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

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Here is the source code for org.apache.lucene.search.similarities.ClassicSimilarity.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.CollectionStatistics;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.TermStatistics;

/**
 * Expert: Historical scoring implementation. You might want to consider using
 * {@link BM25Similarity} instead, which is generally considered superior to
 * TF-IDF.
 */
public class ClassicSimilarity extends TFIDFSimilarity {

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

    /** Implemented as
     *  <code>1/sqrt(length)</code>.
     *
     *  @lucene.experimental */
    @Override
    public float lengthNorm(int numTerms) {
        return (float) (1.0 / Math.sqrt(numTerms));
    }

    /** Implemented as <code>sqrt(freq)</code>. */
    @Override
    public float tf(float freq) {
        return (float) Math.sqrt(freq);
    }

    @Override
    public Explanation idfExplain(CollectionStatistics collectionStats, TermStatistics termStats) {
        final long df = termStats.docFreq();
        final long docCount = collectionStats.docCount();
        final float idf = idf(df, docCount);
        return Explanation.match(idf, "idf, computed as log((docCount+1)/(docFreq+1)) + 1 from:",
                Explanation.match(df, "docFreq, number of documents containing term"),
                Explanation.match(docCount, "docCount, total number of documents with field"));
    }

    /** Implemented as <code>log((docCount+1)/(docFreq+1)) + 1</code>. */
    @Override
    public float idf(long docFreq, long docCount) {
        return (float) (Math.log((docCount + 1) / (double) (docFreq + 1)) + 1.0);
    }

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