//Animation Applet, version 2 Last updated 2/21/99
//M.Dubson, Feb. 1999, ENVD 3252

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;
import java.lang.Thread;

public class Animation2Balls extends Applet 
        implements Runnable, MouseListener, MouseMotionListener
{ 
  Thread theThread;
  Image offscreen;
  double w,h; //width and height of screen,  will need to do floating-point arithmatic
  double a;  //acceleration of gravity
  //int x1, y1, x2, y2;
  //int vx1, vy1, vx2, vy2;
  double x1, y1, x2, y2;                      //present positions
  double x1_Last, y1_Last, x2_Last, y2_Last;  //previous positions
  double vx1, vy1, vx2, vy2;                  //present velocities
  double vx1_Last, vy1_Last, vx2_Last, vy2_Last;  //previous velocities
  double dt;    //time step
  double dR;    // distance between ball centers
  double dV;    // magnitude of velocity difference
  double dRx, dRy, dVx, dVy; // x- and y-components of position difference and 
                            //velocity difference vectors
  
  double r1;  //radius of ball
  double d1;  //diameter of ball

  

  public void init()
  {
    addMouseMotionListener(this);
    addMouseListener(this);
    w = (double) getSize().width; h = (double) getSize().height;
    a = 0.3;
    r1 = w/15.0;
    d1 = 2*r1;
    dt = 0.2;  
    x1 = 2*r1; y1 = 3*r1;
    x2 =  w - 2*r1; y2 = 3*r1;
    vx1 = 15; vy1 =0; vx2 = -4; vy2 = 0.5;
    dR = Math.sqrt(Math.pow((x2-x1),2)+Math.pow((y2-y1),2));
    //dV = Math.sqrt(Math.pow((vx2-vx1),2)+Math.pow((vy2-vy1),2));
  }
  



  public void step()
  {
    Graphics g = getGraphics();
             
    dR = Math.sqrt(Math.pow((x2-x1),2)+Math.pow((y2-y1),2));  //distance between balls
    if (dR < d1) //if balls collide
      {
      x1 -= vx1*dt; //if collision has occured, back up a step
      y1 -= vy1*dt;
      x2 -= vx2*dt;
      y2 -= vy2*dt; 
      dR = Math.sqrt(Math.pow((x2-x1),2)+Math.pow((y2-y1),2)); //recompute distance
      double b = (x2-x1)*(vx2-vx1)+(y2-y1)*(vy2-vy1);
      double dvx = b*(x2-x1)/Math.pow(dR,2);
      double dvy = b*(y2-y1)/Math.pow(dR,2);
      vx1 += dvx;
      vy1 += dvy;
      vx2 += -dvx;
      vy2 += -dvy;     
      System.out.println("Collision has occurred.");
      }
    if ((x1 > w-r1 && vx1 > 0) || (x1 < r1 && vx1 < 0) )
      {
        vx1 *= -1;
      }
    if ((y1 > h-r1 && vy1 > 0) ||  (y1 < r1 && vy1 < 0))
      {
        vy1 *= -1;
      }
    if ((x2 > w-r1 && vx2 > 0) || (x2 < r1 && vx2 < 0))
      {
        vx2 *= -1;
      }
    if ((y2 > h-r1 && vy2 > 0) ||  (y2 < r1 && vy2 <0))
      {
        vy2 *= -1;
      } 
    
    vy1 += a*dt;
    vy2 += a*dt;
    x1 += vx1*dt;
    y1 += vy1*dt;
    x2 += vx2*dt;
    y2 += vy2*dt;
    repaint();  
    
  
  }//End of step method

  public void paint(Graphics g)
  {
    g.setColor(Color.yellow);
    g.fillRect(0,0,(int)w,(int)h);
    
    g.setColor(Color.red);
    g.fillOval( (int)(x1 - r1), (int)(y1 - r1), (int)(2*r1),(int) (2*r1));
    g.setColor(Color.blue);
    g.fillOval( (int)(x2 - r1), (int)(y2 - r1), (int)(2*r1), (int)(2*r1));
      
  }//end of paint method

  public void update( Graphics g )
  {
    if (offscreen == null || 
      offscreen.getHeight(this) != h || offscreen.getWidth(this) != w)
      { offscreen = createImage((int)w, (int)h);}
    Graphics g2 = offscreen.getGraphics();
    paint(g2);
    g.drawImage( offscreen, 0, 0, this);
      
  }// end of update method
  
  public void start()
      {
         if( theThread == null ){theThread = new Thread( this );}
         theThread.start();
      }
             
  public void run()
  {
    while( true )
      {
        step();
        try
        {
          theThread.sleep( 10 );
        }
        catch( Exception e ) {;}

      }
  }//end of run method
             
  public void stop()
  {
    if( theThread != null && theThread.isAlive() )
    {
      theThread.stop();
    }
      theThread = null;
  }

  public void mousePressed( MouseEvent e) {;}
  public void mouseDragged( MouseEvent e) {;}
  public void mouseReleased( MouseEvent e) {;}
  public void mouseMoved( MouseEvent e) {;}
  public void mouseClicked( MouseEvent e) {;}
  public void mouseEntered( MouseEvent e) {;} 
  public void mouseExited( MouseEvent e) {;}  

} // End of class Animation