Allow for traversing OpenEdges

Three new methods, EdgeValue, FromValue, and FromTuple, have been added
to OpenEdge in order to allow future users of the gg package to traverse
the graph.

gg/gg.go

@@ -74,46 +74,56 @@ func (v Value) String() string {
 	}
 }
 
-// VertexType enumerates the different possible vertex types.
-type VertexType string
-
-const (
-	// ValueVertex is a Vertex which contains exactly one value and has at least
-	// one edge (either input or output).
-	ValueVertex VertexType = "val"
-
-	// TupleVertex is a Vertex which contains two or more in edges and
-	// exactly one out edge
-	//
-	// TODO ^ what about 0 or 1 in edges?
-	TupleVertex VertexType = "tup"
-)
-
 ////////////////////////////////////////////////////////////////////////////////
 
 // OpenEdge is an un-realized Edge which can't be used for anything except
 // constructing graphs. It has no meaning on its own.
 type OpenEdge struct {
-	fromV vertex
-	val   Value
+	fromV   vertex
+	edgeVal Value
+}
+
+func (oe OpenEdge) String() string {
+
+	vertexType := "tup"
+
+	if oe.fromV.val != nil {
+		vertexType = "val"
+	}
+
+	return fmt.Sprintf("%s(%s, %s)", vertexType, oe.fromV.String(), oe.edgeVal.String())
 }
 
-// WithEdgeVal returns a copy of the OpenEdge with the edge value replaced by
-// the given one.
-func (oe OpenEdge) WithEdgeVal(val Value) OpenEdge {
-	oe.val = val
-	return oe
+// EdgeValue returns the Value which lies on the edge itself.
+func (oe OpenEdge) EdgeValue() Value {
+	return oe.edgeVal
 }
 
-func (oe OpenEdge) String() string {
-	return fmt.Sprintf("%s(%s, %s)", oe.fromV.VertexType, oe.fromV.String(), oe.val.String())
+// FromValue returns the Value from which the OpenEdge was created via ValueOut,
+// or false if it wasn't created via ValueOut.
+func (oe OpenEdge) FromValue() (Value, bool) {
+	if oe.fromV.val == nil {
+		return ZeroValue, false
+	}
+
+	return *oe.fromV.val, true
+}
+
+// FromTuple returns the tuple of OpenEdges from which the OpenEdge was created
+// via TupleOut, or false if it wasn't created via TupleOut.
+func (oe OpenEdge) FromTuple() ([]OpenEdge, bool) {
+	if oe.fromV.val != nil {
+		return nil, false
+	}
+
+	return oe.fromV.tup, true
 }
 
 // ValueOut creates a OpenEdge which, when used to construct a Graph, represents
 // an edge (with edgeVal attached to it) coming from the ValueVertex containing
 // val.
 func ValueOut(val, edgeVal Value) OpenEdge {
-	return OpenEdge{fromV: mkVertex(ValueVertex, val), val: edgeVal}
+	return OpenEdge{fromV: vertex{val: &val}, edgeVal: edgeVal}
 }
 
 // TupleOut creates an OpenEdge which, when used to construct a Graph,
@@ -126,48 +136,38 @@ func TupleOut(ins []OpenEdge, edgeVal Value) OpenEdge {
 
 	if len(ins) == 1 {
 
+		in := ins[0]
+
 		if edgeVal.IsZero() {
-			return ins[0]
+			return in
 		}
 
-		if ins[0].val.IsZero() {
-			return ins[0].WithEdgeVal(edgeVal)
+		if in.edgeVal.IsZero() {
+			in.edgeVal = edgeVal
+			return in
 		}
 
 	}
 
 	return OpenEdge{
-		fromV: mkVertex(TupleVertex, ZeroValue, ins...),
-		val:   edgeVal,
+		fromV:   vertex{tup: ins},
+		edgeVal: edgeVal,
 	}
 }
 
 func (oe OpenEdge) equal(oe2 OpenEdge) bool {
-	return oe.val.Equal(oe2.val) && oe.fromV.equal(oe2.fromV)
+	return oe.edgeVal.Equal(oe2.edgeVal) && oe.fromV.equal(oe2.fromV)
 }
 
 type vertex struct {
-	VertexType
-	val Value
+	val *Value
 	tup []OpenEdge
 }
 
-func mkVertex(typ VertexType, val Value, tupIns ...OpenEdge) vertex {
-	return vertex{
-		VertexType: typ,
-		val:        val,
-		tup:        tupIns,
-	}
-}
-
 func (v vertex) equal(v2 vertex) bool {
 
-	if v.VertexType != v2.VertexType {
-		return false
-	}
-
-	if v.VertexType == ValueVertex {
-		return v.val.Equal(v2.val)
+	if v.val != nil {
+		return v2.val != nil && v.val.Equal(*v2.val)
 	}
 
 	if len(v.tup) != len(v2.tup) {
@@ -185,25 +185,17 @@ func (v vertex) equal(v2 vertex) bool {
 
 func (v vertex) String() string {
 
-	switch v.VertexType {
-
-	case ValueVertex:
+	if v.val != nil {
 		return v.val.String()
+	}
 
-	case TupleVertex:
-
-		strs := make([]string, len(v.tup))
-
-		for i := range v.tup {
-			strs[i] = v.tup[i].String()
-		}
-
-		return fmt.Sprintf("[%s]", strings.Join(strs, ", "))
+	strs := make([]string, len(v.tup))
 
-	default:
-		panic("unknown vertix kind")
+	for i := range v.tup {
+		strs[i] = v.tup[i].String()
 	}
 
+	return fmt.Sprintf("[%s]", strings.Join(strs, ", "))
 }
 
 type graphValueIn struct {