package org.apache.lucene.search;
/**
* 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 java.io.IOException;
import org.apache.lucene.index.IndexReader;
/**
* <p>Expert: Collectors are primarily meant to be used to
* gather raw results from a search, and implement sorting
* or custom result filtering, collation, etc. </p>
*
* <p>Lucene's core collectors are derived from Collector.
* Likely your application can use one of these classes, or
* subclass {@link TopDocsCollector}, instead of
* implementing Collector directly:
*
* <ul>
*
* <li>{@link TopDocsCollector} is an abstract base class
* that assumes you will retrieve the top N docs,
* according to some criteria, after collection is
* done. </li>
*
* <li>{@link TopScoreDocCollector} is a concrete subclass
* {@link TopDocsCollector} and sorts according to score +
* docID. This is used internally by the {@link
* IndexSearcher} search methods that do not take an
* explicit {@link Sort}. It is likely the most frequently
* used collector.</li>
*
* <li>{@link TopFieldCollector} subclasses {@link
* TopDocsCollector} and sorts according to a specified
* {@link Sort} object (sort by field). This is used
* internally by the {@link IndexSearcher} search methods
* that take an explicit {@link Sort}.
*
* <li>{@link TimeLimitingCollector}, which wraps any other
* Collector and aborts the search if it's taken too much
* time.</li>
*
* <li>{@link PositiveScoresOnlyCollector} wraps any other
* Collector and prevents collection of hits whose score
* is <= 0.0</li>
*
* </ul>
*
* <p>Collector decouples the score from the collected doc:
* the score computation is skipped entirely if it's not
* needed. Collectors that do need the score should
* implement the {@link #setScorer} method, to hold onto the
* passed {@link Scorer} instance, and call {@link
* Scorer#score()} within the collect method to compute the
* current hit's score. If your collector may request the
* score for a single hit multiple times, you should use
* {@link ScoreCachingWrappingScorer}. </p>
*
* <p><b>NOTE:</b> The doc that is passed to the collect
* method is relative to the current reader. If your
* collector needs to resolve this to the docID space of the
* Multi*Reader, you must re-base it by recording the
* docBase from the most recent setNextReader call. Here's
* a simple example showing how to collect docIDs into a
* BitSet:</p>
*
* <pre>
* Searcher searcher = new IndexSearcher(indexReader);
* final BitSet bits = new BitSet(indexReader.maxDoc());
* searcher.search(query, new Collector() {
* private int docBase;
*
* <em>// ignore scorer</em>
* public void setScorer(Scorer scorer) {
* }
*
* <em>// accept docs out of order (for a BitSet it doesn't matter)</em>
* public boolean acceptsDocsOutOfOrder() {
* return true;
* }
*
* public void collect(int doc) {
* bits.set(doc + docBase);
* }
*
* public void setNextReader(IndexReader reader, int docBase) {
* this.docBase = docBase;
* }
* });
* </pre>
*
* <p>Not all collectors will need to rebase the docID. For
* example, a collector that simply counts the total number
* of hits would skip it.</p>
*
* <p><b>NOTE:</b> Prior to 2.9, Lucene silently filtered
* out hits with score <= 0. As of 2.9, the core Collectors
* no longer do that. It's very unusual to have such hits
* (a negative query boost, or function query returning
* negative custom scores, could cause it to happen). If
* you need that behavior, use {@link
* PositiveScoresOnlyCollector}.</p>
*
* @lucene.experimental
*
* @since 2.9
*/
public abstract class Collector {
/**
* Called before successive calls to {@link #collect(int)}. Implementations
* that need the score of the current document (passed-in to
* {@link #collect(int)}), should save the passed-in Scorer and call
* scorer.score() when needed.
*/
public abstract void setScorer(Scorer scorer) throws IOException;
/**
* Called once for every document matching a query, with the unbased document
* number.
*
* <p>
* Note: This is called in an inner search loop. For good search performance,
* implementations of this method should not call {@link Searcher#doc(int)} or
* {@link org.apache.lucene.index.IndexReader#document(int)} on every hit.
* Doing so can slow searches by an order of magnitude or more.
*/
public abstract void collect(int doc) throws IOException;
/**
* Called before collecting from each IndexReader. All doc ids in
* {@link #collect(int)} will correspond to reader.
*
* Add docBase to the current IndexReaders internal document id to re-base ids
* in {@link #collect(int)}.
*
* @param reader
* next IndexReader
* @param docBase
*/
public abstract void setNextReader(IndexReader reader, int docBase) throws IOException;
/**
* Return <code>true</code> if this collector does not
* require the matching docIDs to be delivered in int sort
* order (smallest to largest) to {@link #collect}.
*
* <p> Most Lucene Query implementations will visit
* matching docIDs in order. However, some queries
* (currently limited to certain cases of {@link
* BooleanQuery}) can achieve faster searching if the
* <code>Collector</code> allows them to deliver the
* docIDs out of order.</p>
*
* <p> Many collectors don't mind getting docIDs out of
* order, so it's important to return <code>true</code>
* here.
*/
public abstract boolean acceptsDocsOutOfOrder();
}