code: /**
   * Copyright (c) 2015 YCSB contributors. All rights reserved.
   *
   * 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. See accompanying
   * LICENSE file.
   */
  package com.yahoo.ycsb.generator;
  
  import java.util.concurrent.locks.ReentrantLock;
  
  /**
   * A CounterGenerator that reports generated integers via lastInt()
   * only after they have been acknowledged.
   */
  public class AcknowledgedCounterGenerator extends CounterGenerator
  {
  	/** The size of the window of pending id ack's. 2^20 = {@value} */
  	static final int WINDOW_SIZE = Integer.rotateLeft(1, 20);
  
  	/** The mask to use to turn an id into a slot in {@link #window}. */
  	private static final int WINDOW_MASK = WINDOW_SIZE - 1;
  
  	private final ReentrantLock lock;
  	private final boolean[] window;
  	private volatile int limit;
  
  	/**
  	 * Create a counter that starts at countstart.
  	 */
  	public AcknowledgedCounterGenerator(int countstart)
  	{
  		super(countstart);
  		lock = new ReentrantLock();
  		window = new boolean[WINDOW_SIZE];
  		limit = countstart - 1;
  	}
  
  	/**
  	 * In this generator, the highest acknowledged counter value
  	 * (as opposed to the highest generated counter value).
  	 */
  	@Override
  	public Integer lastValue()
  	{
  		return limit;
  	}
  
  	/**
  	 * Make a generated counter value available via lastInt().
  	 */
  	public void acknowledge(int value)
  	{
  		final int currentSlot = (value & WINDOW_MASK);
  		if (window[currentSlot] == true) {
  			throw new RuntimeException("Too many unacknowledged insertion keys.");
  		}
  
  		window[currentSlot] = true;
  
  		if (lock.tryLock()) {
  			// move a contiguous sequence from the window
  			// over to the "limit" variable
  			try {
  			  // Only loop through the entire window at most once.
  			  int beforeFirstSlot = (limit & WINDOW_MASK);
  				int index;
  				for (index = limit + 1; index != beforeFirstSlot; ++index) {
  					int slot = (index & WINDOW_MASK);
  					if (!window[slot]) {
  						break;
  					}
  
  					window[slot] = false;
  				}
  
  				limit = index - 1;
  			} finally {
  				lock.unlock();
  			}
  		}
  	}
  }
  