Fill undefined keys with default value

See: <http://graphml.graphdrawing.org/primer/graphml-primer.html#AttributesExample>

data/test_graph_default_key_type.xml

@@ -7,6 +7,9 @@
     <key id="key1" for="node" attr.name="integer-key" attr.type="int">
         <default>10</default>
     </key>
+    <key id="key2" for="node" attr.name="color" attr.type="string">
+        <default>yellow</default>
+    </key>
     <graph edgedefault="directed" parse.nodeids="canonical" parse.edgeids="canonical" parse.order="nodesfirst">
         <node id="n0">
             <desc>test node #1</desc>

graphml/graphml.go

@@ -414,26 +414,26 @@ func (gr *Graph) GetEdge(sourceId, targetId string) *Edge {
 
 // GetAttributes return data attributes map associated with GraphML
 func (gml *GraphML) GetAttributes() (map[string]interface{}, error) {
-	return attributesForData(gml.Data, gml)
+	return attributesForData(gml.Data, KeyForGraphML, gml)
 }
 
 // GetAttributes return data attributes map associated with Graph
 func (gr *Graph) GetAttributes() (map[string]interface{}, error) {
-	return attributesForData(gr.Data, gr.parent)
+	return attributesForData(gr.Data, KeyForGraph, gr.parent)
 }
 
 // GetAttributes returns data attributes map associated with Node
 func (n *Node) GetAttributes() (map[string]interface{}, error) {
-	return attributesForData(n.Data, n.graph.parent)
+	return attributesForData(n.Data, KeyForNode, n.graph.parent)
 }
 
 // GetAttributes returns data attributes map associated with Edge
 func (e *Edge) GetAttributes() (map[string]interface{}, error) {
-	return attributesForData(e.Data, e.graph.parent)
+	return attributesForData(e.Data, KeyForEdge, e.graph.parent)
 }
 
 // builds attributes map for specified data array
-func attributesForData(data []*Data, gml *GraphML) (map[string]interface{}, error) {
+func attributesForData(data []*Data, target KeyForElement, gml *GraphML) (map[string]interface{}, error) {
 	attr := make(map[string]interface{})
 	for _, d := range data {
 		key, ok := gml.keysById[d.Key]
@@ -456,6 +456,16 @@ func attributesForData(data []*Data, gml *GraphML) (map[string]interface{}, erro
 			attr[key.Name] = value
 		}
 	}
+	// fill defaults for undefined keys
+	for _, k := range keysForElement(gml.Keys, target) {
+		if _, ok := attr[k.Name]; !ok {
+			val, err := valueByType(k.DefaultValue, k.KeyType, gml.keyTypeDefault)
+			if err != nil {
+				return nil, err
+			}
+			attr[k.Name] = val
+		}
+	}
 	return attr, nil
 }
 
@@ -616,3 +626,13 @@ func valueByType(val string, keyType DataType, keyTypeDefault DataType) (interfa
 		return valueByType(val, keyTypeDefault, keyTypeDefault)
 	}
 }
+
+// keysForElement returns all the keys from allKeys that apply to a certain element
+func keysForElement(allKeys []*Key, target KeyForElement) (keys []*Key) {
+	for _, k := range allKeys {
+		if k.Target == target || k.Target == KeyForAll {
+			keys = append(keys, k)
+		}
+	}
+	return keys
+}

graphml/graphml_test.go

@@ -50,6 +50,7 @@ func TestGraphML_Decode_keyTypeDefault(t *testing.T) {
 	attributes := make(map[string]interface{})
 	attributes["integer-key"] = 10
 	attributes["test-key"] = "test data"
+	attributes["color"] = "yellow"
 
 	// check Graph element
 	//