(function() {
  d3.force_labels = function force_labels() {    
    var labels = d3.layout.force();
      
    // Update the position of the anchor based on the center of bounding box
    function updateAnchor() {
      if (!labels.selection) return;
      labels.selection.each(function(d) {
        var bbox = this.getBBox(),
            x = bbox.x + bbox.width / 2,
            y = bbox.y + bbox.height / 2;

        d.anchorPos.x = x;
        d.anchorPos.y = y;
       
        // If a label position does not exist, set it to be the anchor position 
        if (d.labelPos.x == null) {
          d.labelPos.x = x;
          d.labelPos.y = y;
        }
      });
    }
    
    //The anchor position should be updated on each tick
    labels.on("tick.labels", updateAnchor);
    
    // This updates all nodes/links - retaining any previous labelPos on updated nodes
    labels.update = function(selection) {
      labels.selection = selection;
      var nodes = [], links = [];
      selection[0].forEach(function(d) {    
        if(d && d.__data__) {
          var data = d.__data__;
          
          if (!d.labelPos) d.labelPos = {fixed: false};
          if (!d.anchorPos) d.anchorPos = {fixed: true};
          
          // Place position objects in __data__ to make them available through 
          // d.labelPos/d.anchorPos for different elements
          data.labelPos = d.labelPos;
          data.anchorPos = d.anchorPos;
          
          links.push({target: d.anchorPos, source: d.labelPos});
          nodes.push(d.anchorPos);
          nodes.push(d.labelPos);
        }
      });
      labels
          .stop()
          .nodes(nodes)
          .links(links);
      updateAnchor();
      labels.start();
    };
    return labels;
  };
})();