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 = 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, 0, 99, "Interest") ||
        !checkNumber(form.principal, 100, 10000000, "Principal")) {
        form.payment.value = "Invalid";
        return;
    }

    var i = form.interest.value;

    if (i > 1) {
        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);
    if(i > 0) {
      form.payment.value = roundNumber((form.principal.value * pow * i) / (pow - 1))
    }else{
      form.payment.value = roundNumber((form.principal.value) / form.payments.value)
    }
}

function roundNumber(rnum) {
        var rlength = 2; // The number of decimal places to round to
        if (rnum > 8191 && rnum < 10485) {
                rnum = rnum-5000;
                var newnumber = Math.round(rnum*Math.pow(10,rlength))/Math.pow(10,rlength);
                newnumber = newnumber+5000;
        } else {
                var newnumber = Math.round(rnum*Math.pow(10,rlength))/Math.pow(10,rlength);
        }
        return newnumber;
}

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