/*
* 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;
/**
* Implements the Poisson approximation for the binomial model for DFR.
* @lucene.experimental
* <p>
* WARNING: for terms that do not meet the expected random distribution
* (e.g. stopwords), this model may give poor performance, such as
* abnormally high scores for low tf values.
*/
public class BasicModelP extends BasicModel {
/** {@code log2(Math.E)}, precomputed. */
protected static double LOG2_E = log2(Math.E);
/** Sole constructor: parameter-free */
public BasicModelP() {}
@Override
public final float score(BasicStats stats, float tfn) {
float lambda = (float)(stats.getTotalTermFreq()+1) / (stats.getNumberOfDocuments()+1);
return (float)(tfn * log2(tfn / lambda)
+ (lambda + 1 / (12 * tfn) - tfn) * LOG2_E
+ 0.5 * log2(2 * Math.PI * tfn));
}
@Override
public String toString() {
return "P";
}
}