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' || ch > '9') && ch != '.')
		{
			alert(msg);
			return false;
		}
	}
	var num = 0 + 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 * (i+1);
  form.payment.value = (form.principal.value * pow * i) / (pow - 1)
}

function clearForm(form)
{
	form.payments.value = "";
	form.interest.value = "";
	form.principal.value = "";
}