diff --git a/api/resource/idset.go b/api/resource/idset.go
new file mode 100644
index 000000000..eaa253381
--- /dev/null
+++ b/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)
+}
diff --git a/api/resource/idset_test.go b/api/resource/idset_test.go
new file mode 100644
index 000000000..79cef5a0a
--- /dev/null
+++ b/api/resource/idset_test.go
@@ -0,0 +1,32 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package resource_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	. "sigs.k8s.io/kustomize/api/resource"
+)
+
+func TestIdSet_Empty(t *testing.T) {
+	s := MakeIdSet([]*Resource{})
+	assert.Equal(t, 0, s.Size())
+	assert.False(t, s.Contains(testDeployment.CurId()))
+	assert.False(t, s.Contains(testConfigMap.CurId()))
+}
+
+func TestIdSet_One(t *testing.T) {
+	s := MakeIdSet([]*Resource{testDeployment})
+	assert.Equal(t, 1, s.Size())
+	assert.True(t, s.Contains(testDeployment.CurId()))
+	assert.False(t, s.Contains(testConfigMap.CurId()))
+}
+
+func TestIdSet_Two(t *testing.T) {
+	s := MakeIdSet([]*Resource{testDeployment, testConfigMap})
+	assert.Equal(t, 2, s.Size())
+	assert.True(t, s.Contains(testDeployment.CurId()))
+	assert.True(t, s.Contains(testConfigMap.CurId()))
+}