/*
* 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;
/**
* Geometric as limiting form of the Bose-Einstein model. The formula used in Lucene differs
* slightly from the one in the original paper: {@code F} is increased by {@code 1}
* and {@code N} is increased by {@code F}.
* @lucene.experimental
*/
public class BasicModelG extends BasicModel {
/** Sole constructor: parameter-free */
public BasicModelG() {}
@Override
public final float score(BasicStats stats, float tfn) {
// just like in BE, approximation only holds true when F << N, so we use lambda = F / (N + F)
double F = stats.getTotalTermFreq() + 1;
double N = stats.getNumberOfDocuments();
double lambda = F / (N + F);
// -log(1 / (lambda + 1)) -> log(lambda + 1)
return (float)(log2(lambda + 1) + tfn * log2((1 + lambda) / lambda));
}
@Override
public String toString() {
return "G";
}
}