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.elements[0].value == null || form.elements[0].value.length == 0) ||
        (form.elements[1].value == null || form.elements[1].value.length == 0) ||
        (form.elements[2].value == null || form.elements[2].value.length == 0)) {
        return;
        }

    if (!checkNumber(form.elements[0], 1, 480, "# of payments") ||
        !checkNumber(form.elements[1], .001, 99, "Interest") ||
        !checkNumber(form.elements[2], 100, 10000000, "Principal")) {
        form.payment.value = "Invalid";
        return;
    }

    var i = form.elements[1].value;
    if (i > 1.0) {
        i = i / 100.0;
        form.elements[1].value = i;
    }
    i /= 12;

    var pow = 1;
    var loop = eval(form.elements[0].value);
    for (var j = 0; j < loop; j++) 
        pow = pow * (1 + i);
        Result = (eval(form.elements[2].value) * pow * i) / (pow - 1)
        form.elements[4].value = Result;        
}

function clearForm(form)
{
	form.elements[0].value = "";
	form.elements[1].value = "";
	form.elements[2].value = "";
	form.elements[4].value = "";
	form.payments.focus();
}