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              // define document width and height
var width = 450, height = 300

// create SVG document and set its size
var draw = SVG('pong').size(width, height)
draw.viewbox(0,0,450,300)


// draw background
var background = draw.rect(width, height).fill('#dde3e1')

// draw line
var line = draw.line(width/2, 0, width/2, height)
line.stroke({ width: 5, color: '#fff', dasharray: '5,5' })


// define paddle width and height
var paddleWidth = 15, paddleHeight = 80

// create and position left paddle
var paddleLeft = draw.rect(paddleWidth, paddleHeight)
paddleLeft.x(0).cy(height/2).fill('#00ff99')

// create and position right paddle
var paddleRight = paddleLeft.clone()
paddleRight.x(width-paddleWidth).fill('#ff0066')


// define ball size
var ballSize = 10

// create ball
var ball = draw.circle(ballSize)
ball.center(width/2, height/2).fill('#7f7f7f')


// define inital player score
var playerLeft = playerRight = 0

// create text for the score, set font properties
var scoreLeft = draw.text(playerLeft+'').font({
  size: 32,
  family: 'Menlo, sans-serif',
  anchor: 'end',
  fill: '#fff'
}).move(width/2-10, 10)

// cloning rocks!
var scoreRight = scoreLeft.clone()
  .text(playerRight+'')
  .font('anchor', 'start')
  .x(width/2+10)


// random velocity for the ball at start
var vx = 0, vy = 0

// AI difficulty
var difficulty = 2

// update is called on every animation step
function update(dt) {
  // move the ball by its velocity
  ball.dmove(vx*dt, vy*dt)

  // get position of ball
  var cx = ball.cx()
    , cy = ball.cy()
  
  // get position of ball and paddle
  var paddleLeftCy = paddleLeft.cy()

  // move the left paddle in the direction of the ball
  var dy = Math.min(difficulty, Math.abs(cy - paddleLeftCy))
  paddleLeftCy += cy > paddleLeftCy ? dy : -dy

  // constraint the move to the canvas area
  paddleLeft.cy(Math.max(paddleHeight/2, Math.min(height-paddleHeight/2, paddleLeftCy)))

  // check if we hit top/bottom borders
  if ((vy < 0 && cy <= 0) || (vy > 0 && cy >= height)) {
    vy = -vy
  }
  
  var paddleLeftY = paddleLeft.y()
    , paddleRightY = paddleRight.y()

  // check if we hit the paddle
  if((vx < 0 && cx <= paddleWidth && cy > paddleLeftY && cy < paddleLeftY + paddleHeight) ||
     (vx > 0 && cx >= width - paddleWidth && cy > paddleRightY && cy < paddleRightY + paddleHeight)) {
    // depending on where the ball hit we adjust y velocity
    // for more realistic control we would need a bit more math here
    // just keep it simple
    vy = (cy - ((vx < 0 ? paddleLeftY : paddleRightY) + paddleHeight/2)) * 7 // magic factor

    // make the ball faster on hit
    vx = -vx * 1.05
  } else
  
  // check if we hit left/right borders
  if ((vx < 0 && cx <= 0) || (vx > 0 && cx >= width)) {
    // when x-velocity is negative, it's a point for player 2, otherwise player 1
    if(vx < 0) { ++playerRight }
    else { ++playerLeft }
    
    reset()

    scoreLeft.text(playerLeft+'')
    scoreRight.text(playerRight+'')
  }
  
  
  // move player paddle
  var playerPaddleY = paddleRight.y()

  if (playerPaddleY <= 0 && paddleDirection == -1) {
    paddleRight.cy(paddleHeight/2)
  } else if (playerPaddleY >= height-paddleHeight && paddleDirection == 1) {
    paddleRight.y(height-paddleHeight)
  } else {
    paddleRight.dy(paddleDirection*paddleSpeed)
  }
  
  
  // update ball color based on position
  ball.fill(ballColor.at(1/width*ball.x()))
  
}


var lastTime, animFrame

function callback(ms) {
  // we get passed a timestamp in milliseconds
  // we use it to determine how much time has passed since the last call
  if (lastTime) {
    update((ms-lastTime)/1000) // call update and pass delta time in seconds
  }

  lastTime = ms
  animFrame = requestAnimationFrame(callback)
}


callback()


var paddleDirection = 0
  , paddleSpeed = 5

SVG.on(document, 'keydown', function(e) {
  paddleDirection = e.keyCode == 40 ? 1 : e.keyCode == 38 ? -1 : 0
  e.preventDefault()
})

SVG.on(document, 'keyup', function(e) {
  paddleDirection = 0
  e.preventDefault()
})

draw.on('click', function() {
  if(vx === 0 && vy === 0) {
    vx = Math.random() * 500 - 150
    vy = Math.random() * 500 - 150
  }
})

function reset() {
  // visualize boom
  boom()
  
  // reset speed values
  vx = 0
  vy = 0

  // position the ball back in the middle
  ball.animate(100).center(width/2, height/2)

  // reset the position of the paddles
  paddleLeft.animate(100).cy(height/2)
  paddleRight.animate(100).cy(height/2)
}


// ball color update
var ballColor = new SVG.Color('#ff0066')
ballColor.morph('#00ff99')


// show visual explosion 
function boom() {
  // detect winning player
  var paddle = ball.cx() > width/2 ? paddleLeft : paddleRight

  // create the gradient
  var gradient = draw.gradient('radial', function(stop) {
    stop.at(0, paddle.attr('fill'), 1)
    stop.at(1, paddle.attr('fill'), 0)
  })

  // create circle to carry the gradient
  var blast = draw.circle(300)
  blast.center(ball.cx(), ball.cy()).fill(gradient)

  // animate to invisibility
  blast.animate(1000, '>').opacity(0).after(function() {
    blast.remove()
  })
}
            
          
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