function put_commas(input) {
    var result, len;
    input += "";
    len = input.indexOf(".");
    if(len == -1) {
        len = input.length;
        result = "";
    }
    else {
        result = input.substring(len, input.length);
    }
    while(len > 3) {
        result = input.substring(len-3, len) + result;
        result = "," + result;
        len -= 3;
    }
    result = input.substring(0, len) + result;
    return result;
}

function strip_commas(input) {
        var newnum;
        var ch;

        newnum ="";
        for(var i=0; i<input.length; i++) {
                ch = input.charAt(i);
                if(ch != ",") {
                        newnum += ch;
                }
        }
        return parseFloat(newnum);
}


function checkNum(input) {
	var oldnum;
	var newnum;
	var ch;
	
	oldnum = input.value;
	newnum ="";
	for(var i=0; i<oldnum.length; i++) {
		ch = oldnum.charAt(i);
		if(ch != ",") {
			newnum += ch;
		}
	}
	return parseFloat(newnum);
}


function checkNumPeriods(input) {
	input.value = checkNum(input);
	input.value = parseInt(input.value);
	if(input.value == null ||          
         input.value.length == 0 ||
	   input.value == "NaN") {
		input.value = "";
	}
	else {
		if(parseFloat(input.value) <= 0) {
			input.value = "";
		}
	}
	calcMonthly(input.form);
	input.value = put_commas(input.value);
}

function checkTotal(input) {
	input.value = checkNum(input);
	input.value = parseFloat(input.value);
	if(input.value == null ||          
         input.value.length == 0 ||
	   input.value == "NaN") {
		input.value = "";
	}
	else {
		if(parseFloat(input.value) < 0) {
			input.value = "";
		}
	}
	calcMonthly(input.form);
	input.value = put_commas(input.value);
}

function checkInterest(input) {
	input.value = parseFloat(input.value);
	if(input.value == null ||          
         input.value.length == 0 ||
	   input.value == "NaN") {
		input.value = "";
	}
	else {
		if(parseFloat(input.value) <= 0 || 
		   parseFloat(input.value) > 100.0) {
		   input.value = "";
		}
	}
	calcMonthly(input.form);
}

function calcMonthly(input) {
	if(input.interest.value == "" || 
	   input.nper.value == "" ||
	   input.loan.value == "") {
		input.monthly.value = "";
	}
	else {
		i = strip_commas(input.interest.value) / 100;
		i = i / 12;
		n = strip_commas(input.nper.value) * 12;
		tmp = (1/i) * (1 - Math.pow(1/(1+i), n));
		tmp = strip_commas(input.loan.value) / tmp;
		tmp = Math.round(tmp*100) / 100;
		input.monthly.value = put_commas(tmp);
		if(input.monthly.value == null ||          
	         input.monthly.value.length == 0 ||
		   input.monthly.value == "NaN") {
			input.total.value = "";
		}
	}
}