gistfile1.js

  function offset_current_by(offset) {
    return _slides[offset_index(offset)];
  };

  function offset_index(offset) {
    var last  = _slides.length - 1;
    var idx   = index_of(_current) + offset;

    idx = (idx >= 0)    ? idx : last + idx;
    idx = (idx <= last) ? idx : idx - last;

    return idx;
  };

  function next() {
    change_slide(offset_current_by(1));
  };

  function prev() {
    change_slide(offset_current_by(-1));
  };