org.ninit.models.bm25.BM25TermScorer.java Source code

Introduction

Here is the source code for org.ninit.models.bm25.BM25TermScorer.java

Source

package org.ninit.models.bm25;

/**
 * BM25TermScorer.java
 *
 * Copyright (c) 2008 "Joaqun Prez-Iglesias"
 *
 * 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.
 */
import java.io.IOException;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.TermDocs;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.Scorer;
import org.apache.lucene.search.Similarity;
import org.apache.lucene.search.TermQuery;

/**
 * Calculate the relevance value of a single term applying BM25 function
 * ranking. The {@link BM25Parameters} k1, and b are used.<BR>
 * 
 * @author "Joaquin Perez-Iglesias"
 * @see BM25Parameters
 * 
 */
public class BM25TermScorer extends Scorer {

    private TermQuery term;
    private IndexReader reader;
    private TermDocs termDocs;
    private float idf;
    private float av_length;
    private byte[] norm;
    private float b;
    private float k1;

    public BM25TermScorer(IndexReader reader, TermQuery term, Similarity similarity) throws IOException {
        super(similarity);
        this.reader = reader;
        this.term = term;
        this.idf = this.getSimilarity().idf(reader.docFreq(term.getTerm()), reader.numDocs());
        this.norm = this.reader.norms(this.term.getTerm().field());
        this.av_length = BM25Parameters.getAverageLength(this.term.getTerm().field());
        this.b = BM25Parameters.getB();
        this.k1 = BM25Parameters.getK1();
        this.termDocs = this.reader.termDocs(this.term.getTerm());

    }

    @Override
    public int doc() {
        return this.termDocs.doc();
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.apache.lucene.search.Scorer#explain(int)
     */
    @Override
    public Explanation explain(int doc) throws IOException {
        // Init termDocs
        if (this.termDocs != null) {
            this.termDocs.close();
        }
        this.termDocs = this.reader.termDocs(this.term.getTerm());
        // skipTo doc

        //
        if (!this.skipTo(doc)) {
            return null;
        }
        float length = 0f;
        byte[] norm = this.reader.norms(this.term.getTerm().field());

        float av_length = BM25Parameters.getAverageLength(this.term.getTerm().field());
        length = 1 / ((Similarity.decodeNorm(norm[this.doc()])) * (Similarity.decodeNorm(norm[this.doc()])));

        float tf = this.termDocs.freq();

        float result = BM25Parameters.getB() * (length / av_length);
        result = result + 1 - BM25Parameters.getB();
        result = tf / result;
        // FREQ SATURATION
        result = result / (result + BM25Parameters.getK1());

        Explanation idfE = new Explanation(this.idf, " idf (docFreq:" + this.reader.docFreq(this.term.getTerm())
                + ",numDocs:" + this.reader.numDocs() + ")");
        Explanation bE = new Explanation(result, "B:" + BM25Parameters.getB() + ",Length:" + length + ",AvgLength:"
                + av_length + ",Freq:" + tf + ",K1:" + BM25Parameters.getK1());

        Explanation resultE = new Explanation(this.idf * result,
                "BM25(" + this.term.getTerm().field() + ":" + this.term.getTerm().text());
        resultE.addDetail(idfE);
        resultE.addDetail(bE);

        return resultE;
    }

    @Override
    public boolean next() throws IOException {

        boolean result = this.termDocs.next();
        if (!result) {
            this.termDocs.close();
        }
        return result;

    }

    @Override
    public float score() throws IOException {

        //IDF refers to the inverse document frequency (idf(qi,d)) and

        //TF25 refers to the second factor in the definition of the BM25 scoring function

        float TF25;

        float num25;

        float den25;

        float length;

        float norm = Similarity.decodeNorm(this.norm[this.doc()]);
        length = 1 / (norm * norm);
        den25 = this.b * (length / this.av_length);
        den25 = 1 - this.b + den25;
        den25 = this.k1 * den25;
        den25 = this.termDocs.freq() + den25;
        num25 = this.k1 + 1;
        num25 = num25 * this.termDocs.freq();
        TF25 = num25 / den25;

        return TF25 * this.idf;

    }
    //end of score

    /*
     * (non-Javadoc)
     * 
     * @see org.apache.lucene.search.Scorer#skipTo(int)
     */
    @Override
    public boolean skipTo(int target) throws IOException {
        while (this.next() && this.doc() < target) {
        }

        return this.doc() == target;
    }
}