/*
* 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;
/**
* The smoothed power-law (SPL) distribution for the information-based framework
* that is described in the original paper.
* <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.</p>
* WARNING: this model currently returns infinite scores for very small
* tf values and negative scores for very large tf values
* @lucene.experimental
*/
public class DistributionSPL extends Distribution {
/** Sole constructor: parameter-free */
public DistributionSPL() {}
@Override
public final float score(BasicStats stats, float tfn, float lambda) {
if (lambda == 1f) {
lambda = 0.99f;
}
return (float)-Math.log(
(Math.pow(lambda, (tfn / (tfn + 1))) - lambda) / (1 - lambda));
}
@Override
public String toString() {
return "SPL";
}
}