org.apache.lucene.search.similarities.LambdaDF.java Source code

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Here is the source code for org.apache.lucene.search.similarities.LambdaDF.java

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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
 *
 * 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 org.apache.lucene.search.Explanation;

/**
 * Computes lambda as {@code docFreq+1 / numberOfDocuments+1}.
 * @lucene.experimental
 */
public class LambdaDF extends Lambda {

    /** 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";
    }
}