org.apache.lucene.search.similarities

Class Similarity

java.lang.Object

org.apache.lucene.search.similarities.Similarity

Direct Known Subclasses:

BM25Similarity, BooleanSimilarity, MultiSimilarity, PerFieldSimilarityWrapper, SimilarityBase, TFIDFSimilarity

public abstract class Similarity
extends Object

Similarity defines the components of Lucene scoring.

Expert: Scoring API.

This is a low-level API, you should only extend this API if you want to implement an information retrieval model. If you are instead looking for a convenient way to alter Lucene's scoring, consider extending a higher-level implementation such as TFIDFSimilarity, which implements the vector space model with this API, or just tweaking the default implementation: BM25Similarity.

Similarity determines how Lucene weights terms, and Lucene interacts with this class at both index-time and query-time.

Indexing Time At indexing time, the indexer calls computeNorm(FieldInvertState), allowing the Similarity implementation to set a per-document value for the field that will be later accessible via LeafReader.getNormValues(String). Lucene makes no assumption about what is in this norm, but it is most useful for encoding length normalization information.

Implementations should carefully consider how the normalization is encoded: while Lucene's BM25Similarity encodes a combination of index-time boost and length normalization information with SmallFloat into a single byte, this might not be suitable for all purposes.

Many formulas require the use of average document length, which can be computed via a combination of CollectionStatistics.sumTotalTermFreq() and CollectionStatistics.maxDoc() or CollectionStatistics.docCount(), depending upon whether the average should reflect field sparsity.

Additional scoring factors can be stored in named NumericDocValuesFields and accessed at query-time with LeafReader.getNumericDocValues(String).

Finally, using index-time boosts (either via folding into the normalization byte or via DocValues), is an inefficient way to boost the scores of different fields if the boost will be the same for every document, instead the Similarity can simply take a constant boost parameter C, and PerFieldSimilarityWrapper can return different instances with different boosts depending upon field name.

Query time At query-time, Queries interact with the Similarity via these steps:

1. The computeWeight(float, CollectionStatistics, TermStatistics...) method is called a single time, allowing the implementation to compute any statistics (such as IDF, average document length, etc) across the entire collection. The TermStatistics and CollectionStatistics passed in already contain all of the raw statistics involved, so a Similarity can freely use any combination of statistics without causing any additional I/O. Lucene makes no assumption about what is stored in the returned Similarity.SimWeight object.
2. For each segment in the index, the Query creates a simScorer(SimWeight, org.apache.lucene.index.LeafReaderContext) The score() method is called for each matching document.

Explanations When IndexSearcher.explain(org.apache.lucene.search.Query, int) is called, queries consult the Similarity's DocScorer for an explanation of how it computed its score. The query passes in a the document id and an explanation of how the frequency was computed.

See Also:

IndexWriterConfig.setSimilarity(Similarity), IndexSearcher.setSimilarity(Similarity)

WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	Similarity.SimScorer API for scoring "sloppy" queries such as TermQuery, SpanQuery, and PhraseQuery.
static class	Similarity.SimWeight Stores the weight for a query across the indexed collection.

Constructor Summary

Constructors

Constructor and Description
Similarity() Sole constructor.

Method Summary

Modifier and Type	Method and Description
abstract long	computeNorm(FieldInvertState state) Computes the normalization value for a field, given the accumulated state of term processing for this field (see FieldInvertState).
abstract Similarity.SimWeight	computeWeight(float boost, CollectionStatistics collectionStats, TermStatistics... termStats) Compute any collection-level weight (e.g.
abstract Similarity.SimScorer	simScorer(Similarity.SimWeight weight, LeafReaderContext context) Creates a new Similarity.SimScorer to score matching documents from a segment of the inverted index.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Similarity

public Similarity()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

computeNorm

public abstract long computeNorm(FieldInvertState state)

Computes the normalization value for a field, given the accumulated state of term processing for this field (see FieldInvertState).

Matches in longer fields are less precise, so implementations of this method usually set smaller values when state.getLength() is large, and larger values when state.getLength() is small.

Parameters:

state - current processing state for this field

Returns:

computed norm value

WARNING: This API is experimental and might change in incompatible ways in the next release.

computeWeight

public abstract Similarity.SimWeight computeWeight(float boost,
                                                   CollectionStatistics collectionStats,
                                                   TermStatistics... termStats)

Compute any collection-level weight (e.g. IDF, average document length, etc) needed for scoring a query.

Parameters:

boost - a multiplicative factor to apply to the produces scores

collectionStats - collection-level statistics, such as the number of tokens in the collection.

termStats - term-level statistics, such as the document frequency of a term across the collection.

Returns:

SimWeight object with the information this Similarity needs to score a query.

simScorer

public abstract Similarity.SimScorer simScorer(Similarity.SimWeight weight,
                                               LeafReaderContext context)
                                        throws IOException

Creates a new Similarity.SimScorer to score matching documents from a segment of the inverted index.

Parameters:

weight - collection information from computeWeight(float, CollectionStatistics, TermStatistics...)

context - segment of the inverted index to be scored.

Returns:

SloppySimScorer for scoring documents across context

Throws:

IOException - if there is a low-level I/O error