Fix for [ONOS-3590]

Compare two devices instead of one with itself
Add unit test for root node

Change-Id: If109dd8b05cd77825e182399b321c321865b9786

@@ -519,7 +519,7 @@ public class DefaultTopology extends AbstractModel implements Topology {
     private TopologyVertex findRoot(Set<TopologyVertex> vertexSet) {
     private TopologyVertex findRoot(Set<TopologyVertex> vertexSet) {
         TopologyVertex minVertex = null;
         TopologyVertex minVertex = null;
         for (TopologyVertex vertex : vertexSet) {
         for (TopologyVertex vertex : vertexSet) {
-            if ((minVertex == null) || (minVertex.deviceId()
+            if ((minVertex == null) || (vertex.deviceId()
                     .toString().compareTo(minVertex.deviceId().toString()) < 0)) {
                     .toString().compareTo(minVertex.deviceId().toString()) < 0)) {
                 minVertex = vertex;
                 minVertex = vertex;
             }
             }

@@ -29,9 +29,11 @@ import org.onosproject.net.PortNumber;
 import org.onosproject.net.provider.ProviderId;
 import org.onosproject.net.provider.ProviderId;
 import org.onosproject.net.topology.ClusterId;
 import org.onosproject.net.topology.ClusterId;
 import org.onosproject.net.topology.DefaultGraphDescription;
 import org.onosproject.net.topology.DefaultGraphDescription;
+import org.onosproject.net.topology.DefaultTopologyVertex;
 import org.onosproject.net.topology.GraphDescription;
 import org.onosproject.net.topology.GraphDescription;
 import org.onosproject.net.topology.LinkWeight;
 import org.onosproject.net.topology.LinkWeight;
 import org.onosproject.net.topology.TopologyCluster;
 import org.onosproject.net.topology.TopologyCluster;
+import org.onosproject.net.topology.TopologyVertex;
 
 
 import java.util.Set;
 import java.util.Set;
 
 
@@ -53,6 +55,9 @@ public class DefaultTopologyTest {
     public static final DeviceId D4 = deviceId("of:4");
     public static final DeviceId D4 = deviceId("of:4");
     public static final DeviceId D5 = deviceId("of:5");
     public static final DeviceId D5 = deviceId("of:5");
 
 
+    public static final TopologyVertex V1 = new DefaultTopologyVertex(D1);
+    public static final TopologyVertex V5 = new DefaultTopologyVertex(D5);
+
     public static final PortNumber P1 = portNumber(1);
     public static final PortNumber P1 = portNumber(1);
     public static final PortNumber P2 = portNumber(2);
     public static final PortNumber P2 = portNumber(2);
 
 
@@ -62,6 +67,9 @@ public class DefaultTopologyTest {
 
 
     private DefaultTopology dt;
     private DefaultTopology dt;
 
 
+    public static final ClusterId C0 = ClusterId.clusterId(0);
+    public static final ClusterId C1 = ClusterId.clusterId(1);
+
     @Before
     @Before
     public void setUp() {
     public void setUp() {
         long now = System.currentTimeMillis();
         long now = System.currentTimeMillis();
@@ -81,8 +89,9 @@ public class DefaultTopologyTest {
         assertEquals("incorrect device count", 5, dt.deviceCount());
         assertEquals("incorrect device count", 5, dt.deviceCount());
         assertEquals("incorrect link count", 8, dt.linkCount());
         assertEquals("incorrect link count", 8, dt.linkCount());
         assertEquals("incorrect cluster count", 2, dt.clusterCount());
         assertEquals("incorrect cluster count", 2, dt.clusterCount());
-        assertEquals("incorrect broadcast set size", 6,
-                     dt.broadcastSetSize(ClusterId.clusterId(0)));
+        assertEquals("incorrect broadcast set size", 6, dt.broadcastSetSize(C0));
+        assertEquals("incorrect root node", V1, dt.getCluster(C0).root());
+        assertEquals("incorrect root node", V5, dt.getCluster(C1).root());
     }
     }
 
 
     @Test
     @Test