rewrite and simplification

pkg/graph/graph.go

@@ -0,0 +1,31 @@
+package graph
+
+type ID string
+
+// Delta represent the changes a Node made to its follow list.
+// It Removed some nodes, and Added some others.
+// This means the old follow list is Removed + Common, while the new is Common + Added
+type Delta struct {
+	Node    ID
+	Removed []ID
+	Common  []ID
+	Added   []ID
+}
+
+func (d Delta) Old() []ID {
+	return append(d.Common, d.Removed...)
+}
+
+func (d Delta) New() []ID {
+	return append(d.Common, d.Added...)
+}
+
+// Inverse of the delta. If a delta and it's inverse are applied, the graph returns to its original state.
+func (d Delta) Inverse() Delta {
+	return Delta{
+		Node:    d.Node,
+		Common:  d.Common,
+		Removed: d.Added,
+		Added:   d.Removed,
+	}
+}