import java.util.LinkedList;
import java.util.List;

// This class maintains a statically-allocated pool of memory buffers
public class BufferPool {

   public static final int BUFFER_POOL_SIZE = 512;
   
   public static class Buffer {
      public static final int BUFFER_SIZE = 1024;
      
      private byte[] data;
      
      public Buffer(){
         data = new byte[BUFFER_SIZE];
      }
      public byte[] getData() {
         return data;
      }      
   }
   
   // Note: the list is not internally thread-safe.  
   // So, every read and write must be synchronized
   private final List<Buffer> freeBuffers = new LinkedList<Buffer>();
   
   public BufferPool() {
      for (int i=0;i<BUFFER_POOL_SIZE; i++) {
         Buffer buf = new Buffer();
         freeBuffers.add(buf);
      }
   }
   
   public synchronized int getNumFreeBuffers() {
      return freeBuffers.size();
   }
   
   public synchronized Buffer getFreeBuffer() {
      while (freeBuffers.isEmpty()) {
         try {
            wait();
         }
         catch (InterruptedException iex) {} // ignored
      }
   
      assert (!freeBuffers.isEmpty());
      return freeBuffers.remove(0);
   }
   
   public synchronized void returnFreeBuffer(Buffer buf) {
      freeBuffers.add(buf);      
      notify(); 
   }
}