function getElementsByClass(searchClass, node, tag) {
    var classElements = new Array();
    if (node == null)
        node = document;
    if (tag == null)
        tag = '*';
    var els = node.getElementsByTagName(tag);
    var elsLen = els.length;
    var pattern = new RegExp('(^| )' + searchClass + '( |$)');
    for (i = 0, j = 0; i < elsLen; i++) {
        if (pattern.test(els[i].className)) {
            classElements[j] = els[i];
            j++;
        }
    }
    return classElements;
}

var res = getElementsByClass("filelink");
for (var i = 0; i < res.length; i++) {
    var a = Math.floor(Math.random() * 9);
    var b = Math.floor(Math.random() * 9);
    res[i].attributes['href'].value += ('&a=' + a + '&b=' + b + '&c=' + (a + b));
}