## org.apache.lucene.search.similarities
Class Similarity

    java.lang.Object
      org.apache.lucene.search.similarities.Similarity

Direct Known Subclasses:
  BM25Similarity, 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: `DefaultSimilarity`.

Similarity determines how Lucene weights terms, and Lucene interacts with this class at both index-time and query-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 `AtomicReader.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 classical `TFIDFSimilarity` 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 `NumericDocValuesField`s and accessed at query-time with `AtomicReader.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.

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. The query normalization process occurs a single time: `Similarity.SimWeight.getValueForNormalization()` is called for each query leaf node, `queryNorm(float)` is called for the top-level query, and finally `Similarity.SimWeight.normalize(float, float)` passes down the normalization value and any top-level boosts (e.g. from enclosing `BooleanQuery`s).
3. For each segment in the index, the Query creates a `exactSimScorer(SimWeight, AtomicReaderContext)` (for queries with exact frequencies such as TermQuerys and exact PhraseQueries) or a `sloppySimScorer(SimWeight, AtomicReaderContext)` (for queries with sloppy frequencies such as SpanQuerys and sloppy PhraseQueries). The score() method is called for each matching document.

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
`static class` `Similarity.ExactSimScorer` API for scoring exact queries such as `TermQuery` and exact `PhraseQuery`.
`static class` `Similarity.SimWeight` Stores the weight for a query across the indexed collection.
`static class` `Similarity.SloppySimScorer` API for scoring "sloppy" queries such as `SpanQuery` and sloppy `PhraseQuery`.

Constructor Summary
`Similarity()` Sole constructor.

Method Summary
`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 queryBoost, CollectionStatistics collectionStats, TermStatistics... termStats)` Compute any collection-level weight (e.g.
`float` `coord(int overlap, int maxOverlap)` Hook to integrate coordinate-level matching.
`abstract Similarity.ExactSimScorer` `exactSimScorer(Similarity.SimWeight weight, AtomicReaderContext context)` Creates a new `Similarity.ExactSimScorer` to score matching documents from a segment of the inverted index.
`float` `queryNorm(float valueForNormalization)` Computes the normalization value for a query given the sum of the normalized weights `Similarity.SimWeight.getValueForNormalization()` of each of the query terms.
`abstract Similarity.SloppySimScorer` `sloppySimScorer(Similarity.SimWeight weight, AtomicReaderContext context)` Creates a new `Similarity.SloppySimScorer` 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

### coord

    public float coord(int overlap,
                       int maxOverlap)

  Hook to integrate coordinate-level matching.

  By default this is disabled (returns `1`), as with most modern models this will only skew performance, but some implementations such as `TFIDFSimilarity` override this.

  Parameters:
    `overlap` - the number of query terms matched in the document
    `maxOverlap` - the total number of terms in the query
  Returns:
    a score factor based on term overlap with the query

### queryNorm

    public float queryNorm(float valueForNormalization)

  Computes the normalization value for a query given the sum of the normalized weights `Similarity.SimWeight.getValueForNormalization()` of each of the query terms. This value is passed back to the weight (`Similarity.SimWeight.normalize(float, float)` of each query term, to provide a hook to attempt to make scores from different queries comparable.

  By default this is disabled (returns `1`), but some implementations such as `TFIDFSimilarity` override this.

  Parameters:
    `valueForNormalization` - the sum of the term normalization values
  Returns:
    a normalization factor for query weights

### 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 queryBoost,
                                                       CollectionStatistics collectionStats,
                                                       TermStatistics... termStats)

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

  Parameters:
    `queryBoost` - the query-time boost.
    `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.

### exactSimScorer

    public abstract Similarity.ExactSimScorer exactSimScorer(Similarity.SimWeight weight,
                                                             AtomicReaderContext context)
                                                      throws IOException

  Creates a new `Similarity.ExactSimScorer` 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:
    ExactSimScorer for scoring documents across `context`
  Throws:
    `IOException` - if there is a low-level I/O error

### sloppySimScorer

    public abstract Similarity.SloppySimScorer sloppySimScorer(Similarity.SimWeight weight,
                                                               AtomicReaderContext context)
                                                        throws IOException

  Creates a new `Similarity.SloppySimScorer` 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

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