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Diffstat (limited to 'include/leveldb/filter_policy.h')
-rw-r--r-- | include/leveldb/filter_policy.h | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/include/leveldb/filter_policy.h b/include/leveldb/filter_policy.h new file mode 100644 index 0000000..1fba080 --- /dev/null +++ b/include/leveldb/filter_policy.h @@ -0,0 +1,70 @@ +// 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. +// +// A database can be configured with a custom FilterPolicy object. +// This object is responsible for creating a small filter from a set +// of keys. These filters are stored in leveldb and are consulted +// automatically by leveldb to decide whether or not to read some +// information from disk. In many cases, a filter can cut down the +// number of disk seeks form a handful to a single disk seek per +// DB::Get() call. +// +// Most people will want to use the builtin bloom filter support (see +// NewBloomFilterPolicy() below). + +#ifndef STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_ +#define STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_ + +#include <string> + +namespace leveldb { + +class Slice; + +class FilterPolicy { + public: + virtual ~FilterPolicy(); + + // Return the name of this policy. Note that if the filter encoding + // changes in an incompatible way, the name returned by this method + // must be changed. Otherwise, old incompatible filters may be + // passed to methods of this type. + virtual const char* Name() const = 0; + + // keys[0,n-1] contains a list of keys (potentially with duplicates) + // that are ordered according to the user supplied comparator. + // Append a filter that summarizes keys[0,n-1] to *dst. + // + // Warning: do not change the initial contents of *dst. Instead, + // append the newly constructed filter to *dst. + virtual void CreateFilter(const Slice* keys, int n, std::string* dst) + const = 0; + + // "filter" contains the data appended by a preceding call to + // CreateFilter() on this class. This method must return true if + // the key was in the list of keys passed to CreateFilter(). + // This method may return true or false if the key was not on the + // list, but it should aim to return false with a high probability. + virtual bool KeyMayMatch(const Slice& key, const Slice& filter) const = 0; +}; + +// Return a new filter policy that uses a bloom filter with approximately +// the specified number of bits per key. A good value for bits_per_key +// is 10, which yields a filter with ~ 1% false positive rate. +// +// Callers must delete the result after any database that is using the +// result has been closed. +// +// Note: if you are using a custom comparator that ignores some parts +// of the keys being compared, you must not use NewBloomFilterPolicy() +// and must provide your own FilterPolicy that also ignores the +// corresponding parts of the keys. For example, if the comparator +// ignores trailing spaces, it would be incorrect to use a +// FilterPolicy (like NewBloomFilterPolicy) that does not ignore +// trailing spaces in keys. +extern const FilterPolicy* NewBloomFilterPolicy(int bits_per_key); + +} + +#endif // STORAGE_LEVELDB_INCLUDE_FILTER_POLICY_H_ |