Bitcoin userscript for Mt.Gox (MtGoxFeeCalculator.user.js)

// ==UserScript==
// @name          MtGoxFeeCalculator
// @description   Shows final Mt.Gox value (including fee) as you type.
// @screenshot    http://img707.imageshack.us/img707/4795/screenshot2012010614181.png
// @namespace     http://pastebin.com/Zzqncbrb
// @version       1.0.1
// @include       https://mtgox.com/*
// @installation  chrome-compatible (just open *.user.js in a browser)
// ==/UserScript==

(function(window, undefined ) {
    var w;
    if (typeof unsafeWindow != undefined) {
        w = unsafeWindow
    } else {
        w = window;
    }
    if (w.self != w.top) {
        return;
    }

    function getElementsByClassName(classname, node)  {
        if(!node) node = document.getElementsByTagName("body")[0];
        var a = [];
        var re = new RegExp('\\b' + classname + '\\b');
        var els = node.getElementsByTagName("*");
        for(var i=0,j=els.length; i<j; i++)
            if(re.test(els[i].className))a.push(els[i]);
        return a;
    }

    function update_fee(edit, label, fee) {
        var amount = parseFloat(document.getElementById(edit).value);
        if (isNaN(amount))
            amount = 0.0;
        result = amount * (1 - fee * 0.01);
        result = Math.round(result * 100000000) / 100000000;
        document.getElementById(label).innerHTML = 'Total (-' + fee + '%): ' + result;
    }

    function update() {
        var elements = document.getElementsByClassName('progressStart');
        var fee = parseFloat(elements[0].innerHTML);
        update_fee('buyAmount','totalBuyAmount', fee);
        update_fee('sellCost','totalSellCost', fee);
    }

    function add_label(parent, id) {
        var div = document.createElement('div');
        div.setAttribute('id', id);
        div.style.position = 'relative';
        div.style.lineHeight = '0';
        div.style.color = '#aaa';
        div.style.top = '10px';
        div.style.fontSize = '11px';
        document.getElementById(parent).parentNode.appendChild(div);
    }

    if (/https:\/\/mtgox.com/.test(w.location.href)) {
        add_label('buyAmount', 'totalBuyAmount');
        add_label('sellCost', 'totalSellCost');
        window.setInterval(update, 1000);
        update();
    }

})(window);