/*
 * 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;


import java.io.IOException;

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 setScorer method, to hold onto the passed Scorer instance, and call Scorable.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 ScoreCachingWrappingScorer. 
NOTE: 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:
IndexSearcher searcher = new IndexSearcher(indexReader);
final BitSet bits = new BitSet(indexReader.maxDoc());
searcher.search(query, new Collector() {
  public LeafCollector getLeafCollector(LeafReaderContext context)
      throws IOException {
    final int docBase = context.docBase;
    return new LeafCollector() {
      // ignore scorer
      public void setScorer(Scorer scorer) throws IOException {
      }
      public void collect(int doc) throws IOException {
        bits.set(docBase + doc);
      }
    };
  }
});

Not all collectors will need to rebase the docID.  For
example, a collector that simply counts the total number
of hits would skip it.
@lucene.experimental
/**
 * <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 class="prettyprint">
 * IndexSearcher searcher = new IndexSearcher(indexReader);
 * final BitSet bits = new BitSet(indexReader.maxDoc());
 * searcher.search(query, new Collector() {
 *
 *   public LeafCollector getLeafCollector(LeafReaderContext context)
 *       throws IOException {
 *     final int docBase = context.docBase;
 *     return new LeafCollector() {
 *
 *       <em>// ignore scorer</em>
 *       public void setScorer(Scorer scorer) throws IOException {
 *       }
 *
 *       public void collect(int doc) throws IOException {
 *         bits.set(docBase + doc);
 *       }
 *
 *     };
 *   }
 *
 * });
 * </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>
 *
 * @lucene.experimental
 */
public interface LeafCollector {

  
Called before successive calls to collect(int). Implementations that need the score of the current document (passed-in to collect(int)), should save the passed-in Scorer and call scorer.score() when needed. /**
   * 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.
   */
  void setScorer(Scorable scorer) throws IOException;
  
  
Called once for every document matching a query, with the unbased document
number.
Note: The collection of the current segment can be terminated by throwing a CollectionTerminatedException. In this case, the last docs of the current LeafReaderContext will be skipped and IndexSearcher will swallow the exception and continue collection with the next leaf. 
 Note: This is called in an inner search loop. For good search performance, implementations of this method should not call IndexSearcher.doc(int) or IndexReader.document(int) on every hit. Doing so can slow searches by an order of magnitude or more. /**
   * Called once for every document matching a query, with the unbased document
   * number.
   * <p>Note: The collection of the current segment can be terminated by throwing
   * a {@link CollectionTerminatedException}. In this case, the last docs of the
   * current {@link org.apache.lucene.index.LeafReaderContext} will be skipped and {@link IndexSearcher}
   * will swallow the exception and continue collection with the next leaf.
   * <p>
   * Note: This is called in an inner search loop. For good search performance,
   * implementations of this method should not call {@link IndexSearcher#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.
   */
  void collect(int doc) throws IOException;

}