public class RecvBuffer {
   private Object[] buf;
   private int MAX_BUF;
   private int Nfe = 0;

   public RecvBuffer (int size) {
      buf = new Object[size];
      MAX_BUF  = size;
   }
  
   /* return index position in our array for a given sequence number */
   private int idx (int s) {
      return s%MAX_BUF;
   }

  /* return data of next packet in sequence */
   public Object getnextpacket() {
      if (buf[idx(Nfe)] != null) {
	Object data = buf[idx(Nfe)];
        buf[idx(Nfe++)] = null;
	return data;
      } else {
        return null;
      }
   }

   /* return sequence number if this packet is acknowledged, 
    * or -1 if no ACK is sent. 
    */
   public int storepacket (int s, Object data) {
      if (s < Nfe) {
        return s; 
      } 
      if (s >= (Nfe+MAX_BUF)) {
        return -1; 
      }
      buf[idx(s)] = data;
      return s;  
   }

   /* Optional: return buffer state as string representation */
   private int step = 0;
   public String show () {
      String status = "("+ step++ +")  ";
      
      int Nfe_idx = idx(Nfe);
      for (int i = 0; i < MAX_BUF ; i++) {
	  int offset = (i - Nfe_idx+MAX_BUF)%MAX_BUF;
	  status = status + ((i == Nfe_idx)? "*" : " ")+i + ":";
	  int s = Nfe+offset;
          if (buf[idx(i)] != null) {
	    status = status + "["+s+"],";
	  } else {
	    status = status + " "+s+", ";
	  }
      }
      return status;
   }
}