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

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

/**
 * This class acts as the base class for the implementations of the term
 * frequency normalization methods in the DFR framework.
 * 
 * @see DFRSimilarity
 * @lucene.experimental
 */
public abstract class Normalization {

    /**
     * Sole constructor. (For invocation by subclass 
     * constructors, typically implicit.)
     */
    public Normalization() {
    }

    /** Returns the normalized term frequency.
     * @param len the field length. */
    public abstract double tfn(BasicStats stats, double tf, double len);

    /** Returns an explanation for the normalized term frequency.
     * <p>The default normalization methods use the field length of the document
     * and the average field length to compute the normalized term frequency.
     * This method provides a generic explanation for such methods.
     * Subclasses that use other statistics must override this method.</p>
     */
    public Explanation explain(BasicStats stats, double tf, double len) {
        return Explanation.match((float) tfn(stats, tf, len), getClass().getSimpleName() + ", computed from:",
                Explanation.match((float) tf, "tf, number of occurrences of term in the document"),
                Explanation.match((float) stats.getAvgFieldLength(),
                        "avgfl, average length of field across all documents"),
                Explanation.match((float) len, "fl, field length of the document"));
    }

    /** Implementation used when there is no normalization. */
    public static final class NoNormalization extends Normalization {

        /** Sole constructor: parameter-free */
        public NoNormalization() {
        }

        @Override
        public double tfn(BasicStats stats, double tf, double len) {
            return tf;
        }

        @Override
        public Explanation explain(BasicStats stats, double tf, double len) {
            return Explanation.match(1, "no normalization");
        }

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

    /**
     * Subclasses must override this method to return the code of the
     * normalization formula. Refer to the original paper for the list. 
     */
    @Override
    public abstract String toString();
}