Introduce Pi Register model to PI Framework

Change-Id: I7fae87d5b5ed5fff34b3addfc148cee6fc98137c
diff --git a/core/api/src/main/java/org/onosproject/net/pi/model/PiRegisterId.java b/core/api/src/main/java/org/onosproject/net/pi/model/PiRegisterId.java
new file mode 100644
index 0000000..ea8ca6f
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/pi/model/PiRegisterId.java
@@ -0,0 +1,46 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * 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.pi.model;
+
+import com.google.common.annotations.Beta;
+import org.onlab.util.Identifier;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * Identifier of a register in a protocol-independent pipeline, unique within the scope of a pipeline model.
+ */
+@Beta
+public final class PiRegisterId extends Identifier<String> {
+
+    private PiRegisterId(String name) {
+        super(name);
+    }
+
+    /**
+     * Returns an identifier for the given register name.
+     *
+     * @param name register name
+     * @return register ID
+     */
+    public static PiRegisterId of(String name) {
+        checkNotNull(name);
+        checkArgument(!name.isEmpty(), "Name can't be empty");
+        return new PiRegisterId(name);
+    }
+}