/*
* 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.
*/
package org.apache.lucene.search.similarities;
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;
}
}