Make cluster subgraphs as nodes for edges 2 (#24)

* Make cluster subgraphs as nodes for edges

* Add test for cluster subgraphs

* update grammar to handle subgraphs without statements

dot.bnf

@@ -280,6 +280,8 @@ SubGraphStmt
 	: "{" StmtList "}"                            << ast.NewSubGraph(nil, $1) >>
 	| subgraph "{" StmtList "}"                   << ast.NewSubGraph(nil, $2) >>
 	| subgraph Id "{" StmtList "}"                << ast.NewSubGraph($1, $3) >>
+	| subgraph "{" "}"                   	      << ast.NewSubGraph(nil, nil) >>
+	| subgraph Id "{" "}"                         << ast.NewSubGraph($1, nil) >>
 	;
 
 //An edgeop is -> in directed graphs and -- in undirected graphs.

escape_test.go

@@ -45,3 +45,43 @@ func TestEscape(t *testing.T) {
 		t.Fatalf("should be a node")
 	}
 }
+
+func TestClusterSubgraphs(t *testing.T) {
+	g := NewEscape()
+	g.SetName("G")
+	g.SetDir(false)
+	g.AddSubGraph("G", "cluster0", nil)
+	g.AddSubGraph("cluster0", "cluster_1", nil)
+	g.AddSubGraph("cluster0", "cluster_2", nil)
+	g.AddNode("G", "Code deployment", nil)
+	g.AddPortEdge("cluster_2", "", "cluster_1", "", false, nil)
+	s := g.String()
+	graphStr := `graph G {
+	cluster_2--cluster_1;
+	subgraph cluster0 {
+	subgraph cluster_1 {
+
+}
+;
+	subgraph cluster_2 {
+
+}
+;
+
+}
+;
+	"Code deployment";
+
+}`
+	if !strings.HasPrefix(s, graphStr) {
+		t.Fatalf("%s", s)
+	}
+	g2, err := Parse([]byte(s))
+	if err != nil {
+		t.Fatal(err)
+	}
+	s2 := g2.String()
+	if !strings.HasPrefix(s2, graphStr) {
+		t.Fatalf("%s", s2)
+	}
+}

graph.go

@@ -14,6 +14,8 @@
 
 package gographviz
 
+import "strings"
+
 //The analysed representation of the Graph parsed from the DOT format.
 type Graph struct {
 	Attrs     Attrs
@@ -112,3 +114,9 @@ func (this *Graph) IsSubGraph(name string) bool {
 	_, ok := this.SubGraphs.SubGraphs[name]
 	return ok
 }
+
+func (this *Graph) IsClusterSubGraph(name string) bool {
+	isSubGraph := this.IsSubGraph(name)
+	isCluster := strings.HasPrefix(name, "cluster")
+	return isSubGraph && isCluster
+}