graph/encoding/dot: allow unmarshalling of DOT IDs

graph/encoding/dot/decode.go

@@ -39,6 +39,12 @@ type UnmarshalerAttr interface {
 	UnmarshalDOTAttr(attr Attribute) error
 }
 
+// UnmarshalerID is implemented by types that can unmarshal a DOT ID.
+type UnmarshalerID interface {
+	// UnmarshalDOTID decodes a single DOT ID.
+	UnmarshalDOTID(id string) error
+}
+
 // Unmarshal parses the Graphviz DOT-encoded data and stores the result in dst.
 func Unmarshal(data []byte, dst Builder) error {
 	file, err := dot.ParseBytes(data)
@@ -100,6 +106,12 @@ func (gen *generator) node(dst Builder, id string) graph.Node {
 		return n
 	}
 	n := dst.NewNode()
+	if n, ok := n.(UnmarshalerID); ok {
+		err := n.UnmarshalDOTID(id)
+		if err != nil {
+			panic(err)
+		}
+	}
 	gen.ids[id] = n
 	// Check if within the context of a subgraph, that is to be used as a vertex
 	// of an edge.

graph/encoding/dot/decode_test.go

@@ -65,11 +65,11 @@ const directed = `digraph {
 	];
 
 	// Node definitions.
-	0 [label="foo 2"];
-	1 [label="bar 2"];
+	A [label="foo 2"];
+	B [label="bar 2"];
 
 	// Edge definitions.
-	0 -> 1 [label="baz 2"];
+	A -> B [label="baz 2"];
 }`
 
 const undirected = `graph {
@@ -86,11 +86,11 @@ const undirected = `graph {
 	];
 
 	// Node definitions.
-	0 [label="foo 2"];
-	1 [label="bar 2"];
+	A [label="foo 2"];
+	B [label="bar 2"];
 
 	// Edge definitions.
-	0 -- 1 [label="baz 2"];
+	A -- B [label="baz 2"];
 }`
 
 // Below follows a minimal implementation of a graph capable of validating the
@@ -187,10 +187,22 @@ func (g *dotUndirectedGraph) DOTUnmarshalerAttrs() (graph, node, edge Unmarshale
 // and decoding of node DOT label attributes.
 type dotNode struct {
 	simple.Node
+	dotID string
 	// Node label.
 	Label string
 }
 
+// DOTID returns the node's DOT ID.
+func (n *dotNode) DOTID() string {
+	return n.dotID
+}
+
+// UnmarshalDOTID decodes a DOT ID.
+func (n *dotNode) UnmarshalDOTID(id string) error {
+	n.dotID = id
+	return nil
+}
+
 // UnmarshalDOTAttr decodes a single DOT attribute.
 func (n *dotNode) UnmarshalDOTAttr(attr Attribute) error {
 	if attr.Key != "label" {