function gbinit(start, end) {
  gbcurrentid = start;
  gbcount = end;
  _gbgoto(start);
}

function gbback() {
  if (gbcurrentid > 1) _gbgoto(--gbcurrentid);
}

function gbnext() {
  if (gbcurrentid < gbcount) _gbgoto(++gbcurrentid);
}

function _gbgoto(index) {
  _hideAll();

  var leftItem = document.getElementById("gb-" + index + "-left");
  var rightItem = document.getElementById("gb-" + index + "-right");
  if (leftItem) leftItem.style.display = 'block';
  if (rightItem) rightItem.style.display = 'block';
}

function _hideAll() {
  var leftContainer = document.getElementById("gb-left-container");
  var rightContainer = document.getElementById("gb-right-container");
  _hideItem(leftContainer);
  _hideItem(rightContainer);
  
  var backButton = document.getElementById("gb-back");
  var nextButton = document.getElementById("gb-next");
  nextButton.style.display = 'inline';
  backButton.style.display = 'inline';
  if (gbcurrentid == 1) {
    backButton.style.display = 'none';
  }
  else if (gbcurrentid >= gbcount) {

    nextButton.style.display = 'none';
  }
  
}

function _hideItem(container) {
  for (var i = 0; i < container.childNodes.length; i++) {
    var node = container.childNodes[i];
    if (node.className == "gb-item") {
      node.style.display = 'none';
    }
  }

}