Occlusion culling in Mithril

occluder.js

function occluder(){
	var fresh    = true;
	var item     = 40;
	var offset   = 0;
	var scroller = window;
	var viewport = 0;

	return {
		scroller : function scrollerConfig( el, init, context ){
			if( arguments.length < 2 || !init || fresh ){
				window.removeEventListener( 'scroll', update, false );

				scroller = el;
				scroller.addEventListener( 'scroll', update, false );

				context.onunload = function teardown(){
					scroller.removeEventListener( 'scroll', update, false );
				};

				fresh = false;

				update();
			}
		},
		view     : function mapView( list, template ){
			var total     = ( item * list.length ) || 0;
			var modulo    = ( offset % item ) * -1;

			var start     = Math.floor( offset / item );
			var end       = Math.floor( ( viewport / item ) + start + 2 );
			var occlusion = list.slice( start, end );

			return m( '', {
					config   : containerConfig,
					style    : {
						height   : total + 'px',
						position : 'relative',
						top      : modulo + 'px'
					}
				}, m( '', {
						style  : {
							config   : getItemSize,
							position : 'relative',
							top      : offset + 'px'
						}
					}, occlusion.map( template )
					)
			);
		}
	};

	function containerConfig( el, init, context ){
		if( !init && !fresh ){
			window.addEventListener( 'scroll', update, false );
			window.addEventListener( 'resize', update, false );

			context.onunload = function teardown(){
				window.removeEventListener( 'scroll', update, false );
				window.removeEventListener( 'resize', update, false );
			};
		}
	}

	function getItemSize( el, init ){
		if( !init ) item = el.firstChild.getBoundingClientRect().height;
	}

	function update( event ){
		offset   = scroller.scrollTop    || 0;
		viewport = scroller.offsetHeight || 0;

		m.redraw();
	}
}