package org.apache.maven.plugin.surefire.booterclient;
   
   import java.util.Queue;
   import java.util.concurrent.ConcurrentLinkedQueue;
   import java.util.concurrent.atomic.AtomicInteger;
   
   /**
    * A bucket from which fork numbers can be drawn. Any drawn number needs to be returned to the bucket, in order to keep
    * the range of provided values delivered as small as possible.
   * 
   * @author Andreas Gudian
   */
  public class ForkNumberBucket
  {
  
      private static final ForkNumberBucket INSTANCE = new ForkNumberBucket();
  
      private Queue<Integer> qFree = new ConcurrentLinkedQueue<Integer>();
  
      private AtomicInteger highWaterMark = new AtomicInteger( 1 );
  
      /**
       * Non-public constructor
       */
      protected ForkNumberBucket()
      {
      }
  
      /**
       * @return a fork number that is not currently in use. The value must be returned to the bucket using
       *         {@link #returnNumber(int)}.
       */
      public static int drawNumber()
      {
          return getInstance()._drawNumber();
      }
  
      /**
       * @param number the number to return to the bucket so that it can be reused.
       */
      public static void returnNumber( int number )
      {
          getInstance()._returnNumber( number );
      }
  
      /**
       * @return a singleton instance
       */
      private static ForkNumberBucket getInstance()
      {
          return INSTANCE;
      }
  
      /**
       * @return a fork number that is not currently in use. The value must be returned to the bucket using
       *         {@link #returnNumber(int)}.
       */
      protected int _drawNumber()
      {
          Integer nextFree = qFree.poll();
  
          if ( null == nextFree )
          {
              return highWaterMark.getAndIncrement();
          }
          else
          {
              return nextFree.intValue();
          }
      }
  
      /**
       * @return the highest number that has been drawn
       */
      protected int getHighestDrawnNumber()
      {
          return highWaterMark.get() - 1;
      }
  
      /**
       * @param number the number to return to the bucket so that it can be reused.
       */
      protected void _returnNumber( int number )
      {
          qFree.add( Integer.valueOf( number ) );
      }
  }