it.unimi.di.mg4j.search.score
Class VignaScorer

java.lang.Object
  it.unimi.di.mg4j.search.score.AbstractScorer
      it.unimi.di.mg4j.search.score.AbstractWeightedScorer
          it.unimi.di.mg4j.search.score.VignaScorer

All Implemented Interfaces:

DelegatingScorer, Scorer, FlyweightPrototype<Scorer>

public class VignaScorer
extends AbstractWeightedScorer
implements DelegatingScorer

Computes the Vigna score of all interval iterators of a document.

This scorer progressively moves score from a residual (initialised to 1) to the current score (initialised to 0). For each interval, we move a fraction of the residual equal to the ratio of the extent over the interval length, minimised with 1 and divided by 2. For instance, on a two-term query meeting intervals of length 2 will increase the score from 0 to 1/2, 3/4 and so on. On the other hand, larger intervals take away less from the residual.

When the score exceeds .99, the computation is interrupted. In this way, we exploit the laziness of the algorithms for minimal-interval semantics implemented in it.unimi.di.mg4j.search, greatly improving performance for extremely frequent terms, with no perceivable effect on the score itself.

Field Summary

Fields inherited from class it.unimi.di.mg4j.search.score.AbstractWeightedScorer

index2Weight

Fields inherited from class it.unimi.di.mg4j.search.score.AbstractScorer

documentIterator, indexIterator

Constructor Summary

VignaScorer()

Method Summary

VignaScorer	copy()
double	score(Index index) Returns a score for the current document of the last document iterator given to Scorer.wrap(DocumentIterator), but considering only a given index (optional operation).
String	toString()
boolean	usesIntervals() Returns true.

Methods inherited from class it.unimi.di.mg4j.search.score.AbstractWeightedScorer

getWeights, score, setWeights, wrap

Methods inherited from class it.unimi.di.mg4j.search.score.AbstractScorer

nextDocument

Methods inherited from class java.lang.Object

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

Methods inherited from interface it.unimi.di.mg4j.search.score.Scorer

getWeights, nextDocument, score, setWeights, wrap

Constructor Detail

VignaScorer

public VignaScorer()

Method Detail

score

public double score(Index index)
             throws IOException

Description copied from interface: Scorer

Returns a score for the current document of the last document iterator given to Scorer.wrap(DocumentIterator), but considering only a given index (optional operation).

Specified by:

score in interface Scorer

Parameters:

index - the only index to be considered.

Returns:

the score.

Throws:

IOException

toString

public String toString()

Overrides:

toString in class Object

copy

public VignaScorer copy()

Specified by:

copy in interface DelegatingScorer

Specified by:

copy in interface Scorer

Specified by:

copy in interface FlyweightPrototype<Scorer>

usesIntervals

public boolean usesIntervals()

Returns true.

Specified by:

usesIntervals in interface Scorer

Returns:

true.