import java.io.*;
import java.net.*;
import java.util.*;

/**
 * A simple mobile agent
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public abstract class Agent implements Runnable,
                                       Serializable
{
    private byte[]             byteCodes;
    private LinkedList<String> hosts;
    private String             home;


    public static final int    PORT = 9200;
    public static final String EXT  = ".class";



    /**
     * Explicit Value Constructor
     *
     * @param home   The "home" host
     */
    public Agent(String home)
    {
       FileInputStream        in;


       byteCodes = null;

       try 
       {
          in = new FileInputStream(getName()+EXT);
          byteCodes = new byte[in.available()];
          in.read(byteCodes);
       }
       catch (IOException ioe) 
       {
          byteCodes = null;
       }

       this.home = home;
    }



    /**
     * Add a host to the top of the stack
     *
     * @param host    The host
     */
    public void addHost(String host)
    {
       hosts.addFirst(host);
    }


    /**
     * The code that should be executed before this
     * Agent leaves home
     */
    public abstract void beforeDeparture();


    /**
     * Get the Java byte codes for this Agent
     *
     * @return    The byte codes
     */
    public byte[] getByteCodes()
    {
       return byteCodes;
    }



    /**
     * Get the name of the class for this Agent
     *
     * @return   The name
     */
    public String getName()
    {
       return getClass().getName();
    }



    /**
     * Move this Agent to a Sandbox
     *
     * @param host   The host to go to
     */
    public void goTo(String host)
    {
       ObjectOutputStream    out;
       Socket                s;


       try 
       {
          s = new Socket(host, PORT);
          out = new ObjectOutputStream(s.getOutputStream());

          out.writeObject(getName());
          out.writeObject(getByteCodes());
          out.writeObject(this);
       }
       catch (IOException ioe) 
       {
          host = hosts.getFirst();
          hosts.removeFirst();
          if (host != null) goTo(host);
       }
    }





    /**
     * The code that should be executed when this
     * Agent arrives at a remote Sandbox
     */
    public abstract void onArrival();


    /**
     * The code that should be executed when this
     * Agent returns home
     */
    public abstract void onReturn();








    /**
     * The entry point for the controlling thread
     * of execution
     */
    public void run()
    {
       String         host;


       if (hosts == null) 
       {
          hosts = new LinkedList<String>();
          hosts.addLast(home);
          beforeDeparture();
       }
       else if (hosts.size() == 0) 
       {
          onReturn();
       }
       else 
       {
          onArrival();

          host = hosts.getFirst();
          hosts.removeFirst();
          goTo(host);
       }


       // Drop out of the run method which
       // will terminate the thread on the old
       // Sandbox
    }
    
}