<!--
function checkNumber(input, min, max, msg) {
	msg = msg + " is invalid: " + input.value;
	var str = input.value;
	for (var i = 0; i < str.length; i++) {
		var ch = str.substring(i, i + 1)
		if ((ch < "0" || "9" < ch) && ch != '.') {
			alert(msg);
			return false;
	    }
	}

	var num = parseFloat(str)
	if (num < min || max < num) {
		alert(msg + ", not in range [" + min + ".." + max + "]");
		clearForm();
		return false;
	}
	input.value = str;
	return true;
}

function doCalculation() {
	form = document.calcForm;

	if ((form.payments.value == null || form.payments.value.length == 0) ||
		(form.interest.value == null || form.interest.value.length == 0) ||
		(form.principal.value == null || form.principal.value.length == 0)) {
			clearForm();
			return(false);
	}

	if (!checkNumber(form.payments, 1, 480, "Number of payments") ||
		!checkNumber(form.interest, .001, 99, "Interest Rate") ||
		!checkNumber(form.principal, 100, 100000, "Principal Amount")) {
			clearForm();
			return(false);
	}

	var i = form.interest.value;
	if (i > 1.0) {
		i = i / 100.0;
	}
	i /= 12;
	var pow = 1;
	for (var j = 0; j < form.payments.value; j++) pow = pow * (1 + i);
	money = Math.round((form.principal.value * pow * i) / (pow - 1));
	form.payment.value = Math.ceil(money);
	form.totalcost.value = Math.ceil(money * form.payments.value);
}

function clearForm() {
	document.calcForm.payment.value = "";
	document.calcForm.totalcost.value = "";
}
//-->