// This example is from the book _Java in a Nutshell_ by David Flanagan.
// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
// You may study, use, modify, and distribute this example for any purpose.
// This example is provided WITHOUT WARRANTY either expressed or implied.

import java.applet.*;
import java.awt.*;
import java.net.*;
import java.lang.*;

public class kepler extends Applet implements Runnable {
//    static final int deltax = 4;
//    static final int deltay = 2;
static final int deltax = 2;
static final int deltay = 1;
static final int nsteps = 100;

//  Conversion factors from Kepler natural units to CGS

static final double secyear=3.155815e+7;
static final double AUcm = 1.496e+13;

//  Basic constants

static final double Msolar = 1.989e+33;
static final double G = 6.67259e-8;

//  Input data 

double a = 2.0; 
double acm = AUcm * a;
double M = 3.0;
double Mgrams = M * Msolar;
double e = 0.8;
double P;
double Psec;
double r = a*(1.0 - e);
double rcm = acm*(1.0 - e);
int rpixel;
double pixelScale = 150./a;
double theta = 0.0;
double dtheta;
double t = 0.0;
double dt;
int xshift;
int yshift; 

    static final String message = "Smooth Animation2";
Image sun;
Image planet;
int sizeSun;
int sizePlanet;
    int x = 0; 
    int y = 0;
    Color c1 = new Color(0x000000);
    Color c2 = new Color(0xffffff);
    Font font = new Font("Helvetica", Font.BOLD, 24);
    Image offscreen;
    int imagewidth, imageheight;
    int stringwidth, stringheight, stringascent;
    Thread animator = null;
    boolean please_stop = false;

    // Measure the size of the images we'll be animating.  We need this
    // information later to do clipping.

    public void init() {
URL codeBase = getCodeBase();
sun = getImage(codeBase,"siriusA.jpg");
planet = getImage(codeBase,"siriusB.jpg");
sizeSun = sun.getWidth(this);
sizePlanet = planet.getWidth(this);

//  Calculate period in years from Kepler's 3rd Law and 
//  convert to CGS units (seconds).  Then calculate the length of
//  a timestep in seconds.

P = Math.sqrt(a*a*a/M);
Psec = secyear * P;
dt = Psec/((double) nsteps);

// System.out.println("Psec= " + Psec + "  P= " + P);
    }

    // Start the animation
    public void start() { 
        animator = new Thread(this);
        animator.start();
    }
    // Stop it.
    public void stop() { 
        if (animator != null) animator.stop();
        animator = null;
    }
    // Stop and start animating on mouse clicks.
    public boolean mouseDown(Event e, int x, int y) {
        // if running, stop it.  Otherwise, start it.
        if (animator != null) please_stop = true;
        else { please_stop = false; start(); } 
        return true;
    }


    // Draw applet background 
 
    void drawBackground(Graphics gr, Color c1) {
        Dimension size = this.size();
        int w = size.width, h = size.height;
        gr.setColor(c1);
        gr.fillRect(0, 0, w, h);
xshift = w/2 + (int)(e*a*pixelScale);
yshift = h/2;
//gr.drawImage(sun,w/2,h/2,sizeSun,sizeSun,this);          // resize version 
gr.drawImage(sun, yshift - sizeSun/2, yshift - sizeSun/2, this);   
    }

    // This method draws the background and image at its current position.
    public void paint(Graphics g) {
drawBackground(g, c1);
//g.setColor(Color.white);
//g.fillOval(x,y,sizePlanet,sizePlanet);
// g.drawImage(planet,x,y,sizePlanet,sizePlanet,this);    // resize version 
g.drawImage(planet, x-sizePlanet/2, y-sizePlanet/2, this); // no resize version 
    }

    // The body of the animator thread.
    public void run() {
        while(!please_stop) {
            Dimension d = this.size();
            
            // Make sure the offscreen image is created and is the right size.
            if ((offscreen == null) ||
                ((imagewidth != d.width) || (imageheight != d.height))) {
                // if (offscreen != null) offscreen.flush();
                offscreen = this.createImage(d.width, d.height);
                imagewidth = d.width;
                imageheight = d.height;
            }
            
            // Set up clipping.  We only need to draw within the
            // old rectangle that needs to be cleared and the new
            // one that is being drawn.

            // the old rectangle
            Rectangle oldrect = new Rectangle(x-sizePlanet/2, y-sizePlanet/2, 
                                sizePlanet, sizePlanet);
            // Update the coordinates for animation.

//  Update time and theta

t = t + dt;
dtheta = Math.sqrt(G*Mgrams*acm*(1.0 - e*e))*dt/rcm/rcm;
theta = theta + dtheta; 
System.out.println("t= "+t+"  dt="+dt+"  dtheta="+dtheta+"  theta="+theta);

//  Calculate r
r = a*(1.0 - e*e)/(1.0 + e*Math.cos(theta));
rcm = AUcm * r;
rpixel = (int)(r * pixelScale);
//  Calculate relative x and y in pixels
int xrel = (int)(r * Math.cos(theta) * pixelScale);
int yrel = (int)(r * Math.sin(theta) * pixelScale);
System.out.println("r= " + r +"  rpixel= " + rpixel+"  xrel="+xrel+" yrel="+yrel);

x = xrel + xshift;
y = yrel + yshift;

//            x = ((x + deltax)%d.width);
//            y = ((y + deltay)%d.height);

            // the new rectangle
            Rectangle newrect = new Rectangle(x-sizePlanet/2, y-sizePlanet/2, 
                                sizePlanet, sizePlanet);
            // Compute the union of the rectangles
            Rectangle r = newrect.union(oldrect);
                
            // Use this rectangle as the clipping region when
            // drawing to the offscreen image, and when copying
            // from the offscreen image to the screen.
            Graphics g = offscreen.getGraphics();
//            g.clipRect(r.x, r.y, r.width, r.height);
            // Draw into the off-screen image.
            paint(g);
            // Copy it all at once to the screen, using clipping.
            g = this.getGraphics();
//            g.clipRect(r.x, r.y, r.width, r.height);
//g.setColor(Color.yellow);
//g.drawRect(r.x, r.y, r.width, r.height);
            g.drawImage(offscreen, 0, 0, this);
            
            // wait then draw it again!
            try { Thread.sleep(40); } catch (InterruptedException e) { ; }
        }
        animator = null;
    }
}