import java.awt.*;
import java.applet.*;
import java.util.Vector;


//*************************************************************************************
//**********************   Our Canvas  ************************************************
//*************************************************************************************
class our_canvas extends Canvas
{
  public void update(Graphics g)
   {
   }

  public void paint(Graphics g)
   {
   }
}


//*************************************************************************************
//*************************************************************************************
//***************************  Ball Class *********************************************
//*************************************************************************************
//*************************************************************************************
class Ball
{

  static int box_width;
  static int box_height;

  //Following properties are set in the constructor
  int initial_x;
  int initial_y;
  int initial_xdelta;
  int initial_ydelta;
  int ball_width;
  int radius;
  Image ball_pic;

  //Dynamic properties
  int center_x;      //The current position of the center of the ball
  int center_y;
  int pos_x;         //The current position of the upper left corner of the picture
  int pos_y;
  int delta_x;       //The delta value of the ball(speed and direction)
  int delta_y;


  //Constructor
  Ball(int initial_x_in, int initial_y_in, int initial_xdelta_in, int initial_ydelta_in,
       int ball_width_in, Image ball_pic_in)
   {
     initial_x     = initial_x_in;
     initial_y     = initial_y_in;
     delta_x       = initial_xdelta_in;
     delta_y       = initial_ydelta_in;
     ball_width    = ball_width_in;
     ball_pic      = ball_pic_in;
     radius =  ball_width / 2;
     center_x = initial_x + radius;
     center_y = initial_y + radius;
     pos_x = initial_x;
     pos_y  = initial_y;
   }


  public void move_ball()
   {
     pos_x += delta_x;
     pos_y += delta_y;

     if (  (pos_x > (box_width - ball_width) )  || (pos_x < 0)  )
      { flip_horz_direction(); }
     if (  (pos_y > (box_height - ball_width) )  || (pos_y < 0)  )
      { flip_vert_direction(); }

     center_x = pos_x + radius;
     center_y = pos_y + radius;

   }

  public void flip_horz_direction()
    {
       delta_x *= -1;
       pos_x += delta_x;
       center_x = pos_x + radius;
    }

  public void flip_vert_direction()
    {
       delta_y *= -1;
       pos_y += delta_y;
       center_y = pos_y + radius;
    }

  public void set_delta(int delta_x_in, int delta_y_in)
    {
       delta_x = delta_x_in;
       delta_y = delta_y_in;
    }



}
//*************************************************************************************
//*************************************************************************************
//*************************************************************************************



//*************************************************************************************
//*************************************************************************************
//***************************  The Applet *********************************************
//*************************************************************************************
//*************************************************************************************
public class bouncing_balls extends Applet implements Runnable
{

 private AudioClip boink_sound;
 private Button add_button, subtract_button;
 MediaTracker mt;
 Image ball_img = null;
 Image background_img;
 int ball_width;
 int b_width;
 int b_height;
 Image img_buffer;
 Graphics g_buffer;
 Graphics g_canvas;
 Vector ball_vector;
 private Thread animate;
 our_canvas ball_canvas;


 //************************ Start *************************************
 public void start()
  {

   super.init();
   boink_sound = getAudioClip( getDocumentBase(), "boink.au");
   ball_vector = new Vector (1);
   ball_img = getPicImage("ball.gif");
   ball_width = ball_img.getWidth(null);
   background_img = getPicImage("sky.gif");
   b_width = background_img.getWidth(null);
   b_height = background_img.getHeight(null);
   this.resize(b_width, b_height + 35 );   //Leave room for the buttons
   img_buffer = createImage( b_width, b_height);
   g_buffer = img_buffer.getGraphics();
   Ball.box_width  = b_width;
   Ball.box_height = b_height;

   ball_canvas = new our_canvas();
   ball_canvas.resize(b_width, b_height);

   add(ball_canvas);
   add_button = new Button("Add Ball");
   subtract_button = new Button("Subtract Ball");
   add(add_button);
   add(subtract_button);

   add_ball();   //Start with one ball

   if (animate == null)
    {
     animate = new Thread (this);
     animate.start();
    }

  }

  //*******  Run- this method is called by the thread "animate" and starts an endless loop ***
  public void run()
  {
    try
     {
      for (;;)
       {
        Thread.sleep(15);   //We repaint the screen every 15 micro seconds
        collision_detect();
        move_balls();
        repaint();
       }
     }
    catch(InterruptedException e)
     { return; };
  }


 //************************ Stop *************************************
 public void stop()
  {
    if (animate != null)
     {
       animate.stop();
       animate=null;
     }
  }


 //************************ Action *************************************
  public boolean action(Event evt, Object what)
   {
     if (evt.target == add_button)
      {
       add_ball();
      }
     if (evt.target == subtract_button)
      {
       subtract_ball();
      }
     return true;
   }



 //************************ Update *************************************
  public void update(Graphics g)
   {
    paint(g);
   }

 //************************ Paint *************************************
  public void paint(Graphics g)
  {
   g_buffer.drawImage(background_img, 0, 0, null);
   for ( int i=0; i < ball_vector.size(); i++)
   {
    Ball hold_ball = (Ball) ball_vector.elementAt(i);
    g_buffer.drawImage( hold_ball.ball_pic, hold_ball.pos_x, hold_ball.pos_y, null);
   }
   g_canvas= ball_canvas.getGraphics();
   g_canvas.drawImage(img_buffer, 0, 0, null);
  }

 //*************** Add a bouncing ball to the Applet ******************
  public void add_ball()
   {
    int initial_x = (int) ( Math.random() * (b_width - ball_width) );
    int initial_y = (int) ( Math.random() * (b_height - ball_width) );
    int intial_delta_x = (int) (Math.random() * 2) + 1;
    int intial_delta_y = (int) (Math.random() * 2) + 1;
    ball_vector.addElement( new Ball( initial_x, initial_y, intial_delta_x, intial_delta_y, ball_width,  ball_img)  );
  }

 //*************** Subtract a bouncing ball from the Applet ******************
  public void subtract_ball()
   {
   if ( ball_vector.size() > 0 )
       ball_vector.removeElementAt(0);
  }


 //*************** Move the balls ********************************************
  public void move_balls()
  {
   for ( int i=0; i < ball_vector.size(); i++)
   {
    ((Ball)ball_vector.elementAt(i)).move_ball();
   }
  }


  //********************* Collsion detection ********************
  public void collision_detect()
  {
   int A, B;
   double C;
   int curr_ball;
   int hold_delta_x;
   int hold_delta_y;

   for ( curr_ball = 0; curr_ball < ball_vector.size(); curr_ball++)
    {
     for ( int i=0; i < ball_vector.size(); i++)
     {
      if ( i != curr_ball)
       {
        Ball hold_curr_ball  = (Ball)ball_vector.elementAt(curr_ball);
        Ball hold_check_ball = (Ball)ball_vector.elementAt(i);
        A = Math.abs(hold_curr_ball.center_x - hold_check_ball.center_x);
        B = Math.abs(hold_curr_ball.center_y - hold_check_ball.center_y);
        C = Math.sqrt( (A*A) + (B*B) );
        if (C < ( hold_curr_ball.radius + hold_check_ball.radius)  )
          {
            boink_sound.play();
            hold_delta_x = hold_curr_ball.delta_x;
            hold_delta_y = hold_curr_ball.delta_y;
            hold_curr_ball.set_delta(hold_check_ball.delta_x, hold_check_ball.delta_y);
            hold_check_ball.set_delta(hold_delta_x, hold_delta_y);
            hold_curr_ball.move_ball();
            hold_check_ball.move_ball();
          }
       }
     }
    }
  }



 //*************** getPicImage **************************************
// Use MediaTracker to insure the images are fully loaded before
// we try to use them
 private Image getPicImage(String image_file_name)
 {
  Image img_work = null;
  mt = new MediaTracker(this);
  try
   {
    img_work = getImage(getDocumentBase(),image_file_name);
   }
  catch(Exception e1)
   {
    System.out.println(e1);
   }
  mt.addImage(img_work, 0);
  try
  {
   showStatus("Loading image " + image_file_name );
   mt.checkID(0, true);
   mt.waitForID(0);
  }
  catch(InterruptedException e2)
  {
   System.out.println(e2);
  }
  return img_work;
 }


} //---- End of Applet