org.apache.lucene.search
Class Similarity

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

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

DefaultSimilarity, SimilarityDelegator

public abstract class Similarity
extends Object
implements Serializable

Expert: Scoring API.

Subclasses implement search scoring.

The score of query q for document d correlates to the cosine-distance or dot-product between document and query vectors in a Vector Space Model (VSM) of Information Retrieval. A document whose vector is closer to the query vector in that model is scored higher. The score is computed as follows:

score(q,d) = coord(q,d) · queryNorm(q) ·	∑	( tf(t in d) · idf(t)2 · t.getBoost() · norm(t,d) )
	t in q

where

1. tf(t in d) correlates to the term's frequency, defined as the number of times term t appears in the currently scored document d. Documents that have more occurrences of a given term receive a higher score. The default computation for tf(t in d) in DefaultSimilarity is:
   
   

   tf(t in d) =

   frequency½

   
   
   
2. idf(t) stands for Inverse Document Frequency. This value correlates to the inverse of docFreq (the number of documents in which the term t appears). This means rarer terms give higher contribution to the total score. The default computation for idf(t) in DefaultSimilarity is:
   
   

   idf(t) =

   1 + log (

   numDocs ––––––––– docFreq+1

   )

   
   
   
3. coord(q,d) is a score factor based on how many of the query terms are found in the specified document. Typically, a document that contains more of the query's terms will receive a higher score than another document with fewer query terms. This is a search time factor computed in coord(q,d) by the Similarity in effect at search time.
   
   
4. queryNorm(q) is a normalizing factor used to make scores between queries comparable. This factor does not affect document ranking (since all ranked documents are multiplied by the same factor), but rather just attempts to make scores from different queries (or even different indexes) comparable. This is a search time factor computed by the Similarity in effect at search time. The default computation in DefaultSimilarity is:
   
   

   queryNorm(q) = queryNorm(sumOfSquaredWeights) =

   1 –––––––––––––– sumOfSquaredWeights½

   
   
   The sum of squared weights (of the query terms) is computed by the query Weight object. For example, a boolean query computes this value as:
   
   

   sumOfSquaredWeights = q.getBoost() 2 ·	∑	( idf(t) · t.getBoost() ) 2
   	t in q

   
   
   
5. t.getBoost() is a search time boost of term t in the query q as specified in the query text (see query syntax), or as set by application calls to setBoost(). Notice that there is really no direct API for accessing a boost of one term in a multi term query, but rather multi terms are represented in a query as multi TermQuery objects, and so the boost of a term in the query is accessible by calling the sub-query getBoost().
   
   
6. norm(t,d) encapsulates a few (indexing time) boost and length factors:
   • Document boost - set by calling doc.setBoost() before adding the document to the index.
   • Field boost - set by calling field.setBoost() before adding the field to a document.
   • lengthNorm(field) - computed when the document is added to the index in accordance with the number of tokens of this field in the document, so that shorter fields contribute more to the score. LengthNorm is computed by the Similarity class in effect at indexing.

   When a document is added to the index, all the above factors are multiplied. If the document has multiple fields with the same name, all their boosts are multiplied together:
   
   

   norm(t,d) = doc.getBoost() · lengthNorm(field) ·	∏	f.getBoost()
   	field f in d named as t

   
   
   However the resulted norm value is encoded as a single byte before being stored. At search time, the norm byte value is read from the index directory and decoded back to a float norm value. This encoding/decoding, while reducing index size, comes with the price of precision loss - it is not guaranteed that decode(encode(x)) = x. For instance, decode(encode(0.89)) = 0.75. Also notice that search time is too late to modify this norm part of scoring, e.g. by using a different Similarity for search.
   
   

See Also:

setDefault(Similarity), IndexWriter.setSimilarity(Similarity), Searcher.setSimilarity(Similarity), Serialized Form

Constructor Summary

Similarity()

Method Summary

abstract float	coord(int overlap, int maxOverlap) Computes a score factor based on the fraction of all query terms that a document contains.
static float	decodeNorm(byte b) Decodes a normalization factor stored in an index.
static byte	encodeNorm(float f) Encodes a normalization factor for storage in an index.
static Similarity	getDefault() Return the default Similarity implementation used by indexing and search code.
static float[]	getNormDecoder() Returns a table for decoding normalization bytes.
float	idf(Collection terms, Searcher searcher) Computes a score factor for a phrase.
abstract float	idf(int docFreq, int numDocs) Computes a score factor based on a term's document frequency (the number of documents which contain the term).
float	idf(Term term, Searcher searcher) Computes a score factor for a simple term.
abstract float	lengthNorm(String fieldName, int numTokens) Computes the normalization value for a field given the total number of terms contained in a field.
abstract float	queryNorm(float sumOfSquaredWeights) Computes the normalization value for a query given the sum of the squared weights of each of the query terms.
float	scorePayload(String fieldName, byte[] payload, int offset, int length) Calculate a scoring factor based on the data in the payload.
static void	setDefault(Similarity similarity) Set the default Similarity implementation used by indexing and search code.
abstract float	sloppyFreq(int distance) Computes the amount of a sloppy phrase match, based on an edit distance.
abstract float	tf(float freq) Computes a score factor based on a term or phrase's frequency in a document.
float	tf(int freq) Computes a score factor based on a term or phrase's frequency in a document.

Methods inherited from class java.lang.Object

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

Constructor Detail

Similarity

public Similarity()

Method Detail

setDefault

public static void setDefault(Similarity similarity)

Set the default Similarity implementation used by indexing and search code.

See Also:

Searcher.setSimilarity(Similarity), IndexWriter.setSimilarity(Similarity)

getDefault

public static Similarity getDefault()

Return the default Similarity implementation used by indexing and search code.

This is initially an instance of DefaultSimilarity.

See Also:

Searcher.setSimilarity(Similarity), IndexWriter.setSimilarity(Similarity)

decodeNorm

public static float decodeNorm(byte b)

Decodes a normalization factor stored in an index.

See Also:

encodeNorm(float)

getNormDecoder

public static float[] getNormDecoder()

Returns a table for decoding normalization bytes.

See Also:

encodeNorm(float)

lengthNorm

public abstract float lengthNorm(String fieldName,
                                 int numTokens)

Computes the normalization value for a field given the total number of terms contained in a field. These values, together with field boosts, are stored in an index and multipled into scores for hits on each field by the search code.

Matches in longer fields are less precise, so implementations of this method usually return smaller values when numTokens is large, and larger values when numTokens is small.

That these values are computed under IndexWriter.addDocument(org.apache.lucene.document.Document) and stored then using encodeNorm(float). Thus they have limited precision, and documents must be re-indexed if this method is altered.

Parameters:

fieldName - the name of the field

numTokens - the total number of tokens contained in fields named fieldName of doc.

Returns:

a normalization factor for hits on this field of this document

See Also:

AbstractField.setBoost(float)

queryNorm

public abstract float queryNorm(float sumOfSquaredWeights)

Computes the normalization value for a query given the sum of the squared weights of each of the query terms. This value is then multipled into the weight of each query term.

This does not affect ranking, but rather just attempts to make scores from different queries comparable.

Parameters:

sumOfSquaredWeights - the sum of the squares of query term weights

Returns:

a normalization factor for query weights

encodeNorm

public static byte encodeNorm(float f)

Encodes a normalization factor for storage in an index.

The encoding uses a three-bit mantissa, a five-bit exponent, and the zero-exponent point at 15, thus representing values from around 7x10^9 to 2x10^-9 with about one significant decimal digit of accuracy. Zero is also represented. Negative numbers are rounded up to zero. Values too large to represent are rounded down to the largest representable value. Positive values too small to represent are rounded up to the smallest positive representable value.

See Also:

AbstractField.setBoost(float), SmallFloat

tf

public float tf(int freq)

Computes a score factor based on a term or phrase's frequency in a document. This value is multiplied by the idf(Term, Searcher) factor for each term in the query and these products are then summed to form the initial score for a document.

Terms and phrases repeated in a document indicate the topic of the document, so implementations of this method usually return larger values when freq is large, and smaller values when freq is small.

The default implementation calls tf(float).

Parameters:

freq - the frequency of a term within a document

Returns:

a score factor based on a term's within-document frequency

sloppyFreq

public abstract float sloppyFreq(int distance)

Computes the amount of a sloppy phrase match, based on an edit distance. This value is summed for each sloppy phrase match in a document to form the frequency that is passed to tf(float).

A phrase match with a small edit distance to a document passage more closely matches the document, so implementations of this method usually return larger values when the edit distance is small and smaller values when it is large.

Parameters:

distance - the edit distance of this sloppy phrase match

Returns:

the frequency increment for this match

See Also:

PhraseQuery.setSlop(int)

tf

public abstract float tf(float freq)

Computes a score factor based on a term or phrase's frequency in a document. This value is multiplied by the idf(Term, Searcher) factor for each term in the query and these products are then summed to form the initial score for a document.

Terms and phrases repeated in a document indicate the topic of the document, so implementations of this method usually return larger values when freq is large, and smaller values when freq is small.

Parameters:

freq - the frequency of a term within a document

Returns:

a score factor based on a term's within-document frequency

idf

public float idf(Term term,
                 Searcher searcher)
          throws IOException

Computes a score factor for a simple term.

The default implementation is:

   return idf(searcher.docFreq(term), searcher.maxDoc());
 

Note that Searcher.maxDoc() is used instead of IndexReader.numDocs() because it is proportional to Searcher.docFreq(Term) , i.e., when one is inaccurate, so is the other, and in the same direction.

Parameters:

term - the term in question

searcher - the document collection being searched

Returns:

a score factor for the term

Throws:

IOException

idf

public float idf(Collection terms,
                 Searcher searcher)
          throws IOException

Computes a score factor for a phrase.

The default implementation sums the idf(Term,Searcher) factor for each term in the phrase.

Parameters:

terms - the terms in the phrase

searcher - the document collection being searched

Returns:

a score factor for the phrase

Throws:

IOException

idf

public abstract float idf(int docFreq,
                          int numDocs)

Computes a score factor based on a term's document frequency (the number of documents which contain the term). This value is multiplied by the tf(int) factor for each term in the query and these products are then summed to form the initial score for a document.

Terms that occur in fewer documents are better indicators of topic, so implementations of this method usually return larger values for rare terms, and smaller values for common terms.

Parameters:

docFreq - the number of documents which contain the term

numDocs - the total number of documents in the collection

Returns:

a score factor based on the term's document frequency

coord

public abstract float coord(int overlap,
                            int maxOverlap)

Computes a score factor based on the fraction of all query terms that a document contains. This value is multiplied into scores.

The presence of a large portion of the query terms indicates a better match with the query, so implementations of this method usually return larger values when the ratio between these parameters is large and smaller values when the ratio between them is small.

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

scorePayload

public float scorePayload(String fieldName,
                          byte[] payload,
                          int offset,
                          int length)

Calculate a scoring factor based on the data in the payload. Overriding implementations are responsible for interpreting what is in the payload. Lucene makes no assumptions about what is in the byte array.

The default implementation returns 1.

Parameters:

fieldName - The fieldName of the term this payload belongs to

payload - The payload byte array to be scored

offset - The offset into the payload array

length - The length in the array

Returns:

An implementation dependent float to be used as a scoring factor