var Scroller = Class.create();

Scroller.prototype = {
	initialize: function(id, rate)
	{
		this.element = $(id);
		if ( !this.element )
		{
			throw new Exception("No such element: " + id);
		}
		if ( rate ) 
		{
			this.rate = rate;
		}
		else
		{
			this.rate = 500;
		}
	},
	start: function()
	{
		h = $(this.element.parentNode).getHeight();
		this.offset = h - (3*h/10);
		this.margin = this.offset;
		this.element.style.marginTop = this.offset + "px";
//		this.executer = new PeriodicalExecuter(this.scroll.bind(this), this.rate);
		this.executer = setTimeout(this.scroll.bind(this), this.rate);

	},
	stop: function()
	{
		this.executer.stop();
	},
	scroll: function()
	{
		this.margin--;
		if ( this.element.getHeight() + this.margin <= 0 )
		{
			this.margin = this.offset;
		}
		this.element.style.marginTop = this.margin + "px";
		this.executer = setTimeout(this.scroll.bind(this), this.rate);
	},
	reset: function()
	{
		this.margin = this.offset;
		this.element.style.marginTop = this.margin + "px";
	}	
};