Added a WallClockTimestamp and associated interface/implementation for generatin…

…g it. This will be used by the initial implementation of distributed host store

+package org.onlab.onos.net.host;
+
+import org.onlab.onos.store.Timestamp;
+import org.onlab.packet.MacAddress;
+
+/**
+ * Interface for a logical clock service that issues per host timestamps.
+ */
+public interface HostClockService {
+
+    /**
+     * Returns a new timestamp for the specified host mac address.
+     * @param hostMac host MAC address.
+     * @return timestamp.
+     */
+    public Timestamp getTimestamp(MacAddress hostMac);
+}

+package org.onlab.onos.store.host.impl;
+
+import static org.slf4j.LoggerFactory.getLogger;
+
+import org.apache.felix.scr.annotations.Activate;
+import org.apache.felix.scr.annotations.Component;
+import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Service;
+import org.onlab.onos.net.host.HostClockService;
+import org.onlab.onos.store.Timestamp;
+import org.onlab.onos.store.impl.WallClockTimestamp;
+import org.onlab.packet.MacAddress;
+import org.slf4j.Logger;
+
+/**
+ * HostClockService to issue Timestamps based on local wallclock time.
+ */
+@Component(immediate = true)
+@Service
+public class HostClockManager implements HostClockService {
+
+    private final Logger log = getLogger(getClass());
+
+    @Activate
+    public void activate() {
+        log.info("Started");
+    }
+
+    @Deactivate
+    public void deactivate() {
+        log.info("Stopped");
+    }
+
+    @Override
+    public Timestamp getTimestamp(MacAddress hostMac) {
+        return new WallClockTimestamp();
+    }
+}

@@ -10,8 +10,12 @@ import com.google.common.base.MoreObjects;
 import com.google.common.collect.ComparisonChain;
 
 /**
- * Default implementation of Timestamp.
- * TODO: Better documentation.
+ * A logical timestamp that derives its value from two things:
+ * <ul>
+ * <li> The current mastership term of the device.</li>
+ * <li> The value of the counter used for tracking topology events observed from
+ * the device during that current time of a device. </li>
+ * </ul>
  */
 public final class MastershipBasedTimestamp implements Timestamp {
 

+package org.onlab.onos.store.impl;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import java.util.Objects;
+
+import org.onlab.onos.store.Timestamp;
+
+import com.google.common.base.MoreObjects;
+import com.google.common.collect.ComparisonChain;
+
+/**
+ * A Timestamp that derives its value from the prevailing
+ * wallclock time on the controller where it is generated.
+ */
+public class WallClockTimestamp implements Timestamp {
+
+    private final long unixTimestamp;
+
+    public WallClockTimestamp() {
+        unixTimestamp = System.currentTimeMillis();
+    }
+
+    @Override
+    public int compareTo(Timestamp o) {
+        checkArgument(o instanceof WallClockTimestamp,
+                "Must be WallClockTimestamp", o);
+        WallClockTimestamp that = (WallClockTimestamp) o;
+
+        return ComparisonChain.start()
+                .compare(this.unixTimestamp, that.unixTimestamp)
+                .result();
+    }
+    @Override
+    public int hashCode() {
+        return Objects.hash(unixTimestamp);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+        if (!(obj instanceof WallClockTimestamp)) {
+            return false;
+        }
+        WallClockTimestamp that = (WallClockTimestamp) obj;
+        return Objects.equals(this.unixTimestamp, that.unixTimestamp);
+    }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                    .add("unixTimestamp", unixTimestamp)
+                    .toString();
+    }
+
+    /**
+     * Returns the unixTimestamp.
+     *
+     * @return unix timestamp
+     */
+    public long unixTimestamp() {
+        return unixTimestamp;
+    }
+}

+package org.onlab.onos.store.impl;
+
+import static org.junit.Assert.assertTrue;
+
+import java.nio.ByteBuffer;
+
+import org.junit.Test;
+import org.onlab.onos.store.Timestamp;
+import org.onlab.util.KryoPool;
+
+import com.google.common.testing.EqualsTester;
+
+/**
+ * Tests for {@link WallClockTimestamp}.
+ */
+public class WallClockTimestampTest {
+
+    @Test
+    public final void testBasic() throws InterruptedException {
+        WallClockTimestamp ts1 = new WallClockTimestamp();
+        Thread.sleep(50);
+        WallClockTimestamp ts2 = new WallClockTimestamp();
+
+        assertTrue(ts1.compareTo(ts1) == 0);
+        assertTrue(ts2.compareTo(ts1) > 0);
+        assertTrue(ts1.compareTo(ts2) < 0);
+    }
+
+    @Test
+    public final void testKryoSerializable() {
+        WallClockTimestamp ts1 = new WallClockTimestamp();
+        final ByteBuffer buffer = ByteBuffer.allocate(1 * 1024 * 1024);
+        final KryoPool kryos = KryoPool.newBuilder()
+                .register(WallClockTimestamp.class)
+                .build();
+
+        kryos.serialize(ts1, buffer);
+        buffer.flip();
+        Timestamp copy = kryos.deserialize(buffer);
+
+        new EqualsTester()
+            .addEqualityGroup(ts1, copy)
+            .testEquals();
+    }
+}