/*
 * COPIED FROM APACHE LUCENE 4.7.2
 *
 * Git URL: git@github.com:apache/lucene.git, tag: releases/lucene-solr/4.7.2, path: lucene/core/src/java
 *
 * (see https://issues.apache.org/jira/browse/OAK-10786 for details)
 */

package org.apache.lucene.search.similarities;

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

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

/**
 * 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 <code>c</code>.
   * @param c hyper-parameter that controls the term frequency 
   * normalization with respect to the document length.
   */
  public NormalizationH2(float c) {
    this.c = c;
  }

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

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