var basebookprice  = 13.99;
var baseaudioprice = 19.99;

var shiponethrufour        = 3.75;
var shipfivethrunine       = 6.75;
var shiptenthrutwentyfour  = 10.25;
var shipovertwentyfive     = 12.50;

shiponethrufour       = shiponethrufour.toFixed(2);
shipfivethrunine      = shipfivethrunine.toFixed(2);
shiptenthrutwentyfour = shiptenthrutwentyfour.toFixed(2);
shipovertwentyfive    = shipovertwentyfive.toFixed(2);

var discountonethrufour       = 10;
var discountfivethrunine      = 15;
var discounttenthrutwentyfour = 20;
var discountovertwentyfive    = 25;

function bookunits(units)
{
	/* make sure we have a real number to work with */
	if (isNaN(units))
	{
		alert ("Error: \n\nThis field can only contain numbers.");
		zeroout();
	}
	else
	{	
		/* update the price if a discount applies */
		var newbookprice = basebookprice - discount(basebookprice, units);
		newbookprice = newbookprice.toFixed(2);
		document.forms[0].bookprice.value = newbookprice;
		document.getElementById("book_price").innerHTML = newbookprice;
		
		/* update the total */
		var newbooktotal = newbookprice * units;
		document.getElementById("book_total").innerHTML = newbooktotal.toFixed(2);
		document.forms[0].booktotal.value = newbooktotal.toFixed(2);
		
		/* update total units */
		document.getElementById("totalunits").innerHTML = Math.round(document.getElementById("book_units").value) + Math.round(document.getElementById("audio_units").value);
		
		/* update the subtotal */
		var subt = (document.getElementById("book_total").innerHTML - 0) + (document.getElementById("audio_total").innerHTML - 0);
		document.getElementById("subtotal").innerHTML = subt.toFixed(2);
		document.forms[0].subtotal.value = subt.toFixed(2);
		
		/* update the shipping */
		shipping();
		
		/* update the grand total */
		grandtotal();
	}
}

function audiounits(units)
{
	/* make sure we have a real number to work with */
	if (isNaN(units))
	{
		alert ("Error: \n\nThis field can only contain numbers.");
		zeroout();
	}
	else
	{	
		/* update the price if a discount applies */
		var newaudioprice = baseaudioprice - discount(baseaudioprice, units);
		newaudioprice = newaudioprice.toFixed(2);
		document.forms[0].audioprice.value = newaudioprice;
		document.getElementById("audio_price").innerHTML = newaudioprice;
		
		/* update the total */
		var newaudiototal = newaudioprice * units;
		document.getElementById("audio_total").innerHTML = newaudiototal.toFixed(2);
		document.forms[0].audiototal.value = newaudiototal.toFixed(2);
		
		/* update total units */
		document.getElementById("totalunits").innerHTML = Math.round(document.getElementById("audio_units").value) + Math.round(document.getElementById("book_units").value);
		
		/* update the subtotal */
		var subt = (document.getElementById("book_total").innerHTML - 0) + (document.getElementById("audio_total").innerHTML - 0);
		document.getElementById("subtotal").innerHTML = subt.toFixed(2);
		document.forms[0].subtotal.value = subt.toFixed(2);
		
		/* update the shipping */
		shipping();
		
		/* update the grand total */
		grandtotal();
	}
}

function shipping()
{
	/* figure out the shipping charge */
	var totunits = document.getElementById("totalunits").innerHTML;
	
	if (totunits <= 4)
	{
		document.getElementById("shippingtotal").innerHTML = shiponethrufour;
		document.forms[0].shippingtotal.value = shiponethrufour;
	}
	else if (totunits <= 9)
	{
		document.getElementById("shippingtotal").innerHTML = shipfivethrunine;
		document.forms[0].shippingtotal.value = shipfivethrunine;
	}
	else if (totunits <= 24)
	{
		document.getElementById("shippingtotal").innerHTML = shiptenthrutwentyfour;
		document.forms[0].shippingtotal.value = shiptenthrutwentyfour;
	}
	else if (totunits == 25)
	{
		document.getElementById("shippingtotal").innerHTML = shipovertwentyfive;
		document.forms[0].shippingtotal.value = shipovertwentyfive;
	}
	else if (totunits > 25)
	{
		var numunits = (totunits - 0);
		var cases = (numunits / 25);
		document.getElementById("shippingtotal").innerHTML = (shipovertwentyfive * cases);
		document.forms[0].shippingtotal.value = (shipovertwentyfive * cases);
	}
}

function grandtotal()
{
	var gtotal = (document.getElementById("shippingtotal").innerHTML - 0) + (document.getElementById("subtotal").innerHTML - 0);
	document.getElementById("grandtotal").innerHTML = gtotal.toFixed(2);
	document.forms[0].grandtotal.value = gtotal.toFixed(2);
}

function discount (price, units)
{
	/* figure out the discount applied */
	if (units <= 4)
	{
		var disc = price * 0.1;
		disc = disc.toFixed(2);
		return disc;
	}
	else if (units <= 9)
	{
		var disc = price * 0.15;
		disc = disc.toFixed(2);
		return disc;
	}
	else if (units <= 24)
	{
		var disc = price * 0.2;
		disc = disc.toFixed(2);
		return disc;
	}
	else if (units >= 25)
	{
		var disc = price * 0.25;
		disc = disc.toFixed(2);
		return disc;
	}
}

function zeroout()
{
	document.getElementById("audio_units").value = "";
	document.getElementById("book_units").value = "";
	document.getElementById("book_price").innerHTML = basebookprice;
	document.getElementById("audio_price").innerHTML = baseaudioprice;
	document.forms[0].bookprice.value = basebookprice;
	document.forms[0].audioprice.value = baseaudioprice;
	document.getElementById("grandtotal").innerHTML = "0";
	document.getElementById("shippingtotal").innerHTML = "0";
	document.forms[0].shippingtotal.value = "0";
	document.forms[0].subtotal.value = "0";
	document.getElementById("audio_total").innerHTML = "0";
	document.forms[0].audiototal.value = "0";
	document.getElementById("book_total").innerHTML = "0";
	document.forms[0].booktotal.value = "0";
	document.getElementById("subtotal").innerHTML = "0";
	document.getElementById("totalunits").innerHTML = "0";
}