/*
 * Copyright 2015 Terracotta, Inc., a Software AG company.
 *
 * Licensed 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.terracotta.offheapstore.paging;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.nio.ByteBuffer;
import java.util.Collection;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicLong;

A PhantomReference based limited byte buffer source. 
 This buffer source tracks 'freeing' of allocated byte buffers using phantom references to the allocated buffers and an associated reference queue. An AtomicLong is then used to track number of available bytes for allocation. 
Author:
Chris Dennis/**
 * A {@link PhantomReference} based limited byte buffer source.
 * <p>
 * This buffer source tracks 'freeing' of allocated byte buffers using phantom
 * references to the allocated buffers and an associated reference queue.  An
 * {@link AtomicLong} is then used to track number of available bytes for
 * allocation.
 *
 * @author Chris Dennis
 */
public class PhantomReferenceLimitedPageSource implements PageSource {

  private final ReferenceQueue<ByteBuffer> allocatedBuffers = new ReferenceQueue<>();
  private final Map<PhantomReference<ByteBuffer>, Integer> bufferSizes = new ConcurrentHashMap<>();

  private final AtomicLong max;

  
Create a source that will allocate at most max bytes. 
Params:
max – the maximum total size of all available buffers/**
   * Create a source that will allocate at most {@code max} bytes.
   *
   * @param max the maximum total size of all available buffers
   */
  public PhantomReferenceLimitedPageSource(long max) {
      this.max = new AtomicLong(max);
  }

  
Allocates a byte buffer of the given size.
 This BufferSource places no restrictions on the requested size of the buffer. /**
   * Allocates a byte buffer of the given size.
   * <p>
   * This {@code BufferSource} places no restrictions on the requested size of
   * the buffer.
   */
  @Override
  public Page allocate(int size, boolean thief, boolean victim, OffHeapStorageArea owner) {
      while (true) {
          processQueue();
          long now = max.get();
          if (now < size) {
            return null;
          } else if (max.compareAndSet(now, now - size)) {
            ByteBuffer buffer;
            try {
              buffer = ByteBuffer.allocateDirect(size);
            } catch (OutOfMemoryError e) {
              return null;
            }
            bufferSizes.put(new PhantomReference<>(buffer, allocatedBuffers), size);
            return new Page(buffer, owner);
          }
      }
  }

  
Frees the supplied buffer.

This implementation is a no-op, no validation of the supplied buffer is
attempted, as freeing of allocated buffers is monitored via phantom
references.
/**
   * Frees the supplied buffer.
   * <p>
   * This implementation is a no-op, no validation of the supplied buffer is
   * attempted, as freeing of allocated buffers is monitored via phantom
   * references.
   */
  @Override
  public void free(Page buffer) {
    //no-op
  }

  private void processQueue() {
      while (true) {
          Reference<?> ref = allocatedBuffers.poll();

          if (ref == null) {
              return;
          } else {
              Integer size = bufferSizes.remove(ref);
              max.addAndGet(size.longValue());
          }
      }
  }

  @Override
  public String toString() {
    StringBuilder sb = new StringBuilder("PhantomReferenceLimitedBufferSource\n");
    Collection<Integer> buffers = bufferSizes.values();
    sb.append("Bytes Available   : ").append(max.get()).append('\n');
    sb.append("Buffers Allocated : (count=").append(buffers.size()).append(") ").append(buffers);
    return sb.toString();
  }
}