graph/encoding/dot: add multigraph serialization and deserialization support

graph/encoding/dot/decode.go

@@ -50,6 +50,19 @@ func Unmarshal(data []byte, dst encoding.Builder) error {
 	return copyGraph(dst, file.Graphs[0])
 }
 
+// UnmarshalMulti parses the Graphviz DOT-encoded data as a multigraph and
+// stores the result in dst.
+func UnmarshalMulti(data []byte, dst encoding.MultiBuilder) error {
+	file, err := dot.ParseBytes(data)
+	if err != nil {
+		return err
+	}
+	if len(file.Graphs) != 1 {
+		return fmt.Errorf("invalid number of graphs; expected 1, got %d", len(file.Graphs))
+	}
+	return copyMultigraph(dst, file.Graphs[0])
+}
+
 // copyGraph copies the nodes and edges from the Graphviz AST source graph to
 // the destination graph. Edge direction is maintained if present.
 func copyGraph(dst encoding.Builder, src *ast.Graph) (err error) {
@@ -62,9 +75,41 @@ func copyGraph(dst encoding.Builder, src *ast.Graph) (err error) {
 			panic(e)
 		}
 	}()
-	gen := &generator{
-		directed: src.Directed,
-		ids:      make(map[string]graph.Node),
+	gen := &simpleGraph{
+		generator: generator{
+			directed: src.Directed,
+			ids:      make(map[string]graph.Node),
+		},
+	}
+	if dst, ok := dst.(DOTIDSetter); ok {
+		dst.SetDOTID(src.ID)
+	}
+	if a, ok := dst.(AttributeSetters); ok {
+		gen.graphAttr, gen.nodeAttr, gen.edgeAttr = a.DOTAttributeSetters()
+	}
+	for _, stmt := range src.Stmts {
+		gen.addStmt(dst, stmt)
+	}
+	return err
+}
+
+// copyMultigraph copies the nodes and edges from the Graphviz AST source graph to
+// the destination graph. Edge direction is maintained if present.
+func copyMultigraph(dst encoding.MultiBuilder, src *ast.Graph) (err error) {
+	defer func() {
+		switch e := recover().(type) {
+		case nil:
+		case error:
+			err = e
+		default:
+			panic(e)
+		}
+	}()
+	gen := &multiGraph{
+		generator: generator{
+			directed: src.Directed,
+			ids:      make(map[string]graph.Node),
+		},
 	}
 	if dst, ok := dst.(DOTIDSetter); ok {
 		dst.SetDOTID(src.ID)
@@ -97,7 +142,7 @@ type generator struct {
 
 // node returns the gonum node corresponding to the given dot AST node ID,
 // generating a new such node if none exist.
-func (gen *generator) node(dst encoding.Builder, id string) graph.Node {
+func (gen *generator) node(dst graph.NodeAdder, id string) graph.Node {
 	if n, ok := gen.ids[id]; ok {
 		return n
 	}
@@ -117,8 +162,10 @@ func (gen *generator) node(dst encoding.Builder, id string) graph.Node {
 	return n
 }
 
+type simpleGraph struct{ generator }
+
 // addStmt adds the given statement to the graph.
-func (gen *generator) addStmt(dst encoding.Builder, stmt ast.Stmt) {
+func (gen *simpleGraph) addStmt(dst encoding.Builder, stmt ast.Stmt) {
 	switch stmt := stmt.(type) {
 	case *ast.NodeStmt:
 		n, ok := gen.node(dst, stmt.Node.ID).(encoding.AttributeSetter)
@@ -206,7 +253,7 @@ func applyPortsToEdge(from ast.Vertex, to *ast.Edge, edge graph.Edge) {
 }
 
 // addEdgeStmt adds the given edge statement to the graph.
-func (gen *generator) addEdgeStmt(dst encoding.Builder, stmt *ast.EdgeStmt) {
+func (gen *simpleGraph) addEdgeStmt(dst encoding.Builder, stmt *ast.EdgeStmt) {
 	fs := gen.addVertex(dst, stmt.From)
 	ts := gen.addEdge(dst, stmt.To, stmt.Attrs)
 	for _, f := range fs {
@@ -220,7 +267,7 @@ func (gen *generator) addEdgeStmt(dst encoding.Builder, stmt *ast.EdgeStmt) {
 }
 
 // addVertex adds the given vertex to the graph, and returns its set of nodes.
-func (gen *generator) addVertex(dst encoding.Builder, v ast.Vertex) []graph.Node {
+func (gen *simpleGraph) addVertex(dst encoding.Builder, v ast.Vertex) []graph.Node {
 	switch v := v.(type) {
 	case *ast.Node:
 		n := gen.node(dst, v.ID)
@@ -237,7 +284,7 @@ func (gen *generator) addVertex(dst encoding.Builder, v ast.Vertex) []graph.Node
 }
 
 // addEdge adds the given edge to the graph, and returns its set of nodes.
-func (gen *generator) addEdge(dst encoding.Builder, to *ast.Edge, attrs []*ast.Attr) []graph.Node {
+func (gen *simpleGraph) addEdge(dst encoding.Builder, to *ast.Edge, attrs []*ast.Attr) []graph.Node {
 	if !gen.directed && to.Directed {
 		panic(fmt.Errorf("directed edge to %v in undirected graph", to.Vertex))
 	}
@@ -307,6 +354,123 @@ func (gen *generator) appendSubgraphNode(n graph.Node) {
 	gen.subNodes = append(gen.subNodes, n)
 }
 
+type multiGraph struct{ generator }
+
+// addStmt adds the given statement to the multigraph.
+func (gen *multiGraph) addStmt(dst encoding.MultiBuilder, stmt ast.Stmt) {
+	switch stmt := stmt.(type) {
+	case *ast.NodeStmt:
+		n, ok := gen.node(dst, stmt.Node.ID).(encoding.AttributeSetter)
+		if !ok {
+			return
+		}
+		for _, attr := range stmt.Attrs {
+			a := encoding.Attribute{
+				Key:   attr.Key,
+				Value: attr.Val,
+			}
+			if err := n.SetAttribute(a); err != nil {
+				panic(fmt.Errorf("unable to unmarshal node DOT attribute (%s=%s)", a.Key, a.Value))
+			}
+		}
+	case *ast.EdgeStmt:
+		gen.addEdgeStmt(dst, stmt)
+	case *ast.AttrStmt:
+		var n encoding.AttributeSetter
+		var dst string
+		switch stmt.Kind {
+		case ast.GraphKind:
+			if gen.graphAttr == nil {
+				return
+			}
+			n = gen.graphAttr
+			dst = "graph"
+		case ast.NodeKind:
+			if gen.nodeAttr == nil {
+				return
+			}
+			n = gen.nodeAttr
+			dst = "node"
+		case ast.EdgeKind:
+			if gen.edgeAttr == nil {
+				return
+			}
+			n = gen.edgeAttr
+			dst = "edge"
+		default:
+			panic("unreachable")
+		}
+		for _, attr := range stmt.Attrs {
+			a := encoding.Attribute{
+				Key:   attr.Key,
+				Value: attr.Val,
+			}
+			if err := n.SetAttribute(a); err != nil {
+				panic(fmt.Errorf("unable to unmarshal global %s DOT attribute (%s=%s)", dst, a.Key, a.Value))
+			}
+		}
+	case *ast.Attr:
+		// ignore.
+	case *ast.Subgraph:
+		for _, stmt := range stmt.Stmts {
+			gen.addStmt(dst, stmt)
+		}
+	default:
+		panic(fmt.Sprintf("unknown statement type %T", stmt))
+	}
+}
+
+// addEdgeStmt adds the given edge statement to the multigraph.
+func (gen *multiGraph) addEdgeStmt(dst encoding.MultiBuilder, stmt *ast.EdgeStmt) {
+	fs := gen.addVertex(dst, stmt.From)
+	ts := gen.addLine(dst, stmt.To, stmt.Attrs)
+	for _, f := range fs {
+		for _, t := range ts {
+			edge := dst.NewLine(f, t)
+			dst.SetLine(edge)
+			applyPortsToEdge(stmt.From, stmt.To, edge)
+			addEdgeAttrs(edge, stmt.Attrs)
+		}
+	}
+}
+
+// addVertex adds the given vertex to the multigraph, and returns its set of nodes.
+func (gen *multiGraph) addVertex(dst encoding.MultiBuilder, v ast.Vertex) []graph.Node {
+	switch v := v.(type) {
+	case *ast.Node:
+		n := gen.node(dst, v.ID)
+		return []graph.Node{n}
+	case *ast.Subgraph:
+		gen.pushSubgraph()
+		for _, stmt := range v.Stmts {
+			gen.addStmt(dst, stmt)
+		}
+		return gen.popSubgraph()
+	default:
+		panic(fmt.Sprintf("unknown vertex type %T", v))
+	}
+}
+
+// addLine adds the given edge to the multigraph, and returns its set of nodes.
+func (gen *multiGraph) addLine(dst encoding.MultiBuilder, to *ast.Edge, attrs []*ast.Attr) []graph.Node {
+	if !gen.directed && to.Directed {
+		panic(fmt.Errorf("directed edge to %v in undirected graph", to.Vertex))
+	}
+	fs := gen.addVertex(dst, to.Vertex)
+	if to.To != nil {
+		ts := gen.addLine(dst, to.To, attrs)
+		for _, f := range fs {
+			for _, t := range ts {
+				edge := dst.NewLine(f, t)
+				dst.SetLine(edge)
+				applyPortsToEdge(to.Vertex, to.To, edge)
+				addEdgeAttrs(edge, attrs)
+			}
+		}
+	}
+	return fs
+}
+
 // addEdgeAttrs adds the attributes to the given edge.
 func addEdgeAttrs(edge graph.Edge, attrs []*ast.Attr) {
 	e, ok := edge.(encoding.AttributeSetter)

graph/encoding/dot/decode_test.go

@@ -10,6 +10,7 @@ import (
 
 	"gonum.org/v1/gonum/graph"
 	"gonum.org/v1/gonum/graph/encoding"
+	"gonum.org/v1/gonum/graph/multi"
 	"gonum.org/v1/gonum/graph/simple"
 )
 
@@ -293,6 +294,107 @@ const undirectedNonchained = `strict graph {
 	B -- C [label="baz 2"];
 }`
 
+func TestMultigraphDecoding(t *testing.T) {
+	for i, test := range []struct {
+		directed bool
+		input    string
+		expected string
+	}{
+		{
+			directed: true,
+			input:    directedMultigraph,
+			expected: directedMultigraph,
+		},
+		{
+			directed: false,
+			input:    undirectedMultigraph,
+			expected: undirectedMultigraph,
+		},
+		{
+			directed: true,
+			input: directedSelfLoopMultigraph,
+			expected: directedSelfLoopMultigraph,
+		},
+		{
+			directed: false,
+			input: undirectedSelfLoopMultigraph,
+			expected: undirectedSelfLoopMultigraph,
+		},
+	} {
+		var dst encoding.MultiBuilder
+		if test.directed {
+			dst = multi.NewDirectedGraph()
+		} else {
+			dst = multi.NewUndirectedGraph()
+		}
+
+		if err := UnmarshalMulti([]byte(test.input), dst); err != nil {
+			t.Errorf("i=%d: unable to unmarshal DOT graph; %v", i, err)
+			continue
+		}
+		buf, err := MarshalMulti(dst, "", "", "\t")
+		if err != nil {
+			t.Errorf("i=%d: unable to marshal graph; %v", i, dst)
+			continue
+		}
+		actual := string(buf)
+		if actual != test.expected {
+			t.Errorf("i=%d: graph content mismatch; want:\n%s\n\nactual:\n%s", i, test.expected, actual)
+			continue
+		}
+	}
+}
+
+const directedMultigraph = `digraph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -> 1;
+	0 -> 1;
+	0 -> 2;
+	2 -> 0;
+}`
+
+const undirectedMultigraph = `graph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -- 1;
+	0 -- 1;
+	0 -- 2;
+	0 -- 2;
+}`
+
+const directedSelfLoopMultigraph = `digraph {
+	// Node definitions.
+	0;
+	1;
+
+	// Edge definitions.
+	0 -> 0;
+	0 -> 0;
+	1 -> 1;
+	1 -> 1;
+}`
+
+const undirectedSelfLoopMultigraph = `graph {
+	// Node definitions.
+	0;
+	1;
+
+	// Edge definitions.
+	0 -- 0;
+	0 -- 0;
+	1 -- 1;
+	1 -- 1;
+}`
+
 // Below follows a minimal implementation of a graph capable of validating the
 // round-trip encoding and decoding of DOT graphs with nodes and edges
 // containing DOT attributes.

graph/encoding/dot/encode.go

@@ -55,17 +55,33 @@ type Structurer interface {
 	Structure() []Graph
 }
 
+// MultiStructurer represents a graph.Multigraph that can define subgraphs.
+type MultiStructurer interface {
+	Structure() []Multigraph
+}
+
 // Graph wraps named graph.Graph values.
 type Graph interface {
 	graph.Graph
 	DOTID() string
 }
 
+// Multigraph wraps named graph.Multigraph values.
+type Multigraph interface {
+	graph.Multigraph
+	DOTID() string
+}
+
 // Subgrapher wraps graph.Node values that represent subgraphs.
 type Subgrapher interface {
 	Subgraph() graph.Graph
 }
 
+// MultiSubgrapher wraps graph.Node values that represent subgraphs.
+type MultiSubgrapher interface {
+	Subgraph() graph.Multigraph
+}
+
 // Marshal returns the DOT encoding for the graph g, applying the prefix
 // and indent to the encoding. Name is used to specify the graph name. If
 // name is empty and g implements Graph, the returned string from DOTID
@@ -76,7 +92,7 @@ type Subgrapher interface {
 // implementation of the Node, Attributer, Porter, Attributers, Structurer,
 // Subgrapher and Graph interfaces.
 func Marshal(g graph.Graph, name, prefix, indent string) ([]byte, error) {
-	var p printer
+	var p simpleGraphPrinter
 	p.indent = indent
 	p.prefix = prefix
 	p.visited = make(map[edge]bool)
@@ -87,6 +103,28 @@ func Marshal(g graph.Graph, name, prefix, indent string) ([]byte, error) {
 	return p.buf.Bytes(), nil
 }
 
+// MarshalMulti returns the DOT encoding for the multigraph g, applying the
+// prefix and indent to the encoding. Name is used to specify the graph name. If
+// name is empty and g implements Graph, the returned string from DOTID
+// will be used. If strict is true the output bytes will be prefixed with
+// the DOT "strict" keyword.
+//
+// Graph serialization will work for a graph.Multigraph without modification,
+// however, advanced GraphViz DOT features provided by Marshal depend on
+// implementation of the Node, Attributer, Porter, Attributers, Structurer,
+// MultiSubgrapher and Multigraph interfaces.
+func MarshalMulti(g graph.Multigraph, name, prefix, indent string) ([]byte, error) {
+	var p multiGraphPrinter
+	p.indent = indent
+	p.prefix = prefix
+	p.visited = make(map[line]bool)
+	err := p.print(g, name, false, false)
+	if err != nil {
+		return nil, err
+	}
+	return p.buf.Bytes(), nil
+}
+
 type printer struct {
 	buf bytes.Buffer
 
@@ -94,8 +132,6 @@ type printer struct {
 	indent string
 	depth  int
 
-	visited map[edge]bool
-
 	err error
 }
 
@@ -104,38 +140,17 @@ type edge struct {
 	from, to int64
 }
 
-func (p *printer) print(g graph.Graph, name string, needsIndent, isSubgraph bool) error {
-	nodes := graph.NodesOf(g.Nodes())
-	sort.Sort(ordered.ByID(nodes))
-
-	p.buf.WriteString(p.prefix)
-	if needsIndent {
-		for i := 0; i < p.depth; i++ {
-			p.buf.WriteString(p.indent)
-		}
-	}
-	if !isSubgraph {
-		p.buf.WriteString("strict ")
-	}
-	_, isDirected := g.(graph.Directed)
-	if isSubgraph {
-		p.buf.WriteString("sub")
-	} else if isDirected {
-		p.buf.WriteString("di")
-	}
-	p.buf.WriteString("graph")
 
+func (p *simpleGraphPrinter) print(g graph.Graph, name string, needsIndent, isSubgraph bool) error {
 	if name == "" {
 		if g, ok := g.(Graph); ok {
 			name = g.DOTID()
 		}
 	}
-	if name != "" {
-		p.buf.WriteByte(' ')
-		p.buf.WriteString(name)
-	}
 
-	p.openBlock(" {")
+	_, isDirected := g.(graph.Directed)
+	p.printFrontMatter(name, needsIndent, isSubgraph, isDirected, true)
+
 	if a, ok := g.(Attributers); ok {
 		p.writeAttributeComplex(a)
 	}
@@ -150,6 +165,9 @@ func (p *printer) print(g graph.Graph, name string, needsIndent, isSubgraph bool
 		}
 	}
 
+	nodes := graph.NodesOf(g.Nodes())
+	sort.Sort(ordered.ByID(nodes))
+
 	havePrintedNodeHeader := false
 	for _, n := range nodes {
 		if s, ok := n.(Subgrapher); ok {
@@ -264,11 +282,40 @@ func (p *printer) print(g graph.Graph, name string, needsIndent, isSubgraph bool
 			p.buf.WriteByte(';')
 		}
 	}
+
 	p.closeBlock("}")
 
 	return nil
 }
 
+func (p *printer) printFrontMatter(name string, needsIndent, isSubgraph, isDirected, isStrict bool) error {
+	p.buf.WriteString(p.prefix)
+	if needsIndent {
+		for i := 0; i < p.depth; i++ {
+			p.buf.WriteString(p.indent)
+		}
+	}
+
+	if !isSubgraph && isStrict {
+		p.buf.WriteString("strict ")
+	}
+
+	if isSubgraph {
+		p.buf.WriteString("sub")
+	} else if isDirected {
+		p.buf.WriteString("di")
+	}
+	p.buf.WriteString("graph")
+
+	if name != "" {
+		p.buf.WriteByte(' ')
+		p.buf.WriteString(name)
+	}
+
+	p.openBlock(" {")
+	return nil
+}
+
 func (p *printer) writeNode(n graph.Node) {
 	p.buf.WriteString(nodeID(n))
 }
@@ -293,7 +340,7 @@ func nodeID(n graph.Node) string {
 	}
 }
 
-func graphID(g graph.Graph, n graph.Node) string {
+func graphID(g interface{}, n graph.Node) string {
 	switch g := g.(type) {
 	case Node:
 		return g.DOTID()
@@ -372,3 +419,165 @@ func (p *printer) closeBlock(b string) {
 	p.newline()
 	p.buf.WriteString(b)
 }
+
+type simpleGraphPrinter struct {
+	printer
+	visited map[edge]bool
+}
+
+type multiGraphPrinter struct {
+	printer
+	visited map[line]bool
+}
+
+type line struct {
+	inGraph string
+	id      int64
+}
+
+func (p *multiGraphPrinter) print(g graph.Multigraph, name string, needsIndent, isSubgraph bool) error {
+	if name == "" {
+		if g, ok := g.(Multigraph); ok {
+			name = g.DOTID()
+		}
+	}
+
+	_, isDirected := g.(graph.Directed)
+	p.printFrontMatter(name, needsIndent, isSubgraph, isDirected, false)
+
+	if a, ok := g.(Attributers); ok {
+		p.writeAttributeComplex(a)
+	}
+	if s, ok := g.(MultiStructurer); ok {
+		for _, g := range s.Structure() {
+			_, subIsDirected := g.(graph.Directed)
+			if subIsDirected != isDirected {
+				return errors.New("dot: mismatched graph type")
+			}
+			p.buf.WriteByte('\n')
+			p.print(g, g.DOTID(), true, true)
+		}
+	}
+
+	nodes := graph.NodesOf(g.Nodes())
+	sort.Sort(ordered.ByID(nodes))
+
+	havePrintedNodeHeader := false
+	for _, n := range nodes {
+		if s, ok := n.(MultiSubgrapher); ok {
+			// If the node is not linked to any other node
+			// the graph needs to be written now.
+			if g.From(n.ID()).Len() == 0 {
+				g := s.Subgraph()
+				_, subIsDirected := g.(graph.Directed)
+				if subIsDirected != isDirected {
+					return errors.New("dot: mismatched graph type")
+				}
+				if !havePrintedNodeHeader {
+					p.newline()
+					p.buf.WriteString("// Node definitions.")
+					havePrintedNodeHeader = true
+				}
+				p.newline()
+				p.print(g, graphID(g, n), false, true)
+			}
+			continue
+		}
+		if !havePrintedNodeHeader {
+			p.newline()
+			p.buf.WriteString("// Node definitions.")
+			havePrintedNodeHeader = true
+		}
+		p.newline()
+		p.writeNode(n)
+		if a, ok := n.(encoding.Attributer); ok {
+			p.writeAttributeList(a)
+		}
+		p.buf.WriteByte(';')
+	}
+
+	havePrintedEdgeHeader := false
+	for _, n := range nodes {
+		nid := n.ID()
+		to := graph.NodesOf(g.From(nid))
+		sort.Sort(ordered.ByID(to))
+
+		for _, t := range to {
+			tid := t.ID()
+
+			lines := graph.LinesOf(g.Lines(nid, tid))
+			sort.Sort(ordered.LinesByIDs(lines))
+
+			for _, l := range lines {
+				lid := l.ID()
+				if p.visited[line{inGraph: name, id: lid}] {
+					continue
+				}
+				p.visited[line{inGraph: name, id: lid}] = true
+
+				if !havePrintedEdgeHeader {
+					p.buf.WriteByte('\n')
+					p.buf.WriteString(strings.TrimRight(p.prefix, " \t\n")) // Trim whitespace suffix.
+					p.newline()
+					p.buf.WriteString("// Edge definitions.")
+					havePrintedEdgeHeader = true
+				}
+				p.newline()
+
+				if s, ok := n.(MultiSubgrapher); ok {
+					g := s.Subgraph()
+					_, subIsDirected := g.(graph.Directed)
+					if subIsDirected != isDirected {
+						return errors.New("dot: mismatched graph type")
+					}
+					p.print(g, graphID(g, n), false, true)
+				} else {
+					p.writeNode(n)
+				}
+
+				porter, edgeIsPorter := l.(Porter)
+				if edgeIsPorter {
+					if l.From().ID() == nid {
+						p.writePorts(porter.FromPort())
+					} else {
+						p.writePorts(porter.ToPort())
+					}
+				}
+
+				if isDirected {
+					p.buf.WriteString(" -> ")
+				} else {
+					p.buf.WriteString(" -- ")
+				}
+
+				if s, ok := t.(MultiSubgrapher); ok {
+					g := s.Subgraph()
+					_, subIsDirected := g.(graph.Directed)
+					if subIsDirected != isDirected {
+						return errors.New("dot: mismatched graph type")
+					}
+					p.print(g, graphID(g, t), false, true)
+				} else {
+					p.writeNode(t)
+				}
+				if edgeIsPorter {
+					if l.From().ID() == nid {
+						p.writePorts(porter.ToPort())
+					} else {
+						p.writePorts(porter.FromPort())
+					}
+				}
+
+				if a, ok := l.(encoding.Attributer); ok {
+					p.writeAttributeList(a)
+				}
+
+				p.buf.WriteByte(';')
+			}
+		}
+	}
+
+	p.closeBlock("}")
+
+	return nil
+}

graph/encoding/dot/encode_test.go

@@ -9,6 +9,7 @@ import (
 
 	"gonum.org/v1/gonum/graph"
 	"gonum.org/v1/gonum/graph/encoding"
+	"gonum.org/v1/gonum/graph/multi"
 	"gonum.org/v1/gonum/graph/simple"
 )
 
@@ -1417,3 +1418,258 @@ func TestEncode(t *testing.T) {
 		}
 	}
 }
+
+type intlist []int64
+
+func createMultigraph(g []intlist) graph.Multigraph {
+	dg := multi.NewUndirectedGraph()
+	for u, e := range g {
+		u := int64(u)
+		nu := multi.Node(u)
+		for _, v := range e {
+			nv := multi.Node(v)
+			dg.SetLine(dg.NewLine(nu, nv))
+		}
+	}
+	return dg
+}
+
+func createNamedMultigraph(g []intlist) graph.Multigraph {
+	dg := multi.NewUndirectedGraph()
+	for u, e := range g {
+		u := int64(u)
+		nu := namedNode{id: u, name: alpha[u : u+1]}
+		for _, v := range e {
+			nv := namedNode{id: v, name: alpha[v : v+1]}
+			dg.SetLine(dg.NewLine(nu, nv))
+		}
+	}
+	return dg
+}
+
+func createDirectedMultigraph(g []intlist) graph.Multigraph {
+	dg := multi.NewDirectedGraph()
+	for u, e := range g {
+		u := int64(u)
+		nu := multi.Node(u)
+		for _, v := range e {
+			nv := multi.Node(v)
+			dg.SetLine(dg.NewLine(nu, nv))
+		}
+	}
+	return dg
+}
+
+func createNamedDirectedMultigraph(g []intlist) graph.Multigraph {
+	dg := multi.NewDirectedGraph()
+	for u, e := range g {
+		u := int64(u)
+		nu := namedNode{id: u, name: alpha[u : u+1]}
+		for _, v := range e {
+			nv := namedNode{id: v, name: alpha[v : v+1]}
+			dg.SetLine(dg.NewLine(nu, nv))
+		}
+	}
+	return dg
+}
+
+var encodeMultiTests = []struct {
+	name string
+	g    graph.Multigraph
+
+	prefix string
+
+	want string
+}{
+	{
+		g: createMultigraph([]intlist{}),
+		want: `graph {
+}`,
+	},
+	{
+		g: createMultigraph([]intlist{
+			0: {1},
+			1: {0, 2},
+			2: {},
+		}),
+		want: `graph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -- 1;
+	0 -- 1;
+	1 -- 2;
+}`,
+	},
+	{
+		g: createMultigraph([]intlist{
+			0: {1},
+			1: {2, 2},
+			2: {0, 0, 0},
+		}),
+		want: `graph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -- 1;
+	0 -- 2;
+	0 -- 2;
+	0 -- 2;
+	1 -- 2;
+	1 -- 2;
+}`,
+	},
+	{
+		g: createNamedMultigraph([]intlist{
+			0: {1},
+			1: {2, 2},
+			2: {0, 0, 0},
+		}),
+		want: `graph {
+	// Node definitions.
+	A;
+	B;
+	C;
+
+	// Edge definitions.
+	A -- B;
+	A -- C;
+	A -- C;
+	A -- C;
+	B -- C;
+	B -- C;
+}`,
+	},
+	{
+		g: createMultigraph([]intlist{
+			0: {2, 1, 0},
+			1: {2, 1, 0},
+			2: {2, 1, 0},
+		}),
+		want: `graph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -- 0;
+	0 -- 1;
+	0 -- 1;
+	0 -- 2;
+	0 -- 2;
+	1 -- 1;
+	1 -- 2;
+	1 -- 2;
+	2 -- 2;
+}`,
+	},
+	{
+		g: createDirectedMultigraph([]intlist{}),
+		want: `digraph {
+}`,
+	},
+	{
+		g: createDirectedMultigraph([]intlist{
+			0: {1},
+			1: {0, 2},
+			2: {},
+		}),
+		want: `digraph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -> 1;
+	1 -> 0;
+	1 -> 2;
+}`,
+	},
+	{
+		g: createDirectedMultigraph([]intlist{
+			0: {1},
+			1: {2, 2},
+			2: {0, 0, 0},
+		}),
+		want: `digraph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -> 1;
+	1 -> 2;
+	1 -> 2;
+	2 -> 0;
+	2 -> 0;
+	2 -> 0;
+}`,
+	},
+	{
+		g: createNamedDirectedMultigraph([]intlist{
+			0: {1},
+			1: {2, 2},
+			2: {0, 0, 0},
+		}),
+		want: `digraph {
+	// Node definitions.
+	A;
+	B;
+	C;
+
+	// Edge definitions.
+	A -> B;
+	B -> C;
+	B -> C;
+	C -> A;
+	C -> A;
+	C -> A;
+}`,
+	},
+	{
+		g: createDirectedMultigraph([]intlist{
+			0: {2, 1, 0},
+			1: {2, 1, 0},
+			2: {2, 1, 0},
+		}),
+		want: `digraph {
+	// Node definitions.
+	0;
+	1;
+	2;
+
+	// Edge definitions.
+	0 -> 0;
+	0 -> 1;
+	0 -> 2;
+	1 -> 0;
+	1 -> 1;
+	1 -> 2;
+	2 -> 0;
+	2 -> 1;
+	2 -> 2;
+}`,
+	},
+}
+
+func TestEncodeMulti(t *testing.T) {
+	for i, test := range encodeMultiTests {
+		got, err := MarshalMulti(test.g, test.name, test.prefix, "\t")
+		if err != nil {
+			t.Errorf("unexpected error: %v", err)
+			continue
+		}
+		if string(got) != test.want {
+			t.Errorf("unexpected DOT result for test %d:\ngot: %s\nwant:%s", i, got, test.want)
+		}
+	}
+}

graph/encoding/encoding.go

@@ -12,6 +12,12 @@ type Builder interface {
 	graph.Builder
 }
 
+// MultiBuilder is a graph that can have user-defined nodes and edges added.
+type MultiBuilder interface {
+	graph.Multigraph
+	graph.MultigraphBuilder
+}
+
 // AttributeSetter is implemented by types that can set an encoded graph
 // attribute.
 type AttributeSetter interface {

graph/internal/ordered/sort.go

@@ -74,3 +74,20 @@ func Reverse(nodes []graph.Node) {
 		nodes[i], nodes[j] = nodes[j], nodes[i]
 	}
 }
+
+// LinesByIDs implements the sort.Interface sorting a slice of graph.LinesByIDs
+// lexically by the From IDs, then by the To IDs, finally by the Line IDs.
+type LinesByIDs []graph.Line
+
+func (n LinesByIDs) Len() int { return len(n) }
+func (n LinesByIDs) Less(i, j int) bool {
+	a, b := n[i], n[j]
+	if a.From().ID() != b.From().ID() {
+		return a.From().ID() < b.From().ID()
+	}
+	if a.To().ID() != b.To().ID() {
+		return a.To().ID() < b.To().ID()
+	}
+	return n[i].ID() < n[j].ID()
+}
+func (n LinesByIDs) Swap(i, j int) { n[i], n[j] = n[j], n[i] }