Notes

var ballSpeedX:int = -3;
var ballSpeedY:int = -2;
var cpuPaddleSpeed:int = 3;
var playerScore:int = 0;
var cpuScore:int = 0;

init();

function init():void
{
stage.addEventListener(Event.ENTER_FRAME, loop);
}

function calculateBallAngle(paddleY:Number, ballY:Number):Number
{
var ySpeed:Number = 5 * ( (ballY-paddleY) / 25 );
// (ballY-paddleY) / 25 will be between -1 and 1 depending on where the ball hits

return ySpeed;
}

function updateTextFields():void
{
playerScoreText.text = ("Player Score: " + playerScore);
cpuScoreText.text = ("CPU Score: " + cpuScore);
}

function loop(e:Event):void
{

if( playerPaddle.hitTestObject(ball) == true ){
if(ballSpeedX < 0){
ballSpeedX *= -1;
ballSpeedY = calculateBallAngle(playerPaddle.y, ball.y); //add this
}
} else if(cpuPaddle.hitTestObject(ball) == true ){ //add this
if(ballSpeedX > 0){
ballSpeedX *= -1;
ballSpeedY = calculateBallAngle(cpuPaddle.y, ball.y); //add this
}

}

if(cpuPaddle.y < ball.y - 10){
cpuPaddle.y += cpuPaddleSpeed;
} else if(cpuPaddle.y > ball.y + 10){
cpuPaddle.y -= cpuPaddleSpeed;
}

playerPaddle.y = mouseY;

//check if top of paddle is above top of screen
if(playerPaddle.y - playerPaddle.height/2 < 0){
playerPaddle.y = playerPaddle.height/2;

//check if bottom of paddle is below bottom of screen
} else if(playerPaddle.y + playerPaddle.height/2 > stage.stageHeight){
playerPaddle.y = stage.stageHeight - playerPaddle.height/2;
}

ball.x += ballSpeedX;
ball.y += ballSpeedY;

//because the ball's position is measured by where its CENTER is...
//...we need add or subtract half of its width or height to see if that SIDE is hitting a wall

//first check the left and right boundaries
if(ball.x <= ball.width/2){ //check if the x position of the left side of the ball is less than or equal to the left side of the screen, which would be 0
ball.x = ball.width/2; //then set the ball's x position to that point, in case it already moved off the screen
ballSpeedX *= -1; //and multiply the ball's x speed by -1, which will make it move right instead of left
cpuScore++; //increase cpuScore by 1
updateTextFields();

} else if(ball.x >= stage.stageWidth-ball.width/2){ //check to see the right side of the ball is touching the right boundary, which would be 550
ball.x = stage.stageWidth-ball.width/2; //reposition it, just in case
ballSpeedX *= -1; //multiply the x speed by -1 (now moving left, not right)
playerScore++; //increase playerScore by 1
updateTextFields();

}

//now we do the same with the top and bottom of the screen
if(ball.y <= ball.height/2){ //if the y position of the top of the ball is less than or equal to the top of the screen
ball.y = ball.height/2; //like we did before, set it to that y position...
ballSpeedY *= -1; //...and reverse its y speed so that it is now going down instead of up

} else if(ball.y >= stage.stageHeight-ball.height/2){ //if the bottom of the ball is lower than the bottom of the screen
ball.y = stage.stageHeight-ball.height/2; //reposition it
ballSpeedY *= -1; //and reverse its y speed so that it is moving up now

}
}