encoding/dot: fix ports on undirected graphs

AUTHORS

@@ -29,6 +29,7 @@ Iakov Davydov <iakov.davydov@unil.ch>
 Jalem Raj Rohit <jrajrohit33@gmail.com>
 James Bell <james@stellentus.com>
 James Bowman <james.edward.bowman@gmail.com>
+James Holmes <32bitkid@gmail.com>
 Janne Snabb <snabb@epipe.com>
 Jeff Juozapaitis <jjjuozap@email.arizona.edu>
 Jeremy Atkinson <jchatkinson@gmail.com>

CONTRIBUTORS

@@ -36,6 +36,7 @@ Iakov Davydov <iakov.davydov@unil.ch>
 Jalem Raj Rohit <jrajrohit33@gmail.com>
 James Bell <james@stellentus.com>
 James Bowman <james.edward.bowman@gmail.com>
+James Holmes <32bitkid@gmail.com>
 Janne Snabb <snabb@epipe.com>
 Jeff Juozapaitis <jjjuozap@email.arizona.edu>
 Jeremy Atkinson <jchatkinson@gmail.com>

graph/encoding/dot/encode.go

@@ -41,7 +41,12 @@ type Attributers interface {
 // to the the DOT node port to be used by the edge, compass corresponds
 // to DOT compass point to which the edge will be aimed.
 type Porter interface {
+	// FromPort returns the port and compass for the
+	// From node of a graph.Edge.
 	FromPort() (port, compass string)
+
+	// ToPort returns the port and compass for the
+	// To node of a graph.Edge.
 	ToPort() (port, compass string)
 }
 
@@ -219,9 +224,14 @@ func (p *printer) print(g graph.Graph, name string, needsIndent, isSubgraph bool
 			} else {
 				p.writeNode(n)
 			}
-			e, edgeIsPorter := g.Edge(nid, tid).(Porter)
+			e := g.Edge(nid, tid)
+			porter, edgeIsPorter := e.(Porter)
 			if edgeIsPorter {
-				p.writePorts(e.FromPort())
+				if e.From().ID() == nid {
+					p.writePorts(porter.FromPort())
+				} else {
+					p.writePorts(porter.ToPort())
+				}
 			}
 
 			if isDirected {
@@ -241,7 +251,11 @@ func (p *printer) print(g graph.Graph, name string, needsIndent, isSubgraph bool
 				p.writeNode(t)
 			}
 			if edgeIsPorter {
-				p.writePorts(e.ToPort())
+				if e.From().ID() == nid {
+					p.writePorts(porter.ToPort())
+				} else {
+					p.writePorts(porter.FromPort())
+				}
 			}
 
 			if a, ok := g.Edge(nid, tid).(encoding.Attributer); ok {

graph/encoding/dot/encode_test.go

@@ -256,14 +256,6 @@ func (e portedEdge) From() graph.Node { return e.from }
 func (e portedEdge) To() graph.Node   { return e.to }
 func (e portedEdge) Weight() float64  { return 0 }
 
-// TODO(kortschak): Figure out a better way to handle the fact that
-// headedness is an undefined concept in undirected graphs. We sort
-// nodes by ID, so lower ID nodes are always from nodes in undirected
-// graphs. We can probably do this in the printer, but I am leaving
-// this here as a WARNING.
-// Maybe the approach should be to document that for undirected graphs
-// the low ID node should be returned by the FromPort and the high ID
-// by the ToPort calls.
 func (e portedEdge) FromPort() (port, compass string) {
 	return e.fromPort, e.fromCompass
 }
@@ -1054,14 +1046,7 @@ var encodeTests = []struct {
 				{from: 2, to: 3}: {fromPort: "Zwei", fromCompass: "e"},
 				{from: 2, to: 4}: {fromPort: "Two", fromCompass: "w", toPort: "Four", toCompass: "w"},
 				{from: 3, to: 4}: {toPort: "Four", toCompass: "w"},
-
-				// This definition is reversed (see comment above at portedEdge
-				// definition) so that 4 gets the from port. This is a result
-				// of the fact that we sort nodes by ID, so the lower node
-				// will be always be printed first when the graph is undirected,
-				// thus becoming the from port, but we define the edges here
-				// from a directed adjacency list.
-				{from: 4, to: 0}: {fromCompass: "s", toPort: "Four", toCompass: "_"},
+				{from: 4, to: 0}: {fromPort: "Four", fromCompass: "_", toCompass: "s"},
 			},
 		),
 

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;
+	// }
+}