graph/encoding: move basic encoding API into new encoding package

graph/encoding/encoding.go

@@ -0,0 +1,35 @@
+// Copyright ©2017 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 encoding provides a common graph encoding API.
+package encoding // import "gonum.org/v1/gonum/graph/encoding"
+
+import "gonum.org/v1/gonum/graph"
+
+// Builder is a graph that can have user-defined nodes and edges added.
+type Builder interface {
+	graph.Graph
+	graph.Builder
+	// NewEdge adds a new edge from the source to the destination node to the
+	// graph, or returns the existing edge if already present.
+	NewEdge(from, to graph.Node) graph.Edge
+}
+
+// UnmarshalerAttr is implemented by types that can unmarshal a graph
+// attribute description of themselves.
+type UnmarshalerAttr interface {
+	// UnmarshalAttr decodes a single attribute.
+	UnmarshalAttr(attr Attribute) error
+}
+
+// Attributer defines graph.Node or graph.Edge values that can
+// specify graph attributes.
+type Attributer interface {
+	Attributes() []Attribute
+}
+
+// Attribute is an encoded key value attribute pair use in graph encoding.
+type Attribute struct {
+	Key, Value string
+}