/// <summary>
        /// Adds specific lines as gEdges to the visibility graph
        /// </summary>
        /// <param name="visibilityGraph">VisibilityGraph Graph</param>
        /// <param name="edges">Lines to add as new gEdges</param>
        /// <returns></returns>
        public static VisibilityGraph AddEdges(VisibilityGraph visibilityGraph, List <Edge> edges)
        {
            if (edges == null)
            {
                throw new NullReferenceException("edges");
            }
            List <Vertex> singleVertices = new List <Vertex>();

            foreach (Edge e in edges)
            {
                if (!singleVertices.Contains(e.StartVertex))
                {
                    singleVertices.Add(e.StartVertex);
                }
                if (!singleVertices.Contains(e.EndVertex))
                {
                    singleVertices.Add(e.EndVertex);
                }
            }
            VisibilityGraph updatedGraph = (VisibilityGraph)visibilityGraph.Clone();

            if (singleVertices.Any())
            {
                updatedGraph = AddVertices(visibilityGraph, singleVertices);
            }

            foreach (Edge e in edges)
            {
                updatedGraph.AddEdge(e);
            }

            return(updatedGraph);
        }
        /// <summary>
        /// Adds specific points as gVertices to the VisibilityGraph Graph
        /// </summary>
        /// <param name="visibilityGraph">VisibilityGraph Graph</param>
        /// <param name="vertices">Points to add as gVertices</param>
        /// <returns></returns>
        public static VisibilityGraph AddVertices(VisibilityGraph visibilityGraph, List <Vertex> vertices, bool reducedGraph = true)
        {
            //TODO: Seems that original graph gets updated as well
            if (vertices == null)
            {
                throw new NullReferenceException("vertices");
            }

            VisibilityGraph newVisGraph    = (VisibilityGraph)visibilityGraph.Clone();
            List <Vertex>   singleVertices = new List <Vertex>();

            foreach (Vertex v in vertices)
            {
                if (newVisGraph.Contains(v))
                {
                    continue;
                }
                Edge closestEdge = newVisGraph.baseGraph.edges.OrderBy(e => e.DistanceTo(v)).First();

                if (!closestEdge.DistanceTo(v).AlmostEqualTo(0))
                {
                    singleVertices.Add(v);
                }
                else if (v.OnEdge(closestEdge.StartVertex, closestEdge.EndVertex))
                {
                    v.polygonId = closestEdge.StartVertex.polygonId;
                    newVisGraph.baseGraph.polygons[v.polygonId] = newVisGraph.baseGraph.polygons[v.polygonId].AddVertex(v, closestEdge);
                    singleVertices.Add(v);
                }
            }

            newVisGraph.baseGraph.ResetEdgesFromPolygons();

            foreach (Vertex centre in singleVertices)
            {
                foreach (Vertex v in VisibleVertices(centre, newVisGraph.baseGraph, null, null, singleVertices, false, reducedGraph))
                {
                    newVisGraph.AddEdge(new Edge(centre, v));
                }
            }

            return(newVisGraph);
        }
        public static VisibilityGraph Merge(List <VisibilityGraph> graphs)
        {
            Graph       graph = new Graph();
            List <Edge> edges = new List <Edge>();

            foreach (VisibilityGraph g in graphs)
            {
                Dictionary <int, int> oldNewIds = new Dictionary <int, int>();
                foreach (Polygon p in g.baseGraph.polygons.Values)
                {
                    int nextId = graph.GetNextId();
                    oldNewIds.Add(p.id, nextId);
                    Polygon polygon = (Polygon)p.Clone();
                    polygon.id = nextId;
                    graph.polygons.Add(nextId, polygon);
                }

                foreach (Edge e in g.edges)
                {
                    Vertex start = (Vertex)e.StartVertex.Clone();
                    Vertex end   = (Vertex)e.EndVertex.Clone();
                    //start.polygonId = oldNewIds[start.polygonId];
                    //end.polygonId = oldNewIds[end.polygonId];
                    edges.Add(Edge.ByStartVertexEndVertex(start, end));
                }
            }

            VisibilityGraph visibilityGraph = new VisibilityGraph()
            {
                baseGraph = new Graph(graph.polygons.Values.ToList()),
            };

            foreach (Edge edge in edges)
            {
                visibilityGraph.AddEdge(edge);
            }

            return(visibilityGraph);
        }