function checkNumber(input, min, max, msg) {
	msg = msg + " field has invalid data: " + 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 + "]");
		return false;
	}
	input.value = str;
	return true;
}
function computeField(input) {
	if (input.value != null && input.value.length != 0)
	input.value = "" + eval(input.value);
	computeForm(input.form);
}
function computeForm(form) {
	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)) {
		return;
	}
	if (!checkNumber(form.payments, 1, 480, "# of payments") ||
		!checkNumber(form.interest, .001, 99, "Interest") ||
		!checkNumber(form.principal, 100, 10000000, "Principal")) {
		form.payment.value = "Invalid";
		return;
	}
	var i = form.interest.value;
	if (i > 1.0) {
		i = i / 100.0;
		form.interest.value = i;
	}
	i /= 12;
	var pow = 1;
	for (var j = 0; j < form.payments.value; j++)
	pow = pow * (1 + i);
	form.payment.value = (form.principal.value * pow * i) / (pow - 1)
}
function clearForm(form) {
	form.payments.value = "";
	form.interest.value = "";
	form.principal.value = "";
}