/**
    * 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;
  
  import java.nio.ByteBuffer;
  import java.util.Queue;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.locks.ReentrantLock;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.util.BoundedArrayQueue;
  
  import com.google.common.annotations.VisibleForTesting;
  
  /**
   * Like Hadoops' ByteBufferPool only you do not specify desired size when getting a ByteBuffer.
   * This pool keeps an upper bound on the count of ByteBuffers in the pool and on the maximum size
   * of ByteBuffer that it will retain (Hence the pool is 'bounded' as opposed to, say,
   * Hadoop's ElasticByteBuffferPool).
   * If a ByteBuffer is bigger than the configured threshold, we will just let the ByteBuffer go
   * rather than add it to the pool. If more ByteBuffers than the configured maximum instances,
   * we will not add the passed ByteBuffer to the pool; we will just drop it
   * (we will log a WARN in this case that we are at capacity).
   *
   * <p>The intended use case is a reservoir of bytebuffers that an RPC can reuse; buffers tend to
   * achieve a particular 'run' size over time give or take a few extremes. Set TRACE level on this
   * class for a couple of seconds to get reporting on how it is running when deployed.
   *
   * <p>This class is thread safe.
   */
  @InterfaceAudience.Private
  public class BoundedByteBufferPool {
    private static final Log LOG = LogFactory.getLog(BoundedByteBufferPool.class);
  
    @VisibleForTesting
    final Queue<ByteBuffer> buffers;
  
    // Maximum size of a ByteBuffer to retain in pool
    private final int maxByteBufferSizeToCache;
  
    // A running average only it only rises, it never recedes
    @VisibleForTesting
    volatile int runningAverage;
  
    // Scratch that keeps rough total size of pooled bytebuffers
    private volatile int totalReservoirCapacity;
  
    // For reporting
    private AtomicLong allocations = new AtomicLong(0);
  
    private ReentrantLock lock =  new ReentrantLock();
  
    /**
     * @param maxByteBufferSizeToCache
     * @param initialByteBufferSize
     * @param maxToCache
     */
    public BoundedByteBufferPool(final int maxByteBufferSizeToCache, final int initialByteBufferSize,
        final int maxToCache) {
      this.maxByteBufferSizeToCache = maxByteBufferSizeToCache;
      this.runningAverage = initialByteBufferSize;
      this.buffers = new BoundedArrayQueue<ByteBuffer>(maxToCache);
    }
  
    public ByteBuffer getBuffer() {
      ByteBuffer bb = null;
      lock.lock();
      try {
        bb = this.buffers.poll();
        if (bb != null) {
          this.totalReservoirCapacity -= bb.capacity();
        }
      } finally {
        lock.unlock();
      }
      if (bb != null) {
        // Clear sets limit == capacity. Postion == 0.
        bb.clear();
      } else {
        bb = ByteBuffer.allocate(this.runningAverage);
        this.allocations.incrementAndGet();
      }
     if (LOG.isTraceEnabled()) {
       LOG.trace("runningAverage=" + this.runningAverage +
         ", totalCapacity=" + this.totalReservoirCapacity + ", count=" + this.buffers.size() +
         ", alloctions=" + this.allocations.get());
     }
     return bb;
   }
 
   public void putBuffer(ByteBuffer bb) {
     // If buffer is larger than we want to keep around, just let it go.
     if (bb.capacity() > this.maxByteBufferSizeToCache) return;
     boolean success = false;
     int average = 0;
     lock.lock();
     try {
       success = this.buffers.offer(bb);
       if (success) {
         this.totalReservoirCapacity += bb.capacity();
         average = this.totalReservoirCapacity / this.buffers.size(); // size will never be 0.
       }
     } finally {
       lock.unlock();
     }
     if (!success) {
       if (LOG.isDebugEnabled()) {
         LOG.debug("At capacity: " + this.buffers.size());
       }
     } else {
       if (average > this.runningAverage && average < this.maxByteBufferSizeToCache) {
         this.runningAverage = average;
       }
     }
   }
 }