org.apache.lucene.search.similarities

## Class ClassicSimilarity

- java.lang.Object
- - org.apache.lucene.search.similarities.Similarity
  - - org.apache.lucene.search.similarities.TFIDFSimilarity
    - - org.apache.lucene.search.similarities.ClassicSimilarity

-     public class ClassicSimilarity
      extends TFIDFSimilarity

  Expert: Default scoring implementation which `encodes` norm values 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)) = 0.875.

  Compression of norm values to a single byte saves memory at search time, because once a field is referenced at search time, its norms - for all documents - are maintained in memory.

  The rationale supporting such lossy compression of norm values is that given the difficulty (and inaccuracy) of users to express their true information need by a query, only big differences matter.
  Last, note that search time is too late to modify this norm part of scoring, e.g. by using a different `Similarity` for search.

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity

      `Similarity.SimScorer, Similarity.SimWeight`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected boolean` | `discountOverlaps` True if overlap tokens (tokens with a position of increment of zero) are discounted from the document's length.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `ClassicSimilarity()` Sole constructor: parameter-free

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `float` | `coord(int overlap, int maxOverlap)` Implemented as `overlap / maxOverlap`.
    `float` | `decodeNormValue(long norm)` Decodes the norm value, assuming it is a single byte.
    `long` | `encodeNormValue(float f)` Encodes a normalization factor for storage in an index.
    `boolean` | `getDiscountOverlaps()` Returns true if overlap tokens are discounted from the document's length.
    `float` | `idf(long docFreq, long docCount)` Implemented as `log((docCount+1)/(docFreq+1)) + 1`.
    `Explanation` | `idfExplain(CollectionStatistics collectionStats, TermStatistics termStats)` Computes a score factor for a simple term and returns an explanation for that score factor.
    `float` | `lengthNorm(FieldInvertState state)` Implemented as `state.getBoost()*lengthNorm(numTerms)`, where `numTerms` is `FieldInvertState.getLength()` if `setDiscountOverlaps(boolean)` is false, else it's `FieldInvertState.getLength()` - `FieldInvertState.getNumOverlap()`.
    `float` | `queryNorm(float sumOfSquaredWeights)` Implemented as `1/sqrt(sumOfSquaredWeights)`.
    `float` | `scorePayload(int doc, int start, int end, BytesRef payload)` The default implementation returns `1`
    `void` | `setDiscountOverlaps(boolean v)` Determines whether overlap tokens (Tokens with 0 position increment) are ignored when computing norm.
    `float` | `sloppyFreq(int distance)` Implemented as `1 / (distance + 1)`.
    `float` | `tf(float freq)` Implemented as `sqrt(freq)`.
    `String` | `toString()`

    - ### Methods inherited from class org.apache.lucene.search.similarities.TFIDFSimilarity

      `computeNorm, computeWeight, idfExplain, simScorer`

    - ### Methods inherited from class java.lang.Object

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

- - ### Field Detail

    - #### discountOverlaps

          protected boolean discountOverlaps

      True if overlap tokens (tokens with a position of increment of zero) are discounted from the document's length.

  - ### Constructor Detail

    - #### ClassicSimilarity

          public ClassicSimilarity()

      Sole constructor: parameter-free

  - ### Method Detail

    - #### coord

          public float coord(int overlap,
                             int maxOverlap)

      Implemented as `overlap / maxOverlap`.
      Specified by:
        `coord` in class `TFIDFSimilarity`
      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 sumOfSquaredWeights)

      Implemented as `1/sqrt(sumOfSquaredWeights)`.
      Specified by:
        `queryNorm` in class `TFIDFSimilarity`
      Parameters:
        `sumOfSquaredWeights` - the sum of the squares of query term weights
      Returns:
        a normalization factor for query weights

    - #### encodeNormValue

          public final long encodeNormValue(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.

      Specified by:
        `encodeNormValue` in class `TFIDFSimilarity`
      See Also:
        `Field.setBoost(float)`, `SmallFloat`

    - #### decodeNormValue

          public final float decodeNormValue(long norm)

      Decodes the norm value, assuming it is a single byte.
      Specified by:
        `decodeNormValue` in class `TFIDFSimilarity`
      See Also:
        `encodeNormValue(float)`

    - #### lengthNorm

          public float lengthNorm(FieldInvertState state)

      Implemented as `state.getBoost()*lengthNorm(numTerms)`, where `numTerms` is `FieldInvertState.getLength()` if `setDiscountOverlaps(boolean)` is false, else it's `FieldInvertState.getLength()` - `FieldInvertState.getNumOverlap()`.
      Specified by:
        `lengthNorm` in class `TFIDFSimilarity`
      Parameters:
        `state` - statistics of the current field (such as length, boost, etc)
      Returns:
        an index-time normalization value
      WARNING: This API is experimental and might change in incompatible ways in the next release.

    - #### tf

          public float tf(float freq)

      Implemented as `sqrt(freq)`.
      Specified by:
        `tf` in class `TFIDFSimilarity`
      Parameters:
        `freq` - the frequency of a term within a document
      Returns:
        a score factor based on a term's within-document frequency

    - #### sloppyFreq

          public float sloppyFreq(int distance)

      Implemented as `1 / (distance + 1)`.
      Specified by:
        `sloppyFreq` in class `TFIDFSimilarity`
      Parameters:
        `distance` - the edit distance of this sloppy phrase match
      Returns:
        the frequency increment for this match
      See Also:
        `PhraseQuery.getSlop()`

    - #### scorePayload

          public float scorePayload(int doc,
                                    int start,
                                    int end,
                                    BytesRef payload)

      The default implementation returns `1`
      Specified by:
        `scorePayload` in class `TFIDFSimilarity`
      Parameters:
        `doc` - The docId currently being scored.
        `start` - The start position of the payload
        `end` - The end position of the payload
        `payload` - The payload byte array to be scored
      Returns:
        An implementation dependent float to be used as a scoring factor

    - #### idfExplain

          public Explanation idfExplain(CollectionStatistics collectionStats,
                                        TermStatistics termStats)

      Description copied from class: `TFIDFSimilarity`
      Computes a score factor for a simple term and returns an explanation for that score factor.

      The default implementation uses:

           idf(docFreq, docCount);

      Note that `CollectionStatistics.docCount()` is used instead of `IndexReader#numDocs()` because also `TermStatistics.docFreq()` is used, and when the latter is inaccurate, so is `CollectionStatistics.docCount()`, and in the same direction. In addition, `CollectionStatistics.docCount()` does not skew when fields are sparse.
      Overrides:
        `idfExplain` in class `TFIDFSimilarity`
      Parameters:
        `collectionStats` - collection-level statistics
        `termStats` - term-level statistics for the term
      Returns:
        an Explain object that includes both an idf score factor and an explanation for the term.

    - #### idf

          public float idf(long docFreq,
                           long docCount)

      Implemented as `log((docCount+1)/(docFreq+1)) + 1`.
      Specified by:
        `idf` in class `TFIDFSimilarity`
      Parameters:
        `docFreq` - the number of documents which contain the term
        `docCount` - the total number of documents in the collection
      Returns:
        a score factor based on the term's document frequency

    - #### setDiscountOverlaps

          public void setDiscountOverlaps(boolean v)

      Determines whether overlap tokens (Tokens with 0 position increment) are ignored when computing norm. By default this is true, meaning overlap tokens do not count when computing norms.
      See Also:
        `TFIDFSimilarity.computeNorm(org.apache.lucene.index.FieldInvertState)`
      WARNING: This API is experimental and might change in incompatible ways in the next release.

    - #### getDiscountOverlaps

          public boolean getDiscountOverlaps()

      Returns true if overlap tokens are discounted from the document's length.
      See Also:
        `setDiscountOverlaps(boolean)`

    - #### toString

          public String toString()

      Overrides:
        `toString` in class `Object`

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