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

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

    public class DFRSimilarity

extends SimilarityBase

Implements the divergence from randomness (DFR) framework introduced in Gianni Amati and Cornelis Joost Van Rijsbergen. 2002. Probabilistic models of information retrieval based on measuring the divergence from randomness. ACM Trans. Inf. Syst. 20, 4 (October 2002), 357-389.

The DFR scoring formula is composed of three separate components: the basic model, the aftereffect and an additional normalization component, represented by the classes `BasicModel`, `AfterEffect` and `Normalization`, respectively. The names of these classes were chosen to match the names of their counterparts in the Terrier IR engine.

To construct a DFRSimilarity, you must specify the implementations for all three components of DFR:

1. `BasicModel`: Basic model of information content:
   - `BasicModelBE`: Limiting form of Bose-Einstein
   - `BasicModelG`: Geometric approximation of Bose-Einstein
   - `BasicModelP`: Poisson approximation of the Binomial
   - `BasicModelD`: Divergence approximation of the Binomial
   - `BasicModelIn`: Inverse document frequency
   - `BasicModelIne`: Inverse expected document frequency [mixture of Poisson and IDF]
   - `BasicModelIF`: Inverse term frequency [approximation of I(ne)]
2. `AfterEffect`: First normalization of information gain:
   - `AfterEffectL`: Laplace's law of succession
   - `AfterEffectB`: Ratio of two Bernoulli processes
   - `AfterEffect.NoAfterEffect`: no first normalization
3. `Normalization`: Second (length) normalization:
   - `NormalizationH1`: Uniform distribution of term frequency
   - `NormalizationH2`: term frequency density inversely related to length
   - `NormalizationH3`: term frequency normalization provided by Dirichlet prior
   - `NormalizationZ`: term frequency normalization provided by a Zipfian relation
   - `Normalization.NoNormalization`: no second normalization

Note that qtf, the multiplicity of term-occurrence in the query, is not handled by this implementation.

See Also:
  `BasicModel`, `AfterEffect`, `Normalization`
WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity
`Similarity.ExactSimScorer, Similarity.SimWeight, Similarity.SloppySimScorer`

Field Summary
`protected AfterEffect` | `afterEffect` The first normalization of the information content.
`protected BasicModel` | `basicModel` The basic model for information content.
`protected Normalization` | `normalization` The term frequency normalization.

Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase
`discountOverlaps`

Constructor Summary
`DFRSimilarity(BasicModel basicModel, AfterEffect afterEffect, Normalization normalization)` Creates DFRSimilarity from the three components.

Method Summary
`protected void` | `explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)` Subclasses should implement this method to explain the score.
`AfterEffect` | `getAfterEffect()` Returns the first normalization
`BasicModel` | `getBasicModel()` Returns the basic model of information content
`Normalization` | `getNormalization()` Returns the second normalization
`protected float` | `score(BasicStats stats, float freq, float docLen)` Scores the document `doc`.
`String` | `toString()` Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

Methods inherited from class org.apache.lucene.search.similarities.SimilarityBase
`computeNorm, computeWeight, decodeNormValue, encodeNormValue, exactSimScorer, explain, fillBasicStats, getDiscountOverlaps, log2, newStats, setDiscountOverlaps, sloppySimScorer`

Methods inherited from class org.apache.lucene.search.similarities.Similarity
`coord, queryNorm`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Field Detail

### basicModel

    protected final BasicModel basicModel

  The basic model for information content.

### afterEffect

    protected final AfterEffect afterEffect

  The first normalization of the information content.

### normalization

    protected final Normalization normalization

  The term frequency normalization.

Constructor Detail

### DFRSimilarity

    public DFRSimilarity(BasicModel basicModel,
                         AfterEffect afterEffect,
                         Normalization normalization)

  Creates DFRSimilarity from the three components.

  Note that `null` values are not allowed: if you want no normalization or after-effect, instead pass `Normalization.NoNormalization` or `AfterEffect.NoAfterEffect` respectively.

  Parameters:
    `basicModel` - Basic model of information content
    `afterEffect` - First normalization of information gain
    `normalization` - Second (length) normalization

Method Detail

### score

    protected float score(BasicStats stats,
                          float freq,
                          float docLen)

  Description copied from class: `SimilarityBase`
  Scores the document `doc`.

  Subclasses must apply their scoring formula in this class.

  Specified by:
    `score` in class `SimilarityBase`
  Parameters:
    `stats` - the corpus level statistics.
    `freq` - the term frequency.
    `docLen` - the document length.
  Returns:
    the score.

### explain

    protected void explain(Explanation expl,
                           BasicStats stats,
                           int doc,
                           float freq,
                           float docLen)

  Description copied from class: `SimilarityBase`
  Subclasses should implement this method to explain the score. `expl` already contains the score, the name of the class and the doc id, as well as the term frequency and its explanation; subclasses can add additional clauses to explain details of their scoring formulae.

  The default implementation does nothing.

  Overrides:
    `explain` in class `SimilarityBase`
  Parameters:
    `expl` - the explanation to extend with details.
    `stats` - the corpus level statistics.
    `doc` - the document id.
    `freq` - the term frequency.
    `docLen` - the document length.

### toString

    public String toString()

  Description copied from class: `SimilarityBase`
  Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

  Specified by:
    `toString` in class `SimilarityBase`

### getBasicModel

    public BasicModel getBasicModel()

  Returns the basic model of information content

### getAfterEffect

    public AfterEffect getAfterEffect()

  Returns the first normalization

### getNormalization

    public Normalization getNormalization()

  Returns the second normalization

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