encoding/dot: fix ports on undirected graphs

graph/encoding/dot/example_test.go

@@ -0,0 +1,47 @@
+// Copyright ©2018 The Gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dot_test
+
+import (
+	"fmt"
+
+	"gonum.org/v1/gonum/graph/encoding/dot"
+	"gonum.org/v1/gonum/graph/simple"
+)
+
+type edgeWithPorts struct {
+	simple.Edge
+	fromPort, toPort string
+}
+
+func (e *edgeWithPorts) FromPort() (string, string) {
+	return e.fromPort, ""
+}
+
+func (e *edgeWithPorts) ToPort() (string, string) {
+	return e.toPort, ""
+}
+
+func ExamplePorter() {
+	g := simple.NewUndirectedGraph()
+	g.SetEdge(&edgeWithPorts{
+		Edge:     simple.Edge{simple.Node(1), simple.Node(0)},
+		fromPort: "p1",
+		toPort:   "p2",
+	})
+
+	result, _ := dot.Marshal(g, "", "", "  ", true)
+	fmt.Print(string(result))
+
+	// Output:
+	// strict graph {
+	//   // Node definitions.
+	//   0;
+	//   1;
+	//
+	//   // Edge definitions.
+	//   0:p2 -- 1:p1;
+	// }
+}