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SIREn core module

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/**
 * Copyright (c) 2014, Sindice Limited. All Rights Reserved.
 *
 * This file is part of the SIREn project.
 *
 * SIREn is a free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of
 * the License, or (at your option) any later version.
 *
 * SIREn is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public
 * License along with this program. If not, see .
 */

package com.sindicetech.siren.similarity;

import org.apache.lucene.search.similarities.DefaultSimilarity;

/**
 * Implements the BM25 saturation function for the term frequency.
 */
public class SirenSimilarity extends DefaultSimilarity {

  private static final long serialVersionUID = 1L;

  private final float K1 = 1.2f;

  @Override
  public float tf(final float freq) {
    return freq * (K1 + 1) / (freq + K1);
  }

}