content: 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 &lt;= 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();
    
  }
  