0;
35 // keys[0,n-1] contains a list of keys (potentially with duplicates)
36 // that are ordered according to the user supplied comparator.
37 // Append a filter that summarizes keys[0,n-1] to *dst.
39 // Warning: do not change the initial contents of *dst. Instead,
40 // append the newly constructed filter to *dst.
41 virtual void CreateFilter(const Slice* keys, int n, std::string* dst)
44 // "filter" contains the data appended by a preceding call to
45 // CreateFilter() on this class. This method must return true if
46 // the key was in the list of keys passed to CreateFilter().
47 // This method may return true or false if the key was not on the
48 // list, but it should aim to return false with a high probability.
49 virtual bool KeyMayMatch(const Slice& key, const Slice& filter) const = 0;
52 // Return a new filter policy that uses a bloom filter with approximately
53 // the specified number of bits per key. A good value for bits_per_key
54 // is 10, which yields a filter with ~ 1% false positive rate.
56 // Callers must delete the result after any database that is using the
57 // result has been closed.
59 // Note: if you are using a custom comparator that ignores some parts
60 // of the keys being compared, you must not use NewBloomFilterPolicy()
61 // and must provide your own FilterPolicy that also ignores the
62 // corresponding parts of the keys. For example, if the comparator
63 // ignores trailing spaces, it would be incorrect to use a
64 // FilterPolicy (like NewBloomFilterPolicy) that does not ignore
65 // trailing spaces in keys.
66 extern const FilterPolicy* NewBloomFilterPolicy(int bits_per_key);
70 #endif // STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_