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/*
 * 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.hadoop.hbase.io.hfile;

import org.apache.hadoop.hbase.io.HeapSize;
import org.apache.yetus.audience.InterfaceAudience;

import org.apache.hbase.thirdparty.com.google.common.base.Preconditions;
import org.apache.hbase.thirdparty.com.google.common.collect.MinMaxPriorityQueue;

/**
 * A memory-bound queue that will grow until an element brings total size >= maxSize. From then
 * on, only entries that are sorted larger than the smallest current entry will be
 * inserted/replaced.
 * 
 * Use this when you want to find the largest elements (according to their ordering, not their heap
 * size) that consume as close to the specified maxSize as possible. Default behavior is to grow
 * just above rather than just below specified max.
 * 

 * Object used in this queue must implement {@link HeapSize} as well as {@link Comparable}.
 */
@InterfaceAudience.Private
public class LruCachedBlockQueue implements HeapSize {

  private MinMaxPriorityQueue queue;

  private long heapSize;
  private long maxSize;

  /**
   * @param maxSize   the target size of elements in the queue
   * @param blockSize expected average size of blocks
   */
  public LruCachedBlockQueue(long maxSize, long blockSize) {
    Preconditions.checkArgument(blockSize > 0, "negative blockSize %s", blockSize);
    Preconditions.checkArgument(maxSize > 0, "negative maxSize %s", maxSize);
    int initialSize = (int) (maxSize / blockSize);
    if (initialSize == 0) {
      initialSize++;
    }
    queue = MinMaxPriorityQueue.expectedSize(initialSize).create();
    heapSize = 0;
    this.maxSize = maxSize;
  }

  /**
   * Attempt to add the specified cached block to this queue.
   * 
   * If the queue is smaller than the max size, or if the specified element is ordered before the
   * smallest element in the queue, the element will be added to the queue. Otherwise, there is no
   * side effect of this call.
   * @param cb block to try to add to the queue
   */
  @edu.umd.cs.findbugs.annotations.SuppressWarnings(
      value = "NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE",
      justification = "head can not be null as heapSize is greater than maxSize,"
        + " which means we have something in the queue")
  public void add(LruCachedBlock cb) {
    if (heapSize < maxSize) {
      queue.add(cb);
      heapSize += cb.heapSize();
    } else {
      LruCachedBlock head = queue.peek();
      if (cb.compareTo(head) > 0) {
        heapSize += cb.heapSize();
        heapSize -= head.heapSize();
        if (heapSize > maxSize) {
          queue.poll();
        } else {
          heapSize += head.heapSize();
        }
        queue.add(cb);
      }
    }
  }

  /** Returns The next element in this queue, or {@code null} if the queue is empty. */
  public LruCachedBlock poll() {
    return queue.poll();
  }

  /** Returns The last element in this queue, or {@code null} if the queue is empty. */
  public LruCachedBlock pollLast() {
    return queue.pollLast();
  }

  /**
   * Total size of all elements in this queue.
   * @return size of all elements currently in queue, in bytes
   */
  @Override
  public long heapSize() {
    return heapSize;
  }
}