| /* |
| * 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; |
| |
| import static org.apache.lucene.search.similarities.SimilarityBase.log2; |
| |
| /** |
| * Normalization model in which the term frequency is inversely related to the |
| * length. |
| * <p>While this model is parameterless in the |
| * <a href="http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.101.742"> |
| * original article</a>, the <a href="http://theses.gla.ac.uk/1570/">thesis</a> |
| * introduces the parameterized variant. |
| * The default value for the {@code c} parameter is {@code 1}.</p> |
| * @lucene.experimental |
| */ |
| public class NormalizationH2 extends Normalization { |
| private final float c; |
| |
| /** |
| * Creates NormalizationH2 with the supplied parameter <code>c</code>. |
| * @param c hyper-parameter that controls the term frequency |
| * normalization with respect to the document length. |
| */ |
| public NormalizationH2(float c) { |
| // unbounded but typical range 0..10 or so |
| if (Float.isFinite(c) == false || c < 0) { |
| throw new IllegalArgumentException("illegal c value: " + c + ", must be a non-negative finite value"); |
| } |
| this.c = c; |
| } |
| |
| /** |
| * Calls {@link #NormalizationH2(float) NormalizationH2(1)} |
| */ |
| public NormalizationH2() { |
| this(1); |
| } |
| |
| @Override |
| public final double tfn(BasicStats stats, double tf, double len) { |
| return tf * log2(1 + c * stats.getAvgFieldLength() / len); |
| } |
| |
| @Override |
| public Explanation explain(BasicStats stats, double tf, double len) { |
| return Explanation.match( |
| (float) tfn(stats, tf, len), |
| getClass().getSimpleName() |
| + ", computed as tf * log2(1 + c * avgfl / fl) from:", |
| Explanation.match((float) tf, |
| "tf, number of occurrences of term in the document"), |
| Explanation.match(c, |
| "c, hyper-parameter"), |
| Explanation.match((float) stats.getAvgFieldLength(), |
| "avgfl, average length of field across all documents"), |
| Explanation.match((float) len, "fl, field length of the document")); |
| } |
| |
| @Override |
| public String toString() { |
| return "2"; |
| } |
| |
| /** |
| * Returns the <code>c</code> parameter. |
| * @see #NormalizationH2(float) |
| */ |
| public float getC() { |
| return c; |
| } |
| } |