var oldusername = '';
function check_username(url, username, min_length) {

    if(username==oldusername) {
        return false;
    }
    oldusername=username;
    if(check_username_message.length == 0) {
        check_username_message = $("#check_username_result").html();
    }
    if ($("#username").val().length >= min_length) {
        $("#check_username_result").html(check_username_message);
        $("#check_username_result").removeAttr("class");
        $("#processing_image").show();
        $("#check_username_result").show();
        params = {
            username: username
        };
        ajax.request(url, params, check_username__callback);
    } else {
        $("#check_username_result").attr("class", "message_error");
        $("#check_username_result").html(check_username_invalid);
        $("#check_username_result").show();
    }
}

function check_username__callback(data) {
    $("#processing_image").hide();
    $("#check_username_result").attr("class", data.message_class);
    $("#check_username_result").text(data.message);
}