Add resource id set.

api/resource/idset.go

@@ -0,0 +1,30 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package resource
+
+import "sigs.k8s.io/kustomize/api/resid"
+
+type IdSet struct {
+	ids map[resid.ResId]bool
+}
+
+func MakeIdSet(slice []*Resource) *IdSet {
+	set := make(map[resid.ResId]bool)
+	for _, r := range slice {
+		id := r.CurId()
+		if _, ok := set[id]; !ok {
+			set[id] = true
+		}
+	}
+	return &IdSet{ids: set}
+}
+
+func (s IdSet) Contains(id resid.ResId) bool {
+	_, ok := s.ids[id]
+	return ok
+}
+
+func (s IdSet) Size() int {
+	return len(s.ids)
+}