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

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Here is the source code for org.apache.lucene.search.similarities.LMSimilarity.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 java.util.List;
import java.util.Locale;

import org.apache.lucene.search.CollectionStatistics;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.TermStatistics;

/**
 * Abstract superclass for language modeling Similarities. The following inner
 * types are introduced:
 * <ul>
 *   <li>{@link LMStats}, which defines a new statistic, the probability that
 *   the collection language model generates the current term;</li>
 *   <li>{@link CollectionModel}, which is a strategy interface for object that
 *   compute the collection language model {@code p(w|C)};</li>
 *   <li>{@link DefaultCollectionModel}, an implementation of the former, that
 *   computes the term probability as the number of occurrences of the term in the
 *   collection, divided by the total number of tokens.</li>
 * </ul> 
 * 
 * @lucene.experimental
 */
public abstract class LMSimilarity extends SimilarityBase {
    /** The collection model. */
    protected final CollectionModel collectionModel;

    /** Creates a new instance with the specified collection language model. */
    public LMSimilarity(CollectionModel collectionModel) {
        this.collectionModel = collectionModel;
    }

    /** Creates a new instance with the default collection language model. */
    public LMSimilarity() {
        this(new DefaultCollectionModel());
    }

    @Override
    protected BasicStats newStats(String field, double boost) {
        return new LMStats(field, boost);
    }

    /**
     * Computes the collection probability of the current term in addition to the
     * usual statistics.
     */
    @Override
    protected void fillBasicStats(BasicStats stats, CollectionStatistics collectionStats,
            TermStatistics termStats) {
        super.fillBasicStats(stats, collectionStats, termStats);
        LMStats lmStats = (LMStats) stats;
        lmStats.setCollectionProbability(collectionModel.computeProbability(stats));
    }

    @Override
    protected void explain(List<Explanation> subExpls, BasicStats stats, double freq, double docLen) {
        subExpls.add(
                Explanation.match((float) collectionModel.computeProbability(stats), "collection probability"));
    }

    /**
     * Returns the name of the LM method. The values of the parameters should be
     * included as well.
     * <p>Used in {@link #toString()}</p>.
     */
    public abstract String getName();

    /**
     * Returns the name of the LM method. If a custom collection model strategy is
     * used, its name is included as well.
     * @see #getName()
     * @see CollectionModel#getName()
     * @see DefaultCollectionModel 
     */
    @Override
    public String toString() {
        String coll = collectionModel.getName();
        if (coll != null) {
            return String.format(Locale.ROOT, "LM %s - %s", getName(), coll);
        } else {
            return String.format(Locale.ROOT, "LM %s", getName());
        }
    }

    /** Stores the collection distribution of the current term. */
    public static class LMStats extends BasicStats {
        /** The probability that the current term is generated by the collection. */
        private double collectionProbability;

        /**
         * Creates LMStats for the provided field and query-time boost
         */
        public LMStats(String field, double boost) {
            super(field, boost);
        }

        /**
         * Returns the probability that the current term is generated by the
         * collection.
         */
        public final double getCollectionProbability() {
            return collectionProbability;
        }

        /**
         * Sets the probability that the current term is generated by the
         * collection.
         */
        public final void setCollectionProbability(double collectionProbability) {
            this.collectionProbability = collectionProbability;
        }
    }

    /** A strategy for computing the collection language model. */
    public static interface CollectionModel {
        /**
         * Computes the probability {@code p(w|C)} according to the language model
         * strategy for the current term.
         */
        public double computeProbability(BasicStats stats);

        /** The name of the collection model strategy. */
        public String getName();
    }

    /**
     * Models {@code p(w|C)} as the number of occurrences of the term in the
     * collection, divided by the total number of tokens {@code + 1}.
     */
    public static class DefaultCollectionModel implements CollectionModel {

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

        @Override
        public double computeProbability(BasicStats stats) {
            return (stats.getTotalTermFreq() + 1D) / (stats.getNumberOfFieldTokens() + 1D);
        }

        @Override
        public String getName() {
            return null;
        }
    }
}