package org.apache.lucene.util;
   
   import org.apache.lucene.util.ByteBlockPool.Allocator;
   
   /*
    * 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.
   */
  
  /**
   * A {@link ByteBlockPool.Allocator} implementation that recycles unused byte
   * blocks in a buffer and reuses them in subsequent calls to
   * {@link #getByteBlock()}.
   * <p>
   * Note: This class is not thread-safe
   * </p>
   * @lucene.internal
   */
  public final class RecyclingByteBlockAllocator extends ByteBlockPool.Allocator {
    private byte[][] freeByteBlocks;
    private final int maxBufferedBlocks;
    private int freeBlocks = 0;
    private final Counter bytesUsed;
    public static final int DEFAULT_BUFFERED_BLOCKS = 64;
  
    /**
     * Creates a new {@link RecyclingByteBlockAllocator}
     * 
     * @param blockSize
     *          the block size in bytes
     * @param maxBufferedBlocks
     *          maximum number of buffered byte block
     * @param bytesUsed
     *          {@link Counter} reference counting internally allocated bytes
     */
    public RecyclingByteBlockAllocator(int blockSize, int maxBufferedBlocks,
        Counter bytesUsed) {
      super(blockSize);
      freeByteBlocks = new byte[maxBufferedBlocks][];
      this.maxBufferedBlocks = maxBufferedBlocks;
      this.bytesUsed = bytesUsed;
    }
  
    /**
     * Creates a new {@link RecyclingByteBlockAllocator}.
     * 
     * @param blockSize
     *          the block size in bytes
     * @param maxBufferedBlocks
     *          maximum number of buffered byte block
     */
    public RecyclingByteBlockAllocator(int blockSize, int maxBufferedBlocks) {
      this(blockSize, maxBufferedBlocks, Counter.newCounter(false));
    }
  
    /**
     * Creates a new {@link RecyclingByteBlockAllocator} with a block size of
     * {@link ByteBlockPool#BYTE_BLOCK_SIZE}, upper buffered docs limit of
     * {@link #DEFAULT_BUFFERED_BLOCKS} ({@value #DEFAULT_BUFFERED_BLOCKS}).
     * 
     */
    public RecyclingByteBlockAllocator() {
      this(ByteBlockPool.BYTE_BLOCK_SIZE, 64, Counter.newCounter(false));
    }
  
    @Override
    public byte[] getByteBlock() {
      if (freeBlocks == 0) {
        bytesUsed.addAndGet(blockSize);
        return new byte[blockSize];
      }
      final byte[] b = freeByteBlocks[--freeBlocks];
      freeByteBlocks[freeBlocks] = null;
      return b;
    }
  
    @Override
    public void recycleByteBlocks(byte[][] blocks, int start, int end) {
      final int numBlocks = Math.min(maxBufferedBlocks - freeBlocks, end - start);
      final int size = freeBlocks + numBlocks;
      if (size >= freeByteBlocks.length) {
        final byte[][] newBlocks = new byte[ArrayUtil.oversize(size,
            RamUsageEstimator.NUM_BYTES_OBJECT_REF)][];
        System.arraycopy(freeByteBlocks, 0, newBlocks, 0, freeBlocks);
        freeByteBlocks = newBlocks;
      }
      final int stop = start + numBlocks;
     for (int i = start; i < stop; i++) {
       freeByteBlocks[freeBlocks++] = blocks[i];
       blocks[i] = null;
     }
     for (int i = stop; i < end; i++) {
       blocks[i] = null;
     }
     bytesUsed.addAndGet(-(end - stop) * blockSize);
     assert bytesUsed.get() >= 0;
   }
 
   /**
    * @return the number of currently buffered blocks
    */
   public int numBufferedBlocks() {
     return freeBlocks;
   }
 
   /**
    * @return the number of bytes currently allocated by this {@link Allocator}
    */
   public long bytesUsed() {
     return bytesUsed.get();
   }
 
   /**
    * @return the maximum number of buffered byte blocks
    */
   public int maxBufferedBlocks() {
     return maxBufferedBlocks;
   }
 
   /**
    * Removes the given number of byte blocks from the buffer if possible.
    * 
    * @param num
    *          the number of byte blocks to remove
    * @return the number of actually removed buffers
    */
   public int freeBlocks(int num) {
     assert num >= 0 : "free blocks must be >= 0 but was: "+ num;
     final int stop;
     final int count;
     if (num > freeBlocks) {
       stop = 0;
       count = freeBlocks;
     } else {
       stop = freeBlocks - num;
       count = num;
     }
     while (freeBlocks > stop) {
       freeByteBlocks[--freeBlocks] = null;
     }
     bytesUsed.addAndGet(-count*blockSize);
     assert bytesUsed.get() >= 0;
     return count;
   }
 }