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

/**
 * Log-logistic distribution.
 *
 * <p>Unlike for DFR, the natural logarithm is used, as it is faster to compute and the original
 * paper does not express any preference to a specific base.
 *
 * @lucene.experimental
 */
public class DistributionLL extends Distribution {

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

  @Override
  public final double score(BasicStats stats, double tfn, double lambda) {
    return -Math.log(lambda / (tfn + lambda));
  }

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