| /* |
| * 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 double score(BasicStats stats, double tfn, double lambda) { |
| assert lambda != 1; |
| |
| // tfn/(tfn+1) -> 1 - 1/(tfn+1), guaranteed to be non decreasing when tfn increases |
| double q = 1 - 1 / (tfn + 1); |
| if (q == 1) { |
| q = Math.nextDown(1.0); |
| } |
| |
| double pow = Math.pow(lambda, q); |
| if (pow == lambda) { |
| // this can happen because of floating-point rounding |
| // but then we return infinity when taking the log, so we enforce |
| // that pow is different from lambda |
| if (lambda < 1) { |
| // x^y > x when x < 1 and y < 1 |
| pow = Math.nextUp(lambda); |
| } else { |
| // x^y < x when x > 1 and y < 1 |
| pow = Math.nextDown(lambda); |
| } |
| } |
| |
| return -Math.log((pow - lambda) / (1 - lambda)); |
| } |
| |
| @Override |
| public String toString() { |
| return "SPL"; |
| } |
| } |