/// <summary>
        /// Builds the model.
        /// </summary>
        /// <param name="helper">The helper.</param>
        /// <param name="startingNode">The starting node.</param>
        /// <param name="startingNodeInChildLevel">Renders startingNode in child level if set to <c>true</c>.</param>
        /// <param name="showStartingNode">Show starting node if set to <c>true</c>.</param>
        /// <param name="maxDepth">The max depth.</param>
        /// <param name="drillDownToCurrent">Should the model exceed the maxDepth to reach the current node</param>
        /// <returns>The model.</returns>
        private static NavHelperModel BuildModel(MvcSiteMapHtmlHelper helper, SiteMapNode startingNode, bool startingNodeInChildLevel, bool showStartingNode, int maxDepth, bool drillDownToCurrent)
        {
            // Build model
            var model = new NavHelperModel();
            var node = startingNode;

            // Check if a starting node has been given
            if (node == null)
            {
                return model;
            }

            var mvcNode = node as MvcSiteMapNode;
            bool continueBuilding = ReachedMaximalNodelevel(maxDepth, node, drillDownToCurrent);

            // Check if maximal node level has not been reached
            if (!continueBuilding)
            {
                return model;
            }

            // Check visibility
            bool nodeVisible = true;
            if (mvcNode != null)
            {
                nodeVisible = mvcNode.VisibilityProvider.IsVisible(
                    node, HttpContext.Current, NavSourceMetadata);
            }

            // Check ACL
            if (node.IsAccessibleToUser(HttpContext.Current))
            {
                // Add node?
                var nodeToAdd = SiteMapNodeModelMapper.MapToSiteMapNodeModel(node, mvcNode, NavSourceMetadata);
                if (nodeVisible)
                {
                    if (showStartingNode || !startingNodeInChildLevel)
                    {
                        model.Nodes.Add(nodeToAdd);
                    }
                }

                // Add child nodes
                if (node.HasChildNodes)
                {
                    foreach (SiteMapNode childNode in node.ChildNodes)
                    {
                        var childNodes = BuildModel(helper, childNode, false, true, maxDepth - 1, drillDownToCurrent).Nodes;
                        foreach (var childNodeToAdd in childNodes)
                        {
                            if (!startingNodeInChildLevel)
                            {
                                nodeToAdd.Children.Add(childNodeToAdd);
                            }
                            else
                            {
                                model.Nodes.Add(childNodeToAdd);
                            }
                        }
                    }
                }
            }

            return model;
        }
        /// <summary>
        /// Builds the model.
        /// </summary>
        /// <param name="helper">The helper.</param>
        /// <param name="startingNode">The starting node.</param>
        /// <param name="startingNodeInChildLevel">Renders startingNode in child level if set to <c>true</c>.</param>
        /// <param name="showStartingNode">Show starting node if set to <c>true</c>.</param>
        /// <param name="maxDepth">The max depth.</param>
        /// <param name="drillDownToCurrent">Should the model exceed the maxDepth to reach the current node</param>
        /// <returns>The model.</returns>
        private static NavHelperModel BuildModel(MvcSiteMapHtmlHelper helper, SiteMapNode startingNode, bool startingNodeInChildLevel, bool showStartingNode, int maxDepth, bool drillDownToCurrent)
        {
            // Build model
            var model = new NavHelperModel();
            var node  = startingNode;

            // Check if a starting node has been given
            if (node == null)
            {
                return(model);
            }

            var  mvcNode          = node as MvcSiteMapNode;
            bool continueBuilding = ReachedMaximalNodelevel(maxDepth, node, drillDownToCurrent);

            // Check if maximal node level has not been reached
            if (!continueBuilding)
            {
                return(model);
            }

            // Check visibility
            bool nodeVisible = true;

            if (mvcNode != null)
            {
                nodeVisible = mvcNode.VisibilityProvider.IsVisible(
                    node, HttpContext.Current, NavSourceMetadata);
            }

            // Check ACL
            if (node.IsAccessibleToUser(HttpContext.Current))
            {
                // Add node?
                var nodeToAdd = SiteMapNodeModelMapper.MapToSiteMapNodeModel(node, mvcNode, NavSourceMetadata);
                if (nodeVisible)
                {
                    if (showStartingNode || !startingNodeInChildLevel)
                    {
                        model.Nodes.Add(nodeToAdd);
                    }
                }

                // Add child nodes
                if (node.HasChildNodes)
                {
                    foreach (SiteMapNode childNode in node.ChildNodes)
                    {
                        var childNodes = BuildModel(helper, childNode, false, true, maxDepth - 1, drillDownToCurrent).Nodes;
                        foreach (var childNodeToAdd in childNodes)
                        {
                            if (!startingNodeInChildLevel)
                            {
                                nodeToAdd.Children.Add(childNodeToAdd);
                            }
                            else
                            {
                                model.Nodes.Add(childNodeToAdd);
                            }
                        }
                    }
                }
            }

            return(model);
        }