ro.ranking.technique.bm25.BM25TermScorer.java Source code

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Here is the source code for ro.ranking.technique.bm25.BM25TermScorer.java

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package ro.ranking.technique.bm25;

/**
 * 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.*;

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

    private TermQuery term;
    private IndexReader reader;
    private TermDocs termDocs;
    private float idf;
    private float avgLength;
    private byte[] norm;
    private float b;
    private float k1;
    private int doc = -1;

    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.avgLength = 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 docID() {
        return doc;
    }

    @Override
    public int nextDoc() throws IOException {
        boolean result = this.termDocs.next();
        if (result) {
            doc = this.termDocs.doc();
        } else {
            this.termDocs.close();
            doc = NO_MORE_DOCS;
        }
        return doc;
    }

    @Override
    public float score() throws IOException {
        float fieldNorm = this.getSimilarity().decodeNormValue(this.norm[this.docID()]);
        float length = 1 / (fieldNorm * fieldNorm);

        // LENGTH NORMALIZATION
        float result = this.b * (length / this.avgLength);
        result += 1 - this.b;

        result = (this.term.getBoost() * this.termDocs.freq()) / result;
        // FREQ SATURATION
        result /= (result + this.k1);

        return result * this.idf;

    }

    @Override
    public int advance(int target) throws IOException {
        if (target == NO_MORE_DOCS)
            return NO_MORE_DOCS;
        while (this.nextDoc() != NO_MORE_DOCS && this.docID() < target) {
        }
        return this.docID();
    }
}