Added Edge() getter function

cgraph/cgraph.go

@@ -905,6 +905,9 @@ func (g *Graph) IsSimple() bool {
 	return ccall.Agissimple(g.Agraph)
 }
 
+// Creates a new node with the given name in the graph, and returns the new node.
+//
+// Note: The name is not automatically used as the node's label.
 func (g *Graph) CreateNode(name string) (*Node, error) {
 	node, err := ccall.Agnodef(g.Agraph, name, 1)
 	if err != nil {
@@ -913,6 +916,8 @@ func (g *Graph) CreateNode(name string) (*Node, error) {
 	return toNode(node), nil
 }
 
+// Returns the node with the given name in the graph.
+// If no node with the given name exists in the graph, nil is returned.
 func (g *Graph) Node(name string) (*Node, error) {
 	node, err := ccall.Agnodef(g.Agraph, name, 0)
 	if err != nil {
@@ -959,6 +964,9 @@ func (g *Graph) SubRep(n *Node) *SubNode {
 	}
 }
 
+// Creates a new edge with the given name in the graph, and returns the new edge.
+//
+// Note: The name is not automatically used as the edge's label.
 func (g *Graph) CreateEdge(name string, start *Node, end *Node) (*Edge, error) {
 	edge, err := ccall.Agedgef(g.Agraph, start.Agnode, end.Agnode, name, 1)
 	if err != nil {
@@ -967,6 +975,16 @@ func (g *Graph) CreateEdge(name string, start *Node, end *Node) (*Edge, error) {
 	return toEdge(edge), nil
 }
 
+// Returns the edge with the given name in the graph.
+// If no edge with the given name exists in the graph, nil is returned.
+func (g *Graph) Edge(name string, start *Node, end *Node) (*Edge, error) {
+	edge, err := ccall.Agedgef(g.Agraph, start.Agnode, end.Agnode, name, 0)
+	if err != nil {
+		return nil, err
+	}
+	return toEdge(edge), nil
+}
+
 func (g *Graph) IDEdge(t *Node, h *Node, id IDTYPE, createFlag int) (*Edge, error) {
 	edge, err := ccall.Agidedge(g.Agraph, t.Agnode, h.Agnode, uint64(id), createFlag)
 	if err != nil {

graphviz_test.go

@@ -150,6 +150,93 @@ func TestParseFile(t *testing.T) {
 	}
 }
 
+func TestGetNode(t *testing.T) {
+	g := graphviz.New()
+	graph, err := g.Graph()
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	defer func() {
+		graph.Close()
+		g.Close()
+	}()
+
+	_, err = graph.CreateNode("n")
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+
+	n, err := graph.Node("n")
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	if n == nil {
+		t.Fatal("failed to get node 'n'. Got `nil` instead.")
+	}
+	if name := n.Name(); name != "n" {
+		t.Fatalf("failed to get node 'n'. Got node '%s' instead.", name)
+	}
+
+	m, err := graph.Node("m")
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	if m != nil {
+		t.Fatal("got unexpected node 'm'")
+	}
+	if nodes := graph.NumberNodes(); nodes != 1 {
+		t.Fatalf("expected graph to have 1 node, but found %d", nodes)
+	}
+}
+
+func TestGetEdge(t *testing.T) {
+	g := graphviz.New()
+	graph, err := g.Graph()
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	defer func() {
+		graph.Close()
+		g.Close()
+	}()
+
+	n, err := graph.CreateNode("n")
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	m, err := graph.CreateNode("m")
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+
+	_, err = graph.CreateEdge("e1", n, m)
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+
+	e1, err := graph.Edge("e1", n, m)
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	if e1 == nil {
+		t.Fatal("failed to get edge 'e1'. Got `nil` instead.")
+	}
+	if name := e1.Name(); name != "e1" {
+		t.Fatalf("failed to get node 'n'. Got node '%s' instead.", name)
+	}
+
+	e2, err := graph.Edge("e2", m, n)
+	if err != nil {
+		t.Fatalf("%+v", err)
+	}
+	if e2 != nil {
+		t.Fatal("got unexpected edge 'e2'")
+	}
+	if edges := graph.NumberEdges(); edges != 1 {
+		t.Fatalf("expected graph to have 1 edge, but found %d", edges)
+	}
+}
+
 func TestNodeDegree(t *testing.T) {
 	type test struct {
 		node_name             string