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 * this work for additional information regarding copyright ownership.
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 * (the "License"); you may not use this file except in compliance with
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 *     http://www.apache.org/licenses/LICENSE-2.0
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 * distributed under the License is distributed on an "AS IS" BASIS,
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package org.apache.lucene.search.similarities;


import org.apache.lucene.search.Explanation;

import static org.apache.lucene.search.similarities.SimilarityBase.log2;

Normalization model in which the term frequency is inversely related to the
length.
While this model is parameterless in the

original article, the thesis introduces the parameterized variant. The default value for the c parameter is 1.
@lucene.experimental
/**
 * Normalization model in which the term frequency is inversely related to the
 * length.
 * <p>While this model is parameterless in the
 * <a href="http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.101.742">
 * original article</a>, the <a href="http://theses.gla.ac.uk/1570/">thesis</a>
 * introduces the parameterized variant.
 * The default value for the {@code c} parameter is {@code 1}.</p>
 * @lucene.experimental
 */
public class NormalizationH2 extends Normalization {
  private final float c;
  
  
Creates NormalizationH2 with the supplied parameter c.
Params:
c – hyper-parameter that controls the term frequency 
normalization with respect to the document length./**
   * Creates NormalizationH2 with the supplied parameter <code>c</code>.
   * @param c hyper-parameter that controls the term frequency 
   * normalization with respect to the document length.
   */
  public NormalizationH2(float c) {
    // unbounded but typical range 0..10 or so
    if (Float.isFinite(c) == false || c < 0) {
      throw new IllegalArgumentException("illegal c value: " + c + ", must be a non-negative finite value");
    }
    this.c = c;
  }

  
Calls NormalizationH2(1) /**
   * Calls {@link #NormalizationH2(float) NormalizationH2(1)}
   */
  public NormalizationH2() {
    this(1);
  }
  
  @Override
  public final double tfn(BasicStats stats, double tf, double len) {
    return tf * log2(1 + c * stats.getAvgFieldLength() / len);
  }

  @Override
  public Explanation explain(BasicStats stats, double tf, double len) {
    return Explanation.match(
        (float) tfn(stats, tf, len),
        getClass().getSimpleName()
            + ", computed as tf * log2(1 + c * avgfl / fl) from:",
        Explanation.match((float) tf,
            "tf, number of occurrences of term in the document"),
        Explanation.match(c,
            "c, hyper-parameter"),
        Explanation.match((float) stats.getAvgFieldLength(),
            "avgfl, average length of field across all documents"),
        Explanation.match((float) len, "fl, field length of the document"));
  }

  @Override
  public String toString() {
    return "2";
  }
  
  
Returns the c parameter.
See Also:
NormalizationH2(float)/**
   * Returns the <code>c</code> parameter.
   * @see #NormalizationH2(float)
   */
  public float getC() {
    return c;
  }
}