Implement compact serialization for a set of discrete resources

This is for ONOS-4281.

Change-Id: I08a9fc4fd334c499c7a09d2960145743a798094e
diff --git a/core/api/src/main/java/org/onosproject/net/resource/NoOpCodec.java b/core/api/src/main/java/org/onosproject/net/resource/NoOpCodec.java
new file mode 100644
index 0000000..942848b
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/resource/NoOpCodec.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.net.resource;
+
+/**
+ * Represents no-op codec intended to used in an empty discrete resource set only.
+ * It's not supposed to be used by other classes.
+ */
+public class NoOpCodec implements DiscreteResourceCodec {
+    public static final DiscreteResourceCodec INSTANCE = new NoOpCodec();
+
+    @Override
+    public int encode(DiscreteResource resource) {
+        return 0;
+    }
+
+    @Override
+    public DiscreteResource decode(DiscreteResourceId parent, int value) {
+        return Resource.ROOT;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == this) {
+            return true;
+        }
+
+        if (obj == null || getClass() != obj.getClass()) {
+            return false;
+        }
+
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        return NoOpCodec.class.hashCode();
+    }
+}