Source Code:            Syntax: C#

// Collision (Pong)
// by REAS <http://reas.com>

// Move the mouse up and down to move the paddle. 

// Updated 13 January 2003 by K Pfeiffer


// Global variables for the ball
double ball_x;
double ball_y;
double ball_dir = 1;
double ball_iSize = 5;  // Radius
double dy = 0;  // Direction

// Global variables for the paddle
int paddle_width = 5;
int paddle_height = 20;

int dist_wall = 15;

public override void setup()
{
  size(200, 200);
  rectMode(CENTER_RADIUS);
  ellipseMode(CENTER_RADIUS);
  noStroke();
  ball_y = height/2;
  ball_x = 1;
  framerate(30);
}

public override void draw() 
{
  background(51);
  
  ball_x += ball_dir * 2;
  ball_y += dy;
  if(ball_x > width+ball_iSize) {
    ball_x = -width/2-ball_iSize;
    ball_y = random(0, height);
    dy = 0;
  }
  
  // Constrain paddle to screen
  double paddle_y = constrain(mouseY, paddle_height, height-paddle_height);

  // Test to see if the ball is touching the paddle
  double py = width-dist_wall-paddle_width-ball_iSize;
  if(ball_x == py 
     && ball_y > paddle_y - paddle_height - ball_iSize 
     && ball_y < paddle_y + paddle_height + ball_iSize) {
    ball_dir *= -1;
    if(mouseY != pmouseY) {
      dy = (mouseY-pmouseY)/2.0;
      if(dy >  5) { dy =  5; }
      if(dy < -5) { dy = -5; }
    }
  } 
  
  // If ball hits paddle or back wall, reverse direction
  if(ball_x < ball_iSize && ball_dir == -1) {
    ball_dir *= -1;
  }
  
  // If the ball is touching top or bottom edge, reverse direction
  if(ball_y > height-ball_iSize) {
    dy = dy * -1;
  }
  if(ball_y < ball_iSize) {
    dy = dy * -1;
  }

  // Draw ball
  fill(255);
  ellipse(ball_x, ball_y, ball_iSize, ball_iSize);
  
  // Draw the paddle
  fill(153);
  rect(width-dist_wall, paddle_y, paddle_width, paddle_height);  
}