// Stores the Y position where the touch started
var startY = 0

// Store enabled status
var enabled = false

var supportsPassiveOption = false
try {
	var opts = Object.defineProperty({}, 'passive', {
		get: function() {
			supportsPassiveOption = true
		}
	})
	window.addEventListener('test', null, opts)
} catch (e) {}

var handleTouchmove = function(evt) {
	// Get the element that was scrolled upon
	var el = evt.target

	// Allow zooming
	var zoom = window.innerWidth / window.document.documentElement.clientWidth
	if (evt.touches.length > 1 || zoom !== 1) {
		return
	}

	// Check all parent elements for scrollability
	while (el !== document.body && el !== document) {
		// Get some style properties
		var style = window.getComputedStyle(el)

		if (!style) {
			// If we've encountered an element we can't compute the style for, get out
			break
		}

		// Ignore range input element
		if (el.nodeName === 'INPUT' && el.getAttribute('type') === 'range') {
			return
		}

		var scrolling = style.getPropertyValue('-webkit-overflow-scrolling')
		var overflowY = style.getPropertyValue('overflow-y')
		var height = parseInt(style.getPropertyValue('height'), 10)

		// Determine if the element should scroll
		var isScrollable =
			scrolling === 'touch' &&
			(overflowY === 'auto' || overflowY === 'scroll')
		var canScroll = el.scrollHeight > el.offsetHeight

		if (isScrollable && canScroll) {
			// Get the current Y position of the touch
			var curY = evt.touches ? evt.touches[0].screenY : evt.screenY

			// Determine if the user is trying to scroll past the top or bottom
			// In this case, the window will bounce, so we have to prevent scrolling completely
			var isAtTop = startY <= curY && el.scrollTop === 0
			var isAtBottom =
				startY >= curY && el.scrollHeight - el.scrollTop === height

			// Stop a bounce bug when at the bottom or top of the scrollable element
			if (isAtTop || isAtBottom) {
				evt.preventDefault()
			}

			// No need to continue up the DOM, we've done our job
			return
		}

		// Test the next parent
		el = el.parentNode
	}

	// Stop the bouncing -- no parents are scrollable
	evt.preventDefault()
}

var handleTouchstart = function(evt) {
	// Store the first Y position of the touch
	startY = evt.touches ? evt.touches[0].screenY : evt.screenY
}

const hasScrollSupport = () => {
	// Enable by default if the browser supports -webkit-overflow-scrolling
	// Test this by setting the property with JavaScript on an element that exists in the DOM
	// Then, see if the property is reflected in the computed style
	var testDiv = document.createElement('div')
	document.documentElement.appendChild(testDiv)
	testDiv.style.WebkitOverflowScrolling = 'touch'
	var scrollSupport =
		'getComputedStyle' in window &&
		window.getComputedStyle(testDiv)['-webkit-overflow-scrolling'] ===
			'touch'
	document.documentElement.removeChild(testDiv)
	return scrollSupport
}

export var enable = function() {
	if (!hasScrollSupport()) return

	// Listen to a couple key touch events
	window.addEventListener(
		'touchstart',
		handleTouchstart,
		supportsPassiveOption ? { passive: false } : false
	)
	window.addEventListener(
		'touchmove',
		handleTouchmove,
		supportsPassiveOption ? { passive: false } : false
	)
	enabled = true
}

export var disable = function() {
	if (!hasScrollSupport()) return

	// Stop listening
	window.removeEventListener('touchstart', handleTouchstart, false)
	window.removeEventListener('touchmove', handleTouchmove, false)
	enabled = false
}

var isEnabled = function() {
	return enabled
}