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| * the License. You may obtain a copy of the License at |
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| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
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| |
| /** |
| * This package contains the various ranking models that can be used in Lucene. The abstract class |
| * {@link org.apache.lucene.search.similarities.Similarity} serves as the base for ranking |
| * functions. For searching, users can employ the models already implemented or create their own by |
| * extending one of the classes in this package. |
| * |
| * <h2>Table Of Contents</h2> |
| * |
| * <ol> |
| * <li><a href="#sims">Summary of the Ranking Methods</a> |
| * <li><a href="#changingSimilarity">Changing the Similarity</a> |
| * </ol> |
| * |
| * <a id="sims"></a> |
| * |
| * <h2>Summary of the Ranking Methods</h2> |
| * |
| * <p>{@link org.apache.lucene.search.similarities.BM25Similarity} is an optimized implementation of |
| * the successful Okapi BM25 model. |
| * |
| * <p>{@link org.apache.lucene.search.similarities.ClassicSimilarity} is the original Lucene scoring |
| * function. It is based on the <a href="http://en.wikipedia.org/wiki/Vector_Space_Model">Vector |
| * Space Model</a>. For more information, see {@link |
| * org.apache.lucene.search.similarities.TFIDFSimilarity}. |
| * |
| * <p>{@link org.apache.lucene.search.similarities.SimilarityBase} provides a basic implementation |
| * of the Similarity contract and exposes a highly simplified interface, which makes it an ideal |
| * starting point for new ranking functions. Lucene ships the following methods built on {@link |
| * org.apache.lucene.search.similarities.SimilarityBase}: |
| * |
| * <p><a id="framework"></a> |
| * |
| * <ul> |
| * <li>Amati and Rijsbergen's {@linkplain org.apache.lucene.search.similarities.DFRSimilarity DFR} |
| * framework; |
| * <li>Clinchant and Gaussier's {@linkplain org.apache.lucene.search.similarities.IBSimilarity |
| * Information-based models} for IR; |
| * <li>The implementation of two {@linkplain org.apache.lucene.search.similarities.LMSimilarity |
| * language models} from Zhai and Lafferty's paper. |
| * <li>{@linkplain org.apache.lucene.search.similarities.DFISimilarity Divergence from |
| * independence} models as described in "IRRA at TREC 2012" (Dinçer). |
| * <li> |
| * </ul> |
| * |
| * Since {@link org.apache.lucene.search.similarities.SimilarityBase} is not optimized to the same |
| * extent as {@link org.apache.lucene.search.similarities.ClassicSimilarity} and {@link |
| * org.apache.lucene.search.similarities.BM25Similarity}, a difference in performance is to be |
| * expected when using the methods listed above. However, optimizations can always be implemented in |
| * subclasses; see <a href="#changingSimilarity">below</a>. |
| * |
| * <p><a id="changingSimilarity"></a> |
| * |
| * <h2>Changing Similarity</h2> |
| * |
| * <p>Chances are the available Similarities are sufficient for all your searching needs. However, |
| * in some applications it may be necessary to customize your <a |
| * href="Similarity.html">Similarity</a> implementation. For instance, some applications do not need |
| * to distinguish between shorter and longer documents and could set BM25's {@link |
| * org.apache.lucene.search.similarities.BM25Similarity#BM25Similarity(float,float) b} parameter to |
| * {@code 0}. |
| * |
| * <p>To change {@link org.apache.lucene.search.similarities.Similarity}, one must do so for both |
| * indexing and searching, and the changes must happen before either of these actions take place. |
| * Although in theory there is nothing stopping you from changing mid-stream, it just isn't |
| * well-defined what is going to happen. |
| * |
| * <p>To make this change, implement your own {@link |
| * org.apache.lucene.search.similarities.Similarity} (likely you'll want to simply subclass {@link |
| * org.apache.lucene.search.similarities.SimilarityBase}), and then register the new class by |
| * calling {@link org.apache.lucene.index.IndexWriterConfig#setSimilarity(Similarity)} before |
| * indexing and {@link org.apache.lucene.search.IndexSearcher#setSimilarity(Similarity)} before |
| * searching. |
| * |
| * <h3>Tuning {@linkplain org.apache.lucene.search.similarities.BM25Similarity}</h3> |
| * |
| * <p>{@link org.apache.lucene.search.similarities.BM25Similarity} has two parameters that may be |
| * tuned: |
| * |
| * <ul> |
| * <li><code>k1</code>, which calibrates term frequency saturation and must be positive or null. A |
| * value of {@code 0} makes term frequency completely ignored, making documents scored only |
| * based on the value of the <code>IDF</code> of the matched terms. Higher values of <code>k1 |
| * </code> increase the impact of term frequency on the final score. Default value is {@code |
| * 1.2}. |
| * <li><code>b</code>, which controls how much document length should normalize term frequency |
| * values and must be in {@code [0, 1]}. A value of {@code 0} disables length normalization |
| * completely. Default value is {@code 0.75}. |
| * </ul> |
| * |
| * <h3>Extending {@linkplain org.apache.lucene.search.similarities.SimilarityBase}</h3> |
| * |
| * <p>The easiest way to quickly implement a new ranking method is to extend {@link |
| * org.apache.lucene.search.similarities.SimilarityBase}, which provides basic implementations for |
| * the low level . Subclasses are only required to implement the {@link |
| * org.apache.lucene.search.similarities.SimilarityBase#score(BasicStats, double, double)} and |
| * {@link org.apache.lucene.search.similarities.SimilarityBase#toString()} methods. |
| * |
| * <p>Another option is to extend one of the <a href="#framework">frameworks</a> based on {@link |
| * org.apache.lucene.search.similarities.SimilarityBase}. These Similarities are implemented |
| * modularly, e.g. {@link org.apache.lucene.search.similarities.DFRSimilarity} delegates computation |
| * of the three parts of its formula to the classes {@link |
| * org.apache.lucene.search.similarities.BasicModel}, {@link |
| * org.apache.lucene.search.similarities.AfterEffect} and {@link |
| * org.apache.lucene.search.similarities.Normalization}. Instead of subclassing the Similarity, one |
| * can simply introduce a new basic model and tell {@link |
| * org.apache.lucene.search.similarities.DFRSimilarity} to use it. |
| */ |
| package org.apache.lucene.search.similarities; |