/*
 * 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.bucket;

import java.util.Comparator;
import java.util.Map;
import org.apache.hadoop.hbase.io.hfile.BlockCacheKey;
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 larger than maxSize. From
 * then on, only entries that are sorted larger than the smallest current entry will be
 * inserted/replaced.
 * <p>
 * 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.
 */
@InterfaceAudience.Private
public class CachedEntryQueue {

  private static final Comparator<Map.Entry<BlockCacheKey, BucketEntry>> COMPARATOR =
    (a, b) -> BucketEntry.COMPARATOR.compare(a.getValue(), b.getValue());

  private MinMaxPriorityQueue<Map.Entry<BlockCacheKey, BucketEntry>> queue;

  private long cacheSize;
  private long maxSize;

  /**
   * @param maxSize   the target size of elements in the queue
   * @param blockSize expected average size of blocks
   */
  public CachedEntryQueue(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.orderedBy(COMPARATOR).expectedSize(initialSize).create();
    cacheSize = 0;
    this.maxSize = maxSize;
  }

  /**
   * Attempt to add the specified entry to this queue.
   * <p>
   * If the queue is smaller than the max size, or if the specified element is ordered after the
   * smallest element in the queue, the element will be added to the queue. Otherwise, there is no
   * side effect of this call.
   * @param entry a bucket entry with key 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 cacheSize is greater than maxSize,"
        + " which means we have something in the queue")
  public void add(Map.Entry<BlockCacheKey, BucketEntry> entry) {
    if (cacheSize < maxSize) {
      queue.add(entry);
      cacheSize += entry.getValue().getLength();
    } else {
      BucketEntry head = queue.peek().getValue();
      if (BucketEntry.COMPARATOR.compare(entry.getValue(), head) > 0) {
        cacheSize += entry.getValue().getLength();
        cacheSize -= head.getLength();
        if (cacheSize > maxSize) {
          queue.poll();
        } else {
          cacheSize += head.getLength();
        }
        queue.add(entry);
      }
    }
  }

  /** Returns The next element in this queue, or {@code null} if the queue is empty. */
  public Map.Entry<BlockCacheKey, BucketEntry> poll() {
    return queue.poll();
  }

  /** Returns The last element in this queue, or {@code null} if the queue is empty. */
  public Map.Entry<BlockCacheKey, BucketEntry> pollLast() {
    return queue.pollLast();
  }
}
