| package org.apache.lucene.search.similarities; |
| |
| /* |
| * 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. |
| */ |
| |
| import static org.apache.lucene.search.similarities.SimilarityBase.log2; |
| |
| /** |
| * Limiting form of the Bose-Einstein model. The formula used in Lucene differs |
| * slightly from the one in the original paper: {@code F} is increased by {@code tfn+1} |
| * and {@code N} is increased by {@code F} |
| * @lucene.experimental |
| * NOTE: in some corner cases this model may give poor performance with Normalizations that |
| * return large values for {@code tfn} such as NormalizationH3. Consider using the |
| * geometric approximation ({@link BasicModelG}) instead, which provides the same relevance |
| * but with less practical problems. |
| */ |
| public class BasicModelBE extends BasicModel { |
| |
| /** Sole constructor: parameter-free */ |
| public BasicModelBE() {} |
| |
| @Override |
| public final float score(BasicStats stats, float tfn) { |
| double F = stats.getTotalTermFreq() + 1 + tfn; |
| // approximation only holds true when F << N, so we use N += F |
| double N = F + stats.getNumberOfDocuments(); |
| return (float)(-log2((N - 1) * Math.E) |
| + f(N + F - 1, N + F - tfn - 2) - f(F, F - tfn)); |
| } |
| |
| /** The <em>f</em> helper function defined for <em>B<sub>E</sub></em>. */ |
| private final double f(double n, double m) { |
| return (m + 0.5) * log2(n / m) + (n - m) * log2(n); |
| } |
| |
| @Override |
| public String toString() { |
| return "Be"; |
| } |
| } |
