Added code to establish the time delta between the client and server

clocks immediately following authentication.


git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@1398 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/presents/client/DeltaCalculator.java

@@ -0,0 +1,91 @@
+//
+// $Id: DeltaCalculator.java,v 1.1 2002/05/28 21:56:38 mdb Exp $
+
+package com.threerings.presents.client;
+
+import java.util.Arrays;
+import com.samskivert.util.StringUtil;
+
+import com.threerings.presents.Log;
+import com.threerings.presents.net.PingRequest;
+import com.threerings.presents.net.PongResponse;
+
+/**
+ * Used to compute the client/server time delta, attempting to account for
+ * the network delay experienced when the server sends its current time to
+ * the client.
+ */
+public class DeltaCalculator
+{
+    /**
+     * Constructs a delta calculator which is used to calculate the time
+     * delta between the client and server, accounding reasonably well for
+     * the delay introduced by sending a timestamp over the network from
+     * the server to the client.
+     */
+    public DeltaCalculator ()
+    {
+        _deltas = new long[CLOCK_SYNC_PING_COUNT];
+    }
+
+    /**
+     * Must be called when a ping message is sent to the server.
+     */
+    public void sentPing (PingRequest ping)
+    {
+        _ping = ping;
+    }
+
+    /**
+     * Must be called when the pong response arrives back from the server.
+     *
+     * @return true if we've iterated sufficiently many times to establish
+     * a stable time delta estimate.
+     */
+    public boolean gotPong (PongResponse pong)
+    {
+        // make a note of when the ping message was sent and when the pong
+        // response was received (both in client time)
+        long send = _ping.getPackStamp(), recv = pong.getUnpackStamp();
+
+        // make a note of when the pong response was sent (in server time)
+        // and the processing delay incurred on the server
+        long server = pong.getPackStamp(), delay = pong.getProcessDelay();
+
+        // compute the network delay (round-trip time divided by two)
+        long nettime = (recv - send - delay)/2;
+
+        // the time delta is the client time when the pong was received
+        // minus the server's send time (plus network delay): dT = C - S
+        _deltas[_iter] = recv - (server + nettime);
+
+        return (++_iter >= CLOCK_SYNC_PING_COUNT);
+    }
+
+    /**
+     * Returns the best estimate client/server time-delta.
+     */
+    public long getTimeDelta ()
+    {
+        // sort the estimates and return one from the middle
+        Arrays.sort(_deltas);
+        Log.info("Returning time delta " +
+                 "[deltas=" + StringUtil.toString(_deltas) + "].");
+        return _deltas[_deltas.length/2];
+    }
+
+    /** The number of ping/pong iterations we've made. */
+    protected int _iter;
+
+    /** Client/server time delta estimates. */
+    protected long[] _deltas;
+
+    /** A reference to the most recently sent ping which we use to obtain
+     * the appropriate send stamp when we get the corresponding receive
+     * stamp. */
+    protected PingRequest _ping;
+
+    /** The number of times we PING during clock sync to try to smooth out
+     * network jiggling. */
+    protected static final int CLOCK_SYNC_PING_COUNT = 3;
+}