export const smoothScroll = eID => {
  const currentYPosition = () => {
    // Firefox, Chrome, Opera, Safari
    if (self.pageYOffset) return self.pageYOffset;
    // Internet Explorer 6 - standards mode
    if (document.documentElement && document.documentElement.scrollTop)
      return document.documentElement.scrollTop;
    // Internet Explorer 6, 7 and 8
    if (document.body.scrollTop) return document.body.scrollTop;
    return 0;
  }


  const elmYPosition = (eID) => {
    const elm = document.getElementById(eID);
    const y = elm.offsetTop;
    const node = elm;
    while (node.offsetParent && node.offsetParent != document.body) {
      node = node.offsetParent;
      y += node.offsetTop;
    } return y;
  }

  const startY = currentYPosition();
  const stopY = elmYPosition(eID);
  const distance = stopY > startY ? stopY - startY : startY - stopY;

  if (distance < 100) {
    scrollTo(0, stopY); return;
  }

  const speed = 1000;
  const step = Math.round(distance / 25);
  const leapY = stopY > startY ? startY + step : startY - step;
  const timer = 0;
  if (stopY > startY) {
    for ( let i=startY; i<stopY; i+=step ) {
      setTimeout(`window.scrollTo(0, ${leapY})`, timer * speed);
      leapY += step; if (leapY > stopY) leapY = stopY; timer++;
    } return;
  }
  for ( let i=startY; i>stopY; i-=step ) {
    setTimeout(`window.scrollTo(0, ${leapY})`, timer * speed);
    leapY -= step; if (leapY < stopY) leapY = stopY; timer++;
  }
}