JCSS / yl_miniapp

export const mapVirtualToProps = ({
	items,
	itemHeight
}, {
	startIndex,
	endIndex
}) => {
	const visibleItems = endIndex > -1 ? items.slice(startIndex, endIndex + 1) : []

	// style
	const height = items.length * itemHeight
	const paddingTop = startIndex * itemHeight

	return {
		items: visibleItems,
		style: `height: ${height}px; padding-top: ${paddingTop}px; box-sizing: border-box;`
	}
}

export const getVisibleItemBounds = (viewTop, viewHeight, itemCount, itemHeight, itemBuffer) => {
	// visible list inside view
	const listViewTop = Math.max(0, viewTop)

	// visible item indexes
	const startIndex = Math.max(0, Math.floor(listViewTop / itemHeight) - 9)
	const endIndex = Math.min(startIndex + Math.ceil(viewHeight / itemHeight) + itemBuffer - 1, itemCount)

	return {
		startIndex,
		endIndex,
	}
}