org.apache.lucene.search.similarities.NormalizationH1.java Source code

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Here is the source code for org.apache.lucene.search.similarities.NormalizationH1.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;

/**
 * Normalization model that assumes a uniform distribution of the term frequency.
 * <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>, <a href="http://dl.acm.org/citation.cfm?id=1835490">
 * information-based models</a> (see {@link IBSimilarity}) introduced a
 * multiplying factor.
 * The default value for the {@code c} parameter is {@code 1}.</p>
 * @lucene.experimental
 */
public class NormalizationH1 extends Normalization {
    private final float c;

    /**
     * Creates NormalizationH1 with the supplied parameter <code>c</code>.
     * @param c hyper-parameter that controls the term frequency 
     * normalization with respect to the document length.
     */
    public NormalizationH1(float c) {
        // unbounded but typical range 0..10 or so
        if (Float.isFinite(c) == false || c < 0) {
            throw new IllegalArgumentException("illegal c value: " + c + ", must be a non-negative finite value");
        }
        this.c = c;
    }

    /**
     * Calls {@link #NormalizationH1(float) NormalizationH1(1)}
     */
    public NormalizationH1() {
        this(1);
    }

    @Override
    public final double tfn(BasicStats stats, double tf, double len) {
        return tf * c * (stats.getAvgFieldLength() / len);
    }

    @Override
    public Explanation explain(BasicStats stats, double tf, double len) {
        return Explanation.match((float) tfn(stats, tf, len),
                getClass().getSimpleName() + ", computed as tf * c * (avgfl / fl) from:",
                Explanation.match((float) tf, "tf, number of occurrences of term in the document"),
                Explanation.match(c, "c, hyper-parameter"),
                Explanation.match((float) stats.getAvgFieldLength(),
                        "avgfl, average length of field across all documents"),
                Explanation.match((float) len, "fl, field length of the document"));
    }

    @Override
    public String toString() {
        return "1";
    }

    /**
     * Returns the <code>c</code> parameter.
     * @see #NormalizationH1(float)
     */
    public float getC() {
        return c;
    }
}