/**
 * Smooth scroll to ...
 *
 * @param  {integer} offset Vertical scroll offset (px)
 * @param  {integer} speed  Animation speed (ms)
 */
function smoothScrollTo(offset = 0, speed = 500) {

	// Fallback for old browsers
	if (!performance || !performance.now || !requestAnimationFrame) {
		window.scrollTo(0, 0)
		return
	}

	const cosParameter = window.scrollY / 2
	let scrollCount = 0
	let oldTimestamp = performance.now()

	function step (newTimestamp) {
		scrollCount += Math.PI / (speed = 500 / (newTimestamp - oldTimestamp))

		if (scrollCount >= Math.PI) {
			window.scrollTo(0, 0)
		}

		if (window.scrollY <= offset) {
			return
		}

		window.scrollTo(0, Math.round(cosParameter + cosParameter * Math.cos(scrollCount)))
		oldTimestamp = newTimestamp
		requestAnimationFrame(step)
	}

	requestAnimationFrame(step)
}

export {smoothScrollTo as default}