| /* |
| * 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 java.util.Collections; |
| |
| import org.apache.lucene.document.NumericDocValuesField; |
| import org.apache.lucene.index.FieldInvertState; |
| import org.apache.lucene.search.CollectionStatistics; |
| import org.apache.lucene.search.Explanation; |
| import org.apache.lucene.search.IndexSearcher; |
| import org.apache.lucene.search.TermStatistics; |
| import org.apache.lucene.util.SmallFloat; |
| |
| /** |
| * Similarity defines the components of Lucene scoring. |
| * <p> |
| * Expert: Scoring API. |
| * <p> |
| * This is a low-level API, you should only extend this API if you want to implement |
| * an information retrieval <i>model</i>. If you are instead looking for a convenient way |
| * to alter Lucene's scoring, consider just tweaking the default implementation: |
| * {@link BM25Similarity} or extend {@link SimilarityBase}, which makes it easy to compute |
| * a score from index statistics. |
| * <p> |
| * Similarity determines how Lucene weights terms, and Lucene interacts with |
| * this class at both <a href="#indextime">index-time</a> and |
| * <a href="#querytime">query-time</a>. |
| * <p> |
| * <a name="indextime">Indexing Time</a> |
| * At indexing time, the indexer calls {@link #computeNorm(FieldInvertState)}, allowing |
| * the Similarity implementation to set a per-document value for the field that will |
| * be later accessible via {@link org.apache.lucene.index.LeafReader#getNormValues(String)}. |
| * Lucene makes no assumption about what is in this norm, but it is most useful for |
| * encoding length normalization information. |
| * <p> |
| * Implementations should carefully consider how the normalization is encoded: while |
| * Lucene's {@link BM25Similarity} encodes length normalization information with |
| * {@link SmallFloat} into a single byte, this might not be suitable for all purposes. |
| * <p> |
| * Many formulas require the use of average document length, which can be computed via a |
| * combination of {@link CollectionStatistics#sumTotalTermFreq()} and |
| * {@link CollectionStatistics#docCount()}. |
| * <p> |
| * Additional scoring factors can be stored in named {@link NumericDocValuesField}s and |
| * accessed at query-time with {@link org.apache.lucene.index.LeafReader#getNumericDocValues(String)}. |
| * However this should not be done in the {@link Similarity} but externally, for instance |
| * by using <tt>FunctionScoreQuery</tt>. |
| * <p> |
| * Finally, using index-time boosts (either via folding into the normalization byte or |
| * via DocValues), is an inefficient way to boost the scores of different fields if the |
| * boost will be the same for every document, instead the Similarity can simply take a constant |
| * boost parameter <i>C</i>, and {@link PerFieldSimilarityWrapper} can return different |
| * instances with different boosts depending upon field name. |
| * <p> |
| * <a name="querytime">Query time</a> |
| * At query-time, Queries interact with the Similarity via these steps: |
| * <ol> |
| * <li>The {@link #scorer(float, CollectionStatistics, TermStatistics...)} method is called a single time, |
| * allowing the implementation to compute any statistics (such as IDF, average document length, etc) |
| * across <i>the entire collection</i>. The {@link TermStatistics} and {@link CollectionStatistics} passed in |
| * already contain all of the raw statistics involved, so a Similarity can freely use any combination |
| * of statistics without causing any additional I/O. Lucene makes no assumption about what is |
| * stored in the returned {@link Similarity.SimScorer} object. |
| * <li>Then {@link SimScorer#score(float, long)} is called for every matching document to compute its score. |
| * </ol> |
| * <p> |
| * <a name="explaintime">Explanations</a> |
| * When {@link IndexSearcher#explain(org.apache.lucene.search.Query, int)} is called, queries consult the Similarity's DocScorer for an |
| * explanation of how it computed its score. The query passes in a the document id and an explanation of how the frequency |
| * was computed. |
| * |
| * @see org.apache.lucene.index.IndexWriterConfig#setSimilarity(Similarity) |
| * @see IndexSearcher#setSimilarity(Similarity) |
| * @lucene.experimental |
| */ |
| public abstract class Similarity { |
| |
| /** |
| * Sole constructor. (For invocation by subclass |
| * constructors, typically implicit.) |
| */ |
| public Similarity() {} |
| |
| /** |
| * Computes the normalization value for a field, given the accumulated |
| * state of term processing for this field (see {@link FieldInvertState}). |
| * |
| * <p>Matches in longer fields are less precise, so implementations of this |
| * method usually set smaller values when <code>state.getLength()</code> is large, |
| * and larger values when <code>state.getLength()</code> is small. |
| * |
| * <p>Note that for a given term-document frequency, greater unsigned norms |
| * must produce scores that are lower or equal, ie. for two encoded norms |
| * {@code n1} and {@code n2} so that |
| * {@code Long.compareUnsigned(n1, n2) > 0} then |
| * {@code SimScorer.score(freq, n1) <= SimScorer.score(freq, n2)} |
| * for any legal {@code freq}. |
| * |
| * <p>{@code 0} is not a legal norm, so {@code 1} is the norm that produces |
| * the highest scores. |
| * |
| * @lucene.experimental |
| * |
| * @param state current processing state for this field |
| * @return computed norm value |
| */ |
| public abstract long computeNorm(FieldInvertState state); |
| |
| /** |
| * Compute any collection-level weight (e.g. IDF, average document length, etc) needed for scoring a query. |
| * |
| * @param boost a multiplicative factor to apply to the produces scores |
| * @param collectionStats collection-level statistics, such as the number of tokens in the collection. |
| * @param termStats term-level statistics, such as the document frequency of a term across the collection. |
| * @return SimWeight object with the information this Similarity needs to score a query. |
| */ |
| public abstract SimScorer scorer(float boost, |
| CollectionStatistics collectionStats, TermStatistics... termStats); |
| |
| /** Stores the weight for a query across the indexed collection. This abstract |
| * implementation is empty; descendants of {@code Similarity} should |
| * subclass {@code SimWeight} and define the statistics they require in the |
| * subclass. Examples include idf, average field length, etc. |
| */ |
| public static abstract class SimScorer { |
| |
| /** |
| * Sole constructor. (For invocation by subclass |
| * constructors.) |
| */ |
| protected SimScorer() {} |
| |
| /** |
| * Score a single document. {@code freq} is the document-term sloppy |
| * frequency and must be finite and positive. {@code norm} is the |
| * encoded normalization factor as computed by |
| * {@link Similarity#computeNorm(FieldInvertState)} at index time, or |
| * {@code 1} if norms are disabled. {@code norm} is never {@code 0}. |
| * <p> |
| * Score must not decrease when {@code freq} increases, ie. if |
| * {@code freq1 > freq2}, then {@code score(freq1, norm) >= |
| * score(freq2, norm)} for any value of {@code norm} that may be produced |
| * by {@link Similarity#computeNorm(FieldInvertState)}. |
| * <p> |
| * Score must not increase when the unsigned {@code norm} increases, ie. if |
| * {@code Long.compareUnsigned(norm1, norm2) > 0} then |
| * {@code score(freq, norm1) <= score(freq, norm2)} for any legal |
| * {@code freq}. |
| * <p> |
| * As a consequence, the maximum score that this scorer can produce is bound |
| * by {@code score(Float.MAX_VALUE, 1)}. |
| * @param freq sloppy term frequency, must be finite and positive |
| * @param norm encoded normalization factor or {@code 1} if norms are disabled |
| * @return document's score |
| */ |
| public abstract float score(float freq, long norm); |
| |
| /** |
| * Explain the score for a single document |
| * @param freq Explanation of how the sloppy term frequency was computed |
| * @param norm encoded normalization factor, as returned by {@link Similarity#computeNorm}, or {@code 1} if norms are disabled |
| * @return document's score |
| */ |
| public Explanation explain(Explanation freq, long norm) { |
| return Explanation.match( |
| score(freq.getValue().floatValue(), norm), |
| "score(freq=" + freq.getValue() +"), with freq of:", |
| Collections.singleton(freq)); |
| } |
| |
| } |
| } |