@@ -19,7 +19,8 @@ package org.apache.lucene.search;
 
 import org.apache.lucene.util.PriorityQueue;
 
-final class HitQueue extends PriorityQueue<ScoreDoc> {
+/** Expert: Priority queue containing hit docs */
+public final class HitQueue extends PriorityQueue<ScoreDoc> {
 
   /**
    * Creates a new instance with <code>size</code> elements. If
/*
 * 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
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 */
package org.apache.lucene.search;


import org.apache.lucene.util.PriorityQueue;

final class HitQueue extends PriorityQueue<ScoreDoc> {

  /**
   * Creates a new instance with <code>size</code> elements. If
   * <code>prePopulate</code> is set to true, the queue will pre-populate itself
   * with sentinel objects and set its {@link #size()} to <code>size</code>. In
   * that case, you should not rely on {@link #size()} to get the number of
   * actual elements that were added to the queue, but keep track yourself.<br>
   * <b>NOTE:</b> in case <code>prePopulate</code> is true, you should pop
   * elements from the queue using the following code example:
   * 
   * <pre class="prettyprint">
   * PriorityQueue&lt;ScoreDoc&gt; pq = new HitQueue(10, true); // pre-populate.
   * ScoreDoc top = pq.top();
   * 
   * // Add/Update one element.
   * top.score = 1.0f;
   * top.doc = 0;
   * top = (ScoreDoc) pq.updateTop();
   * int totalHits = 1;
   * 
   * // Now pop only the elements that were *truly* inserted.
   * // First, pop all the sentinel elements (there are pq.size() - totalHits).
   * for (int i = pq.size() - totalHits; i &gt; 0; i--) pq.pop();
   * 
   * // Now pop the truly added elements.
   * ScoreDoc[] results = new ScoreDoc[totalHits];
   * for (int i = totalHits - 1; i &gt;= 0; i--) {
   *   results[i] = (ScoreDoc) pq.pop();
   * }
   * </pre>
   * 
   * <p><b>NOTE</b>: This class pre-allocate a full array of
   * length <code>size</code>.
   * 
   * @param size
   *          the requested size of this queue.
   * @param prePopulate
   *          specifies whether to pre-populate the queue with sentinel values.
   */
  HitQueue(int size, boolean prePopulate) {
    super(size, () -> {
      if (prePopulate) {
        // Always set the doc Id to MAX_VALUE so that it won't be favored by
        // lessThan. This generally should not happen since if score is not NEG_INF,
        // TopScoreDocCollector will always add the object to the queue.
        return new ScoreDoc(Integer.MAX_VALUE, Float.NEGATIVE_INFINITY);
      } else {
        return null;
      }
    });
  }

  @Override
  protected final boolean lessThan(ScoreDoc hitA, ScoreDoc hitB) {
    if (hitA.score == hitB.score)
      return hitA.doc > hitB.doc; 
    else
      return hitA.score < hitB.score;
  }
}

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