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| <article class="content wrap" id="_content" data-uid="Lucene.Net.Codecs.Bloom"> |
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| <h1 id="Lucene_Net_Codecs_Bloom" data-uid="Lucene.Net.Codecs.Bloom" class="text-break">Namespace Lucene.Net.Codecs.Bloom |
| </h1> |
| <div class="markdown level0 summary"><!-- |
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| <p>Codec PostingsFormat for fast access to low-frequency terms such as primary key fields.</p> |
| </div> |
| <div class="markdown level0 conceptual"></div> |
| <div class="markdown level0 remarks"></div> |
| <h3 id="classes">Classes |
| </h3> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.BloomFilterFactory.html">BloomFilterFactory</a></h4> |
| <section><p>Class used to create index-time <a class="xref" href="Lucene.Net.Codecs.Bloom.FuzzySet.html">FuzzySet</a> appropriately configured for |
| each field. Also called to right-size bitsets for serialization. |
| <p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.BloomFilteringPostingsFormat.html">BloomFilteringPostingsFormat</a></h4> |
| <section><p>A <span class="xref">Lucene.Net.Codecs.PostingsFormat</span> useful for low doc-frequency fields such as primary |
| keys. Bloom filters are maintained in a ".blm" file which offers "fast-fail" |
| for reads in segments known to have no record of the key. A choice of |
| delegate <span class="xref">Lucene.Net.Codecs.PostingsFormat</span> is used to record all other Postings data. |
| <p> |
| A choice of <a class="xref" href="Lucene.Net.Codecs.Bloom.BloomFilterFactory.html">BloomFilterFactory</a> can be passed to tailor Bloom Filter |
| settings on a per-field basis. The default configuration is |
| <a class="xref" href="Lucene.Net.Codecs.Bloom.DefaultBloomFilterFactory.html">DefaultBloomFilterFactory</a> which allocates a ~8mb bitset and hashes |
| values using <a class="xref" href="Lucene.Net.Codecs.Bloom.MurmurHash2.html">MurmurHash2</a>. This should be suitable for most purposes. |
| <p> |
| The format of the blm file is as follows:</p> |
| <p><ul><li>BloomFilter (.blm) --> Header, DelegatePostingsFormatName, |
| NumFilteredFields, Filter<sup>NumFilteredFields</sup>, Footer</li><li>Filter --> FieldNumber, FuzzySet</li><li>FuzzySet -->See <a class="xref" href="Lucene.Net.Codecs.Bloom.FuzzySet.html#Lucene_Net_Codecs_Bloom_FuzzySet_Serialize_Lucene_Net_Store_DataOutput_">Serialize(DataOutput)</a></li><li>Header --> CodecHeader (<span class="xref">Lucene.Net.Codecs.CodecUtil.WriteHeader(Lucene.Net.Store.DataOutput,System.String,System.Int32)</span>) </li><li>DelegatePostingsFormatName --> String (<span class="xref">Lucene.Net.Store.DataOutput.WriteString(System.String)</span>) |
| The name of a ServiceProvider registered <span class="xref">Lucene.Net.Codecs.PostingsFormat</span></li><li>NumFilteredFields --> Uint32 (<span class="xref">Lucene.Net.Store.DataOutput.WriteInt32(System.Int32)</span>) </li><li>FieldNumber --> Uint32 (<span class="xref">Lucene.Net.Store.DataOutput.WriteInt32(System.Int32)</span>) The number of the |
| field in this segment</li><li>Footer --> CodecFooter (<span class="xref">Lucene.Net.Codecs.CodecUtil.WriteFooter(Lucene.Net.Store.IndexOutput)</span>) </li></ul> |
| <p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.DefaultBloomFilterFactory.html">DefaultBloomFilterFactory</a></h4> |
| <section><p>Default policy is to allocate a bitset with 10% saturation given a unique term per document. |
| Bits are set via <a class="xref" href="Lucene.Net.Codecs.Bloom.MurmurHash2.html">MurmurHash2</a> hashing function. |
| <p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.FuzzySet.html">FuzzySet</a></h4> |
| <section><p>A class used to represent a set of many, potentially large, values (e.g. many |
| long strings such as URLs), using a significantly smaller amount of memory. |
| <p> |
| The set is "lossy" in that it cannot definitively state that is does contain |
| a value but it <em>can</em> definitively say if a value is <em>not</em> in |
| the set. It can therefore be used as a Bloom Filter. |
| <p> |
| Another application of the set is that it can be used to perform fuzzy counting because |
| it can estimate reasonably accurately how many unique values are contained in the set. |
| <p> |
| This class is NOT threadsafe. |
| <p> |
| Internally a Bitset is used to record values and once a client has finished recording |
| a stream of values the <a class="xref" href="Lucene.Net.Codecs.Bloom.FuzzySet.html#Lucene_Net_Codecs_Bloom_FuzzySet_Downsize_System_Single_">Downsize(Single)</a> method can be used to create a suitably smaller set that |
| is sized appropriately for the number of values recorded and desired saturation levels. |
| <p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.HashFunction.html">HashFunction</a></h4> |
| <section><p>Base class for hashing functions that can be referred to by name. |
| Subclasses are expected to provide threadsafe implementations of the hash function |
| on the range of bytes referenced in the provided <span class="xref">Lucene.Net.Util.BytesRef</span>. |
| <p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.MurmurHash2.html">MurmurHash2</a></h4> |
| <section><p>This is a very fast, non-cryptographic hash suitable for general hash-based |
| lookup. See <a href="http://murmurhash.googlepages.com/">http://murmurhash.googlepages.com/</a> for more details. |
| <p> |
| The C version of MurmurHash 2.0 found at that site was ported to Java by |
| Andrzej Bialecki (ab at getopt org). |
| <p> |
| The code from getopt.org was adapted by Mark Harwood in the form here as one of a pluggable choice of |
| hashing functions as the core function had to be adapted to work with <span class="xref">Lucene.Net.Util.BytesRef</span>s with offsets and lengths |
| rather than raw byte arrays.<br><p> |
| <div class="lucene-block lucene-experimental">This is a Lucene.NET EXPERIMENTAL API, use at your own risk</div></section> |
| <h3 id="enums">Enums |
| </h3> |
| <h4><a class="xref" href="Lucene.Net.Codecs.Bloom.FuzzySet.ContainsResult.html">FuzzySet.ContainsResult</a></h4> |
| <section></section> |
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