lucene/core/src/java/org/apache/lucene/search/similarities/BasicModelIn.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 org.apache.lucene.search.Explanation;
import static org.apache.lucene.search.similarities.SimilarityBase.log2;

/**
 * The basic tf-idf model of randomness.
 * @lucene.experimental
 */ 
public class BasicModelIn extends BasicModel {
  
  /** Sole constructor: parameter-free */
  public BasicModelIn() {}

  @Override
  public final double score(BasicStats stats, double tfn, double aeTimes1pTfn) {
    long N = stats.getNumberOfDocuments();
    long n = stats.getDocFreq();
    double A = log2((N + 1) / (n + 0.5));

    // basic model I(n) should return A * tfn
    // which we rewrite to A * (1 + tfn) - A
    // so that it can be combined with the after effect while still guaranteeing
    // that the result is non-decreasing with tfn

    return A * aeTimes1pTfn * (1 - 1 / (1 + tfn));
  }
  
  @Override
  public final Explanation explain(BasicStats stats, double tfn, double aeTimes1pTfn) {
    return Explanation.match(
        (float) (score(stats, tfn, aeTimes1pTfn) * (1 + tfn) / aeTimes1pTfn),
        getClass().getSimpleName() +
            ", computed as tfn * log2((N + 1) / (n + 0.5)) from:",
        Explanation.match((float) tfn, "tfn, normalized term frequency"),
        Explanation.match(stats.getNumberOfDocuments(),
            "N, total number of documents with field"),
        Explanation.match(stats.getDocFreq(),
            "n, number of documents containing term"));
  }

  @Override
  public String toString() {
    return "I(n)";
  }
}