(function($) {
  $.fn.listenForChange = function(options) {
    settings = $.extend({
      interval: 200 // in microseconds
    }, options);

    var jquery_object = this;
    var current_focus = null;

    jquery_object.filter(":input").add(":input", jquery_object).focus( function() {
      current_focus = this;
    }).blur( function() {
      current_focus = null;
    });

    setInterval(function() {
      // allow
      jquery_object.filter(":input").add(":input", jquery_object).each(function() {
        // set data cache on element to input value if not yet set
        if ($(this).data('change_listener') == undefined) {
          $(this).data('change_listener', $(this).val());
          return;
        }
        // return if the value matches the cache
        if ($(this).data('change_listener') == $(this).val()) {
          return;
        }
        // if we make it here, manually fire the change event and set the new value
        $(this).trigger('change');
        $(this).data('change_listener', $(this).val());
      });
    }, settings.interval);
    return this;
  };
})(jQuery);

$(document).ready(function(){
  $("#dni").listenForChange();
  $("#dni").change(function(){
    var dni = $("#dni").val();
    if (dni != "" && isFinite(dni)) {
      $("#nifletter").val("TRWAGMYFPDXBNJZSQVHLCKE".slice(dni%23,dni%23 + 1));
    } else {
      $("#nifletter").val("");
    }
  });
});