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 * 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
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 * Unless required by applicable law or agreed to in writing, software
 * 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;

Computes lambda as docFreq+1 / numberOfDocuments+1.
@lucene.experimental
/** * Computes lambda as {@code docFreq+1 / numberOfDocuments+1}. * @lucene.experimental */
public class LambdaDF extends Lambda {
Sole constructor: parameter-free
/** Sole constructor: parameter-free */
public LambdaDF() {} @Override public final float lambda(BasicStats stats) { float lambda = (float) ((stats.getDocFreq() + 1.0) / (stats.getNumberOfDocuments() + 1.0)); if (lambda == 1) { // Distribution SPL cannot work with values of lambda that are equal to 1 lambda = Math.nextDown(lambda); } return lambda; } @Override public final Explanation explain(BasicStats stats) { return Explanation.match( lambda(stats), getClass().getSimpleName() + ", computed as (n + 1) / (N + 1) from:", Explanation.match(stats.getDocFreq(), "n, number of documents containing term"), Explanation.match(stats.getNumberOfDocuments(), "N, total number of documents with field")); } @Override public String toString() { return "D"; } }