Fixing port-stats DBZ

Change-Id: I2bd1f9d94c1441d6bdcde00a5d2a3e595e0c5c4f

@@ -17,6 +17,8 @@ package org.onosproject.net.statistic;
 
 import com.google.common.base.MoreObjects;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 /**
  * Implementation of a load.
  */
@@ -62,6 +64,7 @@ public class DefaultLoad implements Load {
      * @param interval poll interval for this load
      */
     public DefaultLoad(long current, long previous, int interval) {
+        checkArgument(interval > 0, "Interval must be greater than 0");
         this.current = current;
         this.previous = previous;
         this.time = System.currentTimeMillis();

@@ -46,8 +46,11 @@ import static org.slf4j.LoggerFactory.getLogger;
 @Component(immediate = true)
 @Service
 public class PortStatisticsManager implements PortStatisticsService {
+
     private final Logger log = getLogger(getClass());
 
+    private static final int SECOND = 1_000;
+
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected DeviceService deviceService;
 
@@ -73,9 +76,9 @@ public class PortStatisticsManager implements PortStatisticsService {
     public Load load(ConnectPoint connectPoint) {
         DataPoint c = current.get(connectPoint);
         DataPoint p = previous.get(connectPoint);
-        if (c != null && p != null) {
+        if (c != null && p != null && (c.time > p.time + SECOND)) {
             return new DefaultLoad(c.stats.bytesSent(), p.stats.bytesSent(),
-                                   (int) (c.time - p.time) / 1000);
+                                   (int) (c.time - p.time) / SECOND);
         }
         return null;
     }