export function scrollTo(element: HTMLElement, target: number, duration: number) {
  target = Math.round(target)
  duration = Math.round(duration)
  if (duration < 0) {
    return Promise.reject("bad duration")
  }
  if (duration === 0) {
    element.scrollTop = target
    return Promise.resolve()
  }

  const startTime = Date.now()
  const endTime = startTime + duration

  const startTop = element.scrollTop
  const distance = target - startTop

  // based on http://en.wikipedia.org/wiki/Smoothstep
  const smoothStep = (start: number, end: number, point: number) => {
    if (point <= start) { return 0 }
    if (point >= end) { return 1 }
    const x = (point - start) / (end - start) // interpolation
    return x * x * (3 - 2 * x)
  }

  return new Promise((resolve, reject) => {
    // This is to keep track of where the element's scrollTop is
    // supposed to be, based on what we're doing
    let previousTop = element.scrollTop

    // This is like a think function from a game loop
    const scrollFrame = () => {
      if (element.scrollTop !== previousTop) {
        return
      }

      // set the scrollTop for this frame
      const now = Date.now()
      const point = smoothStep(startTime, endTime, now)
      const frameTop = Math.round(startTop + (distance * point))
      element.scrollTop = frameTop

      // check if we're done!
      if (now >= endTime) {
        resolve()
        return
      }

      // If we were supposed to scroll but didn't, then we
      // probably hit the limit, so consider it done not
      // interrupted.
      if (element.scrollTop === previousTop
        && element.scrollTop !== frameTop) {
        resolve()
        return
      }
      previousTop = element.scrollTop

      // schedule next frame for execution
      setTimeout(scrollFrame, 0)
    }

    // boostrap the animation process
    setTimeout(scrollFrame, 0)
  })
}