/*
 * y: the y coordinate to scroll, 0 = top
 * duration: scroll duration in milliseconds; default is 0 (no transition)
 * element: the html element that should be scrolled ; default is the main scrolling element
 */
function scrollToY (y, duration = 0, element = document.scrollingElement) {
    // cancel if already on target position
    if (element.scrollTop === y) return;

    const cosParameter = (element.scrollTop - y) / 2;
    let scrollCount = 0, oldTimestamp = null;

    function step (newTimestamp) {
        if (oldTimestamp !== null) {
            // if duration is 0 scrollCount will be Infinity
            scrollCount += Math.PI * (newTimestamp - oldTimestamp) / duration;
            if (scrollCount >= Math.PI) return element.scrollTop = y;
            element.scrollTop = cosParameter + y + cosParameter * Math.cos(scrollCount);
        }
        oldTimestamp = newTimestamp;
        window.requestAnimationFrame(step);
    }
    window.requestAnimationFrame(step);
}

export default function(element, duration = 0, addedOffset = 0) {
    const offset = Math.round(element.getBoundingClientRect().top);
    scrollToY(document.scrollingElement.scrollTop + offset + addedOffset, duration);
}