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

/**
 * This class acts as the base class for the implementations of the <em>first
 * normalization of the informative content</em> in the DFR framework. This
 * component is also called the <em>after effect</em> and is defined by the
 * formula <em>Inf<sub>2</sub> = 1 - Prob<sub>2</sub></em>, where
 * <em>Prob<sub>2</sub></em> measures the <em>information gain</em>.
 * 
 * @see DFRSimilarity
 * @lucene.experimental
 */
public abstract class AfterEffect {
  
  /**
   * Sole constructor. (For invocation by subclass 
   * constructors, typically implicit.)
   */
  public AfterEffect() {}

  /** Returns the product of the after effect with {@code 1+tfn}.
   *  This may not depend on the value of {@code tfn}. */
  public abstract double scoreTimes1pTfn(BasicStats stats);
  
  /** Returns an explanation for the score. */
  public abstract Explanation explain(BasicStats stats, double tfn);
  
  /**
   * Subclasses must override this method to return the code of the
   * after effect formula. Refer to the original paper for the list. 
   */
  @Override
  public abstract String toString();
}