lucene/misc/src/java/org/apache/lucene/search/similarity/LegacyBM25Similarity.java

/*
 * 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.similarity;

import org.apache.lucene.index.FieldInvertState;
import org.apache.lucene.search.CollectionStatistics;
import org.apache.lucene.search.TermStatistics;
import org.apache.lucene.search.similarities.BM25Similarity;
import org.apache.lucene.search.similarities.Similarity;

/**
 * Similarity that behaves like {@link BM25Similarity} while also applying
 * the k1+1 factor to the numerator of the scoring formula
 *
 * @see BM25Similarity
 *
 * @deprecated {@link BM25Similarity} should be used instead
 */
@Deprecated
public final class LegacyBM25Similarity extends Similarity {

  private final BM25Similarity bm25Similarity;

  /** BM25 with these default values:
   * <ul>
   *   <li>{@code k1 = 1.2}</li>
   *   <li>{@code b = 0.75}</li>
   * </ul>
   */
  public LegacyBM25Similarity() {
    this.bm25Similarity = new BM25Similarity();
  }

  /**
   * BM25 with the supplied parameter values.
   * @param k1 Controls non-linear term frequency normalization (saturation).
   * @param b Controls to what degree document length normalizes tf values.
   * @throws IllegalArgumentException if {@code k1} is infinite or negative, or if {@code b} is
   *         not within the range {@code [0..1]}
   */
  public LegacyBM25Similarity(float k1, float b) {
    this.bm25Similarity = new BM25Similarity(k1, b);
  }

  @Override
  public long computeNorm(FieldInvertState state) {
    return bm25Similarity.computeNorm(state);
  }

  @Override
  public SimScorer scorer(float boost, CollectionStatistics collectionStats, TermStatistics... termStats) {
    return bm25Similarity.scorer(boost * (1 + bm25Similarity.getK1()), collectionStats, termStats);
  }

  /**
   * Returns the <code>k1</code> parameter
   * @see #LegacyBM25Similarity(float, float)
   */
  public final float getK1() {
    return bm25Similarity.getK1();
  }

  /**
   * Returns the <code>b</code> parameter
   * @see #LegacyBM25Similarity(float, float)
   */
  public final float getB() {
    return bm25Similarity.getB();
  }

  /** Sets whether overlap tokens (Tokens with 0 position increment) are
   *  ignored when computing norm.  By default this is true, meaning overlap
   *  tokens do not count when computing norms. */
  public void setDiscountOverlaps(boolean v) {
    bm25Similarity.setDiscountOverlaps(v);
  }

  /**
   * Returns true if overlap tokens are discounted from the document's length.
   * @see #setDiscountOverlaps
   */
  public boolean getDiscountOverlaps() {
    return bm25Similarity.getDiscountOverlaps();
  }

  @Override
  public String toString() {
    return bm25Similarity.toString();
  }
}