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, .001, 99, "Interest") ||

        !checkNumber(form.principal, 1, 10000000, "Principal")) {

        form.fv.value = "Invalid";

        return;

    }



    var i = form.interest.value;

    if (i >= 1.0) {

        i = i / 100.0;

    }

    i /= 12;


   var pow = form.principal.value;
    
    for (var j = 0; j < form.payments.value *12; j++)

        pow = (pow * i) + (pow *1);

    form.fv.value = (pow + (form.rate.value - form.principal.value))
		
		form.totalint.value = (form.fv.value - form.rate.value)

    form.comment.value = ("If you work one hour and then invest $" + form.principal.value + " of the $" + form.rate.value + " that you earned from it;  into an investment that earns " + form.interest.value + "% per year;  for a period of " + form.payments.value + " years, you will end up earning $" + parseInt(eval(form.totalint.value) + eval(form.rate.value),10) + " for one hour's work!  So it's up to you, do you want to make $" + form.rate.value + " per hour or $" + parseInt(eval(form.totalint.value) + eval(form.rate.value),10) + " per hour?");

}



function clearForm(form)

{

    form.payments.value = "";

    form.interest.value = "";

    form.principal.value = "";

    form.rate.value = "";

    form.comment.value = "";

}