float x;  // ball's x position  
float y;  // ball's y position
float gravity; // acceleration due to gravity
int ballRadius;
int paddleWidth;
float xv, yv;  // x and y velocities

void setup(){
  size(800,600);
  x = width*0.5;
  y = height*0.5;
  ballRadius = 50;  // nice large ball
  paddleWidth = 100;  // nice wide paddle
  xv = 0.2;
  yv = -4.0; // start with the ball moving up
  gravity = 0.1;  // adjust gravity here
}


void draw(){
  background(200);
  ellipse(x,y,ballRadius,ballRadius);
  rect(mouseX-paddleWidth*0.5,height-20,paddleWidth,10);
  x = x + xv;
  y = y + yv;
  /*
  if(y > height-ballRadius*0.5 || y < 0){
    yv = yv * -1;
    y = y + yv;
  }
  */
  // corrected paddle bounce
  if(abs(x-mouseX) < paddleWidth*0.5 && abs(y-(height-20)) < ballRadius*0.5){
    yv = yv * -1;
    y = y + yv;
  }
  // bounce off sides
  if(x > width || x < 0){
    xv = xv * -1;
    x = x + xv;
  }
  yv = yv + gravity;
}