function update_text_input(elem)
{
    if(elem.value == "")
	{
	    elem.className = elem.className.replace( /(?:^|\s)form_filled_field(?!\S)/ , '' );
	    elem.className = elem.className.replace( /(?:^|\s)form_empty_field(?!\S)/ , '' );
	    elem.className += " form_empty_field";
	}
    else
	{
	    elem.className = elem.className.replace( /(?:^|\s)form_filled_field(?!\S)/ , '' );
	    elem.className = elem.className.replace( /(?:^|\s)form_empty_field(?!\S)/ , '' );
	    elem.className += " form_filled_field";
	}
}

function update_form()
{
    nameelem = document.getElementById('form_name');
    emailelem = document.getElementById('form_email');
    messageelem = document.getElementById('form_message');
    submitelem = document.getElementById('form_submit');

    update_text_input(nameelem);
    update_text_input(emailelem);
    update_text_input(messageelem);
	   
    if(nameelem.value != "" &&
       emailelem.value != "" &&
       messageelem.value != "")
	{
	    submitelem.disabled = false;
	}
    else
	{
	    submitelem.disabled = true;
	}
}