YAHOO.util.Event.onDOMReady(function() {

  $E = YAHOO.util.Event;
  $D = YAHOO.util.Dom;
  $ = YAHOO.util.Dom.get;
  
  $D.setStyle($('controls'), 'display', 'block');
  $D.setStyle($('thumbnails'), 'display', 'block');

  var current = $D.getElementsByClassName('selected', 'li', 'images')[0];
  $D.setStyle($('images').getElementsByTagName('li'), 'display', 'none');
  $D.setStyle(current, 'display', 'block');

  $E.on(['next', 'previous'], 'click', function(e) {

    var current = $D.getElementsByClassName('selected', 'li', 'images')[0];
    var next = Object;

    $D.removeClass($('images').getElementsByTagName('li'), 'selected');
    $D.setStyle(current, 'display', 'none');

    if(this.id == 'next') {
      if($D.getNextSibling(current)) {
        next = $D.getNextSibling(current);
      } else {
        next = $('images').getElementsByTagName('li')[0];
      }
    } else {
      if($D.getPreviousSibling(current)) {
        next = $D.getPreviousSibling(current);
      } else {
        var images = $('images').getElementsByTagName('li');
        next = images[images.length - 1];
      }
    }
    
    $D.addClass(next, 'selected');
    $D.setStyle(next, 'display', 'block');
    
  });

});