/*
 * 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;

Model of the information gain based on the ratio of two Bernoulli processes.
@lucene.experimental
/**
 * Model of the information gain based on the ratio of two Bernoulli processes.
 * @lucene.experimental
 */
public class AfterEffectB extends AfterEffect {

  
Sole constructor: parameter-free /** Sole constructor: parameter-free */
  public AfterEffectB() {}

  @Override
  public final double scoreTimes1pTfn(BasicStats stats) {
    long F = stats.getTotalTermFreq()+1;
    long n = stats.getDocFreq()+1;
    return (F + 1.0) / n;
  }
  
  @Override
  public final Explanation explain(BasicStats stats, double tfn) {
    return Explanation.match(
        (float) (scoreTimes1pTfn(stats) / (1 + tfn)),
        getClass().getSimpleName()
            + ", computed as (F + 1) / (n * (tfn + 1)) from:",
        Explanation.match((float) tfn, "tfn, normalized term frequency"),
        Explanation.match(stats.getTotalTermFreq(),
            "F, total number of occurrences of term across all documents + 1"),
        Explanation.match(stats.getDocFreq(),
            "n, number of documents containing term + 1"),
        Explanation.match((float) tfn, "tfn, normalized term frequency"));
  }

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