@@ -35,7 +35,7 @@ public abstract class TopDocsCollector<T extends ScoreDoc> implements Collector
 
   /** This is used in case topDocs() is called with illegal parameters, or there
    *  simply aren't (enough) results. */
-  protected static final TopDocs EMPTY_TOPDOCS = new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[0]);
+  public static final TopDocs EMPTY_TOPDOCS = new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[0]);
   
   /**
    * The priority queue which holds the top documents. Note that different
/*
 * 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 org.apache.lucene.util.PriorityQueue;

/**
 * A base class for all collectors that return a {@link TopDocs} output. This
 * collector allows easy extension by providing a single constructor which
 * accepts a {@link PriorityQueue} as well as protected members for that
 * priority queue and a counter of the number of total hits.<br>
 * Extending classes can override any of the methods to provide their own
 * implementation, as well as avoid the use of the priority queue entirely by
 * passing null to {@link #TopDocsCollector(PriorityQueue)}. In that case
 * however, you might want to consider overriding all methods, in order to avoid
 * a NullPointerException.
 */
public abstract class TopDocsCollector<T extends ScoreDoc> implements Collector {

  /** This is used in case topDocs() is called with illegal parameters, or there
   *  simply aren't (enough) results. */
  protected static final TopDocs EMPTY_TOPDOCS = new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[0]);
  
  /**
   * The priority queue which holds the top documents. Note that different
   * implementations of PriorityQueue give different meaning to 'top documents'.
   * HitQueue for example aggregates the top scoring documents, while other PQ
   * implementations may hold documents sorted by other criteria.
   */
  protected final PriorityQueue<T> pq;

  /** The total number of documents that the collector encountered. */
  protected int totalHits;

  /** Whether {@link #totalHits} is exact or a lower bound. */
  protected TotalHits.Relation totalHitsRelation = TotalHits.Relation.EQUAL_TO;

  protected TopDocsCollector(PriorityQueue<T> pq) {
    this.pq = pq;
  }
  
  /**
   * Populates the results array with the ScoreDoc instances. This can be
   * overridden in case a different ScoreDoc type should be returned.
   */
  protected void populateResults(ScoreDoc[] results, int howMany) {
    for (int i = howMany - 1; i >= 0; i--) { 
      results[i] = pq.pop();
    }
  }

  /**
   * Returns a {@link TopDocs} instance containing the given results. If
   * <code>results</code> is null it means there are no results to return,
   * either because there were 0 calls to collect() or because the arguments to
   * topDocs were invalid.
   */
  protected TopDocs newTopDocs(ScoreDoc[] results, int start) {
    return results == null ? EMPTY_TOPDOCS : new TopDocs(new TotalHits(totalHits, totalHitsRelation), results);
  }
  
  /** The total number of documents that matched this query. */
  public int getTotalHits() {
    return totalHits;
  }
  
  /** The number of valid PQ entries */
  protected int topDocsSize() {
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    return totalHits < pq.size() ? totalHits : pq.size();
  }
  
  /** Returns the top docs that were collected by this collector. */
  public TopDocs topDocs() {
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    return topDocs(0, topDocsSize());
  }

  /**
   * Returns the documents in the range [start .. pq.size()) that were collected
   * by this collector. Note that if {@code start >= pq.size()}, an empty TopDocs is
   * returned.<br>
   * This method is convenient to call if the application always asks for the
   * last results, starting from the last 'page'.<br>
   * <b>NOTE:</b> you cannot call this method more than once for each search
   * execution. If you need to call it more than once, passing each time a
   * different <code>start</code>, you should call {@link #topDocs()} and work
   * with the returned {@link TopDocs} object, which will contain all the
   * results this search execution collected.
   */
  public TopDocs topDocs(int start) {
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    return topDocs(start, topDocsSize());
  }

  /**
   * Returns the documents in the range [start .. start+howMany) that were
   * collected by this collector. Note that if {@code start >= pq.size()}, an empty
   * TopDocs is returned, and if pq.size() - start &lt; howMany, then only the
   * available documents in [start .. pq.size()) are returned.<br>
   * This method is useful to call in case pagination of search results is
   * allowed by the search application, as well as it attempts to optimize the
   * memory used by allocating only as much as requested by howMany.<br>
   * <b>NOTE:</b> you cannot call this method more than once for each search
   * execution. If you need to call it more than once, passing each time a
   * different range, you should call {@link #topDocs()} and work with the
   * returned {@link TopDocs} object, which will contain all the results this
   * search execution collected.
   */
  public TopDocs topDocs(int start, int howMany) {
    
    // In case pq was populated with sentinel values, there might be less
    // results than pq.size(). Therefore return all results until either
    // pq.size() or totalHits.
    int size = topDocsSize();

    if (howMany < 0) {
      throw new IllegalArgumentException("Number of hits requested must be greater than 0 but value was " + howMany);
    }

    if (start < 0) {
      throw new IllegalArgumentException("Expected value of starting position is between 0 and " + size +
          ", got " + start);
    }

    if (start >= size || howMany == 0) {
      return newTopDocs(null, start);
    }

    // We know that start < pqsize, so just fix howMany. 
    howMany = Math.min(size - start, howMany);
    ScoreDoc[] results = new ScoreDoc[howMany];

    // pq's pop() returns the 'least' element in the queue, therefore need
    // to discard the first ones, until we reach the requested range.
    // Note that this loop will usually not be executed, since the common usage
    // should be that the caller asks for the last howMany results. However it's
    // needed here for completeness.
    for (int i = pq.size() - start - howMany; i > 0; i--) { pq.pop(); }
    
    // Get the requested results from pq.
    populateResults(results, howMany);
    
    return newTopDocs(results, start);
  }

}

This is used in o.a.l.sandbox.search.LargeNumHitsTopDocsCollector.