lucene/core/src/java/org/apache/lucene/search/similarities/Axiomatic.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.similarities;

import java.util.ArrayList;
import java.util.List;
import org.apache.lucene.search.Explanation;

/**
 * Axiomatic approaches for IR. From Hui Fang and Chengxiang Zhai 2005. An Exploration of Axiomatic
 * Approaches to Information Retrieval. In Proceedings of the 28th annual international ACM SIGIR
 * conference on Research and development in information retrieval (SIGIR '05). ACM, New York, NY,
 * USA, 480-487.
 *
 * <p>There are a family of models. All of them are based on BM25, Pivoted Document Length
 * Normalization and Language model with Dirichlet prior. Some components (e.g. Term Frequency,
 * Inverted Document Frequency) in the original models are modified so that they follow some
 * axiomatic constraints.
 *
 * @lucene.experimental
 */
public abstract class Axiomatic extends SimilarityBase {
  /** hyperparam for the growth function */
  protected final float s;

  /** hyperparam for the primitive weighthing function */
  protected final float k;

  /** the query length */
  protected final int queryLen;

  /**
   * Constructor setting all Axiomatic hyperparameters
   *
   * @param s hyperparam for the growth function
   * @param queryLen the query length
   * @param k hyperparam for the primitive weighting function
   */
  public Axiomatic(float s, int queryLen, float k) {
    if (Float.isFinite(s) == false || Float.isNaN(s) || s < 0 || s > 1) {
      throw new IllegalArgumentException("illegal s value: " + s + ", must be between 0 and 1");
    }
    if (Float.isFinite(k) == false || Float.isNaN(k) || k < 0 || k > 1) {
      throw new IllegalArgumentException("illegal k value: " + k + ", must be between 0 and 1");
    }
    if (queryLen < 0 || queryLen > Integer.MAX_VALUE) {
      throw new IllegalArgumentException(
          "illegal query length value: "
              + queryLen
              + ", must be larger 0 and smaller than MAX_INT");
    }
    this.s = s;
    this.queryLen = queryLen;
    this.k = k;
  }

  /**
   * Constructor setting only s, letting k and queryLen to default
   *
   * @param s hyperparam for the growth function
   */
  public Axiomatic(float s) {
    this(s, 1, 0.35f);
  }

  /**
   * Constructor setting s and queryLen, letting k to default
   *
   * @param s hyperparam for the growth function
   * @param queryLen the query length
   */
  public Axiomatic(float s, int queryLen) {
    this(s, queryLen, 0.35f);
  }

  /** Default constructor */
  public Axiomatic() {
    this(0.25f, 1, 0.35f);
  }

  @Override
  public double score(BasicStats stats, double freq, double docLen) {
    double score =
        tf(stats, freq, docLen)
                * ln(stats, freq, docLen)
                * tfln(stats, freq, docLen)
                * idf(stats, freq, docLen)
            - gamma(stats, freq, docLen);
    score *= stats.boost;
    // AxiomaticF3 similarities might produce negative scores due to their gamma component
    return Math.max(0, score);
  }

  @Override
  protected Explanation explain(BasicStats stats, Explanation freq, double docLen) {
    List<Explanation> subs = new ArrayList<>();
    double f = freq.getValue().doubleValue();
    explain(subs, stats, f, docLen);

    double score =
        tf(stats, f, docLen) * ln(stats, f, docLen) * tfln(stats, f, docLen) * idf(stats, f, docLen)
            - gamma(stats, f, docLen);

    Explanation explanation =
        Explanation.match(
            (float) score,
            "score("
                + getClass().getSimpleName()
                + ", freq="
                + freq.getValue()
                + "), computed from:",
            subs);
    if (stats.boost != 1f) {
      explanation =
          Explanation.match(
              (float) (score * stats.boost),
              "Boosted score, computed as (score * boost) from:",
              explanation,
              Explanation.match((float) stats.boost, "Query boost"));
    }
    if (score < 0) {
      explanation =
          Explanation.match(0, "max of:", Explanation.match(0, "Minimum legal score"), explanation);
    }
    return explanation;
  }

  @Override
  protected void explain(List<Explanation> subs, BasicStats stats, double freq, double docLen) {
    if (stats.getBoost() != 1.0d) {
      subs.add(Explanation.match((float) stats.getBoost(), "boost, query boost"));
    }

    subs.add(Explanation.match(this.k, "k, hyperparam for the primitive weighting function"));
    subs.add(Explanation.match(this.s, "s, hyperparam for the growth function"));
    subs.add(Explanation.match(this.queryLen, "queryLen, query length"));
    subs.add(tfExplain(stats, freq, docLen));
    subs.add(lnExplain(stats, freq, docLen));
    subs.add(tflnExplain(stats, freq, docLen));
    subs.add(idfExplain(stats, freq, docLen));
    subs.add(Explanation.match((float) gamma(stats, freq, docLen), "gamma"));
    super.explain(subs, stats, freq, docLen);
  }

  /** Name of the axiomatic method. */
  @Override
  public abstract String toString();

  /** compute the term frequency component */
  protected abstract double tf(BasicStats stats, double freq, double docLen);

  /** compute the document length component */
  protected abstract double ln(BasicStats stats, double freq, double docLen);

  /** compute the mixed term frequency and document length component */
  protected abstract double tfln(BasicStats stats, double freq, double docLen);

  /** compute the inverted document frequency component */
  protected abstract double idf(BasicStats stats, double freq, double docLen);

  /** compute the gamma component (only for F3EXp and F3LOG) */
  protected abstract double gamma(BasicStats stats, double freq, double docLen);

  /**
   * Explain the score of the term frequency component for a single document
   *
   * @param stats the corpus level statistics
   * @param freq number of occurrences of term in the document
   * @param docLen the document length
   * @return Explanation of how the tf component was computed
   */
  protected abstract Explanation tfExplain(BasicStats stats, double freq, double docLen);

  /**
   * Explain the score of the document length component for a single document
   *
   * @param stats the corpus level statistics
   * @param freq number of occurrences of term in the document
   * @param docLen the document length
   * @return Explanation of how the ln component was computed
   */
  protected abstract Explanation lnExplain(BasicStats stats, double freq, double docLen);

  /**
   * Explain the score of the mixed term frequency and document length component for a single
   * document
   *
   * @param stats the corpus level statistics
   * @param freq number of occurrences of term in the document
   * @param docLen the document length
   * @return Explanation of how the tfln component was computed
   */
  protected abstract Explanation tflnExplain(BasicStats stats, double freq, double docLen);

  /**
   * Explain the score of the inverted document frequency component for a single document
   *
   * @param stats the corpus level statistics
   * @param freq number of occurrences of term in the document
   * @param docLen the document length
   * @return Explanation of how the idf component was computed
   */
  protected abstract Explanation idfExplain(BasicStats stats, double freq, double docLen);
}