// start the digest loop
requestAnimationFrame(function loop (t) {
  Kinetic.notify(t)
  requestAnimationFrame(loop)
})

// create kinetic instance
var kinetic = new Kinetic({
  el: document.body,
  Vector: Vector
})

// spawn it
Kinetic.spawn(kinetic)

// implement scroll
var $target = document.getElementById('container')

var position = new Vector(0, 0)

function scrollTo (position) {
  $target.style.transform = `translateY(${position.y}px)`
}

function isValidScroll (position) {
  return position.y <= 0 && position.y > -5000 + window.innerHeight
}

function scrollY (pointers) {
  if (pointers.length === 1) {
    var pointer = pointers[0]
    var next = position.add(pointer.delta)
    if (isValidScroll(next)) {
      scrollTo(next)
      position = next
    }
  }
}

// subscribe to kinetic
kinetic.subscribe(scrollY)