/***** Sun-Earth-Simulation & Runge-Kutta-Integration *****
 ***** @author: Christoph Federrath                   *****/

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class SunEarth extends Applet
				implements Runnable, ActionListener, ItemListener
{
    private static final int Lx = 450, Ly = 340; //graphic window
    private int pxE, pyE, pxS, pyS, spx, spy; //pixelcoordinates
    private double xE, xpE, yE, ypE; //coordinates and velocities
    private double xS, xpS, yS, ypS; //coordinates and velocities
    private double coordnew, vnew; //new coords
	private double helpcoord[] = new double[4]; //help
	private double helpv[] = new double[4]; //help
    private double M = 1.98E30; //sun mass
    private static final double m = 5.977E24; //earth mass
	private static final double G = 6.673E-11; //gravity const.
    private double k1, k2, k3, k4, l1, l2, l3, l4; //Runge-Kutta
    private double dt; //Runge-Kutta-timestep
    private double startwert[] = new double[4]; //save initial values
    private int step;
    Canvas can;
    Choice Chooser;
    Button but_ypE_plus, but_ypE_min;
    Checkbox cb_stop;
    Thread animator;
    private Graphics g, offGraphics;
	private Image offImage, blackstar;

    public void init()
    {
        setLayout(null);
        can = new Canvas();
        Panel pan = new Panel();
        pan.setLayout(null);
        can.setBounds(1,1,Lx,Ly+50);
        can.setBackground(Color.black);
        pan.setBounds(Lx+1,1,100,Ly+50);
        pan.setBackground(new Color(0,200,100));
        pan.setFont(new Font("Verdana",Font.PLAIN,10));

        Choice Chooser = new Choice();
        Chooser.add("Values_1");
        Chooser.add("Values_2");
        Chooser.add("Values_3");
        Chooser.add("Values_4");
        but_ypE_plus = new Button("vy_Earth +");
        but_ypE_min = new Button("vy_Earth -");
		cb_stop = new Checkbox("Stop/Go", false);

        Chooser.setBounds(10,90,80,30);
        but_ypE_plus.setBounds(10,140,80,27);
        but_ypE_min.setBounds(10,175,80,27);
        cb_stop.setBounds(10,220,80,30);
		cb_stop.setBackground(new Color(0,200,100));

        pan.add(Chooser); pan.add(cb_stop);
        pan.add(but_ypE_plus); pan.add(but_ypE_min);
        add(can); add(pan);

		Chooser.addItemListener(this);
        but_ypE_plus.addActionListener(this);
        but_ypE_min.addActionListener(this);

        g = can.getGraphics();
        g.setFont(new Font("Verdana",Font.BOLD,10));

		blackstar = getImage(getCodeBase(),"blackstar.gif");

		startwert[0] = 6000.0; //dt
        startwert[1] = M; //M
		startwert[2] = Math.sqrt(G*M/1.496E11); //ypE
		startwert[3] = -10.0; //ypS
    }//init()

    public void startwerte()  //method for setting initial values
    {
		dt = startwert[0];
		M = startwert[1];
		xE = 1.496E11;
        yE = 0.0;
    	xS = 0.0;
    	yS = 0.0;
		xpE = 0.0;
		ypE = startwert[2];
    	xpS = 0.0;
    	ypS = startwert[3];
		step = 0;
    }//startwerte()

    public void run()
    {
        try{Thread.sleep(100);} catch(InterruptedException e){}
        startwerte();

        while(Thread.currentThread() == animator)
        {
			if(cb_stop.getState()) //stop/go
			{try{Thread.sleep(10);}catch(InterruptedException e){}continue;}

	       	runge_step(M,xE,xS,xpE);
        	helpcoord[0] = coordnew;
        	helpv[0] = vnew;
                runge_step(M,yE,yS,ypE);
                helpcoord[1] = coordnew;
                helpv[1] = vnew;
	       	runge_step(m,xS,xE,xpS);
        	helpcoord[2] = coordnew;
        	helpv[2] = vnew;
                runge_step(m,yS,yE,ypS);
                helpcoord[3] = coordnew;
                helpv[3] = vnew;

			xE = helpcoord[0];
			xpE = helpv[0];
                yE = helpcoord[1];
                ypE = helpv[1];

			xS = helpcoord[2];
			xpS = helpv[2];
                yS = helpcoord[3];
                ypS = helpv[3];

        	if(step % 15 == 0) //paint not so often
        	{
	            try{Thread.sleep(5);} catch(InterruptedException e){}
                pixels(); //calculate pixelcoords
                repaint(); //paint new frame
        	}
			step++;
        }//while currentThread()==animator
    }//run()

    public void paint(Graphics gr)  //double buffer
    {
        if(offImage != null)
        {
			gr = can.getGraphics(); //initialize graphic window
            gr.drawImage(offImage, 0, 0, null);
        }
    } //paint(gr)

    public void update(Graphics gr)  //double buffer
    {
		gr = can.getGraphics();
        if(offGraphics == null)
        {
			offImage = createImage(Lx,Ly+50);
            offGraphics = offImage.getGraphics();
        }
        offGraphics.setFont(new Font("Verdana",Font.BOLD,10));
        offGraphics.setColor(can.getBackground());
        offGraphics.fillRect(0, 0, Lx, Ly+50);
        paintFrame(offGraphics);
        gr.drawImage(offImage, 0, 0, null);
    } //update(gr)

    public void paintFrame(Graphics gr)
    {
		gr.drawImage(blackstar, 0, 0, this);
		print(gr); //print values
        gr.setColor(Color.red);
        gr.drawLine(0,Ly/2,Lx,Ly/2);
        gr.drawLine(Lx/2,0,Lx/2,Ly);
        gr.setColor(Color.yellow);
        gr.drawString("Step " + step, 20, 30);
		pixels(); //calculate pixelcoords
        gr.setColor(Color.yellow);
        gr.fillOval(pxS-11,pyS-11,23,23);
        gr.setColor(Color.blue);
        gr.fillOval(pxE-7,pyE-7,15,15);
        gr.setColor(Color.green);
        gr.fillOval(spx-1,spy-1,3,3);
    } //paintFrame(gr)

    public void print(Graphics gr)
    {
		gr.setColor(Color.yellow);
		gr.drawString("Energy:",20,Ly+20);
		gr.drawString("" +Math.round(energy()/1.E28) + "E28",20,Ly+40);
		gr.drawString("M / m:",110,Ly+20);
		gr.drawString("" +(double)Math.round(M/m),110,Ly+40);
		gr.drawString("Distance:",185,Ly+20);
		gr.drawString("" +(double)Math.round(1.E-8*r())*1.E8 + " m",185,Ly+40);
		gr.drawString("v_Earth:",275,Ly+20);
		gr.drawString("" +(double)Math.round(10*vE())/10 + " m/s",275,Ly+40);
		gr.drawString("Time:",370,Ly+20);
		gr.drawString("" +Math.round(step*dt/3600./24.) + " days",370,Ly+40);
    }//print(gr)

    public double energy()
    {
    	return 0.5*m*vE()*vE() + 0.5*M*vS()*vS() - G*m*M/r();
    }//energy()

    public double r()  //distance between sun and earth
    {
    	return Math.sqrt((xE-xS)*(xE-xS)+(yE-yS)*(yE-yS));
    }//r()

    public double[] Schwerpunkt()  //balance point
    {
    	double R[] = new double[2];
    	R[0] = (M*xS+m*xE)/(M+m);
    	R[1] = (M*yS+m*yE)/(M+m);
    	return R;
    }//r()

    public double vE()
    {
    	return Math.sqrt(xpE*xpE+ypE*ypE);
    }//vE()

    public double vS()
    {
    	return Math.sqrt(xpS*xpS+ypS*ypS);
    }//vS()

    public double force(double mass, double coord1, double coord2)
    {
        return -G*mass/Math.pow(r(),3)*(coord1-coord2);
    }//force(mass, coord1, coord2)

    public void runge_step(double mass, double coord1, double coord2, double v)
    {
        k1 = dt*v;
        l1 = dt*force(mass, coord1, coord2);
        k2 = dt*(v+l1/2);
        l2 = dt*force(mass, coord1+k1/2, coord2);
        k3 = dt*(v+l2/2);
        l3 = dt*force(mass, coord1+k2/2, coord2);
        k4 = dt*(v+l3);
        l4 = dt*force(mass, coord1+k3, coord2);
        coordnew = coord1 + (k1+2*k2+2*k3+k4)/6;
        vnew     = v + (l1+2*l2+2*l3+l4)/6;
    }//runge_step(mass, coord1, coord2, v)

    public void pixels()
    {
        pxE = (int)Math.round((double)(Lx/2) + 1.E-9*xE);
        pyE = (int)Math.round((double)(Ly/2) - 1.E-9*yE);
        pxS = (int)Math.round((double)(Lx/2) + 1.E-9*xS);
        pyS = (int)Math.round((double)(Ly/2) - 1.E-9*yS);
    	spx = (int)Math.round((double)(Lx/2) + 1.E-9*Schwerpunkt()[0]);
    	spy = (int)Math.round((double)(Ly/2) - 1.E-9*Schwerpunkt()[1]);
    }//pixels()

    public void actionPerformed(ActionEvent evt)
    {
        if(evt.getActionCommand()==but_ypE_plus.getActionCommand())
        {
           	stop();
           	startwert[2] = startwert[2]/0.9;
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
        }
        if(evt.getActionCommand()==but_ypE_min.getActionCommand())
        {
           	stop();
           	startwert[2] = startwert[2]*0.9;
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
        }
    }//actionPerformed(evt)

	public void itemStateChanged(ItemEvent evt) //Choice Objekt
	{
        if(evt.getItem() == "Values_1")
        {
			stop();
           	startwert[0] = 6000.0; //dt
            startwert[1] = 1.98E30; //M
            startwert[2] = Math.sqrt(G*startwert[1]/1.496E11); //ypE
            startwert[3] = - 10.0; //ypS
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
            start();
        }
        if(evt.getItem() == "Values_2")
        {
			stop();
   	        startwert[0] = 500000.0; //dt
	       	startwert[1] = 30*m; //M
    	    startwert[2] = Math.sqrt(G*startwert[1]/1.496E11); //ypE
            startwert[3] = - 10.0; //ypS
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
            start();
        }
        if(evt.getItem() == "Values_3")
        {
			stop();
           	startwert[0] = 2000000.0; //dt
            startwert[1] = 4*m; //M
            startwert[2] = Math.sqrt(G*startwert[1]/1.496E11); //ypE
            startwert[3] = - 26.0; //ypS
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
            start();
        }
        if(evt.getItem() == "Values_4")
        {
			stop();
           	startwert[0] = 2000000.0; //dt
            startwert[1] = m; //M
            startwert[2] = Math.sqrt(G*startwert[1]/1.496E11); //ypE
            startwert[3] = -55.0; //ypS
			startwerte(); //set initial values
            pixels(); //calculate pixelcoords
			update(g); //paint initial state
			start();
            start();
        }
	} //itemStateChanged(evt)

    public void start()
    {
	    animator = new Thread(this);
        animator.start();
    }//start()

    public void stop()
    {
		animator = null;
    	offImage = null;
		offGraphics = null;
    }//stop()
}