const easeInOutQuad = (t: number, b: number, c: number, d: number) => {
  t /= d / 2;
  if (t < 1) {
    return c / 2 * t * t + b;
  }
  t--;
  return -c / 2 * (t * (t - 2) - 1) + b;
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function() {
  return window.requestAnimationFrame || window.webkitRequestAnimationFrame || (window as any).mozRequestAnimationFrame || function(callback) { window.setTimeout(callback, 1000 / 60); };
})();

// Because it's so fucking difficult to detect the scrolling element, just move them all
const move = (amount: number) => {
  document.documentElement.scrollTop = amount;
  (document.body.parentNode as HTMLElement).scrollTop = amount;
  document.body.scrollTop = amount;
};

const position = () => {
  return document.documentElement.scrollTop ||
    (document.body.parentNode as HTMLElement).scrollTop || document.body.scrollTop;
};

export const scrollTo = (to: number, duration: number, callback?: Function) => {
  const start = position();
  const change = to - start;
  const increment = 20;
  let currentTime = 0;
  duration = (typeof (duration) === 'undefined') ? 500 : duration;
  const animateScroll = function() {
    // increment the time
    currentTime += increment;
    // find the value with the quadratic in-out easing function
    const val = easeInOutQuad(currentTime, start, change, duration);
    // move the document.body
    move(val);
    // do the animation unless its over
    if (currentTime < duration) {
      requestAnimFrame(animateScroll);
    } else {
      if (callback && typeof (callback) === 'function') {
        // the animation is done so lets callback
        callback();
      }
    }
  };
  animateScroll();
};