public class buffertest {    
   /* make ourselves a 5-slot receiving window */
   private static Ringpuffer rbuf = new Ringpuffer(5);

   /* visualization functions */
   private static void show_getbuf (String retval) {
      System.out.print ("\t\t\t^^^ "+ (retval != null? 
                        retval : "[null]" ) + " ^^^"); 
      System.out.println ("\t\t" + rbuf.show() );
   }
   private static void show_delivery (int ack, int s, String data) {
      System.out.print ("--- ["+ s +": "+ data +"] -->");
      System.out.println ("\t\t\t\t" + rbuf.show() );
      if (ack >= 0) {
	  System.out.println ("<-- [ACK_"+ ack + "] ---");
      }
   }

   public static void main(String[] args) throws Exception {
      int EOT = -1000;
      String[] data = {"And", "then", "the", "quick", "brown", "fox", 
                       "jumped", "over", "the", "lazy", "dog"};
      /* Testing sequence: positive values indicate that the corresponding
       * data element (see above) is delivered, negative values indicate that
       * the upper layer requests a packet. 
       */
      int[] trace = {1, 2, 3, -1, -1, 4, 5, 6, -1, 0, -1, -1, 7, 8, 9, -1,
                     -1, 6, 7, 8, -1, -1, 5, 9, -1, 10, -1, -1, 9, 8, -1, 
		     -1, -1, EOT};

      // commence testing sequence
      int i = 0; String message = "";
      do {
         if (trace[i]<0) {
	    /* upper layer requests next packet */
	    String next = (String)rbuf.getnextpacket();
	    if (next != null) { message = message + next; }
	    /* visualize on screen */
	    show_getbuf(next);
	 } else {
	    /* lower layer delivers next packet */
	    int ack = rbuf.storepacket(trace[i], data[trace[i]]);
	    /* visualize on screen */
	    show_delivery(ack, trace[i], data[trace[i]]);
         }
         i++;
      } while (trace[i] != EOT);
      
      System.out.println(message);
   } // of main
} // of UserAgent class