/*
 * 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;

/**
 * Expert: Priority queue containing hit docs
 *
 * @lucene.internal
 */
public 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.
   */
  public 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;
    }
  }
}
