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// ========================================================================
// Copyright 2006 Mort Bay Consulting Pty. Ltd.
// ------------------------------------------------------------------------
// 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.cometd.server.ext;

import java.util.Map;

import org.cometd.Client;
import org.cometd.Extension;
import org.cometd.Message;
import org.cometd.server.ClientImpl;
import org.eclipse.jetty.util.ajax.JSON;

/* ------------------------------------------------------------ */
/**
 * Timesync extension (server side).
 * 
 * With each handshake or connect, the extension sends timestamps within the ext
 * field like:
 * {ext:{timesync:{tc:12345567890,l:23,o:4567},...},...} where:
 * 
 * tc is the client timestamp in ms since 1970 of when the message was sent.
 * 
l is the network lag that the client has calculated.
 * 
o is the clock offset that the client has calculated.
 * 
 * 
 * 
 * A cometd server that supports timesync, can respond with an ext field like:
 * {ext:{timesync:{tc:12345567890,ts:1234567900,p:123,a:3},...},...}
 * where:
 * 

 * tc is the client timestamp of when the message was sent,
 * 
ts is the server timestamp of when the message was received
 * 
p is the poll duration in ms - ie the time the server took before sending
 * the response.
 * 
a is the measured accuracy of the calculated offset and lag sent by the
 * client
 * 
 * 
 * The relationship between tc, ts, o & l on the server is given by
 * ts=tc+o+l (the time the server received the message is the
 * client time plus the offset plus the network lag). Thus the accuracy of the o
 * and l settings can be determined with a=tc+o+l-ts.
 * 
 * 
 * When the client has received the response, it can make a more accurate
 * estimate of the lag as l2=(now-tc-p)/2 (assuming symmetric lag).
 * A new offset can then be calculated with the relationship on the client that
 * ts=tc+o2+l2, thus o2=ts-tc-l2.
 * 
 * 
 * Since the client also receives the a value calculated on the server, it
 * should be possible to analyse this and compensate for some asymmetry in the
 * lag. But the current client does not do this.
 * 
 */
public class TimesyncExtension implements Extension
{
    private int _accuracyTarget=25;

    public TimesyncExtension()
    {
    }

    /* ------------------------------------------------------------ */
    /**
     * timesync responses are not set if the measured accuracy is less than the
     * accuracyTarget.
     * 
     * @return accuracy target in ms (default 25ms)
     */
    public int getAccuracyTarget()
    {
        return _accuracyTarget;
    }

    /* ------------------------------------------------------------ */
    /**
     * timesync responses are not set if the measured accuracy is less than the
     * accuracyTarget.
     * 
     * @param target
     *            accuracy target in ms
     */
    public void setAccuracyTarget(int target)
    {
        _accuracyTarget=target;
    }

    public Message rcv(Client from, Message message)
    {
        return message;
    }

    public Message rcvMeta(Client from, Message message)
    {
        Map ext=message.getExt(false);
        if (ext != null)
        {
            Map sync=(Map)ext.get("timesync");
            if (sync != null)
            {
                sync.put("ts",new Long(System.currentTimeMillis()));
                Number lag=(Number)sync.get("l");
                if (lag != null && from != null)
                    ((ClientImpl)from).setLag(lag.intValue());
            }
        }
        return message;
    }

    public Message send(Client from, Message message)
    {
        return message;
    }

    public Message sendMeta(Client from, Message message)
    {
        Message associated=message.getAssociated();
        if (associated != null)
        {
            Map extIn=associated.getExt(false);

            if (extIn != null)
            {
                Map sync=(Map)extIn.get("timesync");
                if (sync != null)
                {
                    final long tc=((Number)sync.get("tc")).longValue();
                    final long ts=((Number)sync.get("ts")).longValue();

                    final Number lag=(Number)sync.get("l");
                    if (lag == null)
                    {
                        // old style timesync
                        Map extOut=(Map)message.getExt(true);
                        JSON.Literal timesync=new JSON.Literal("{\"tc\":" + tc + ",\"ts\":" + ts + ",\"p\":" + (System.currentTimeMillis() - ts) + "}");
                        extOut.put("timesync",timesync);
                    }
                    else
                    {
                        final int l=lag.intValue();
                        final int o=((Number)sync.get("o")).intValue();
                        final int a=(int)((tc + o + l) - ts);

                        // is a OK ?
                        if (l == 0 || a >= _accuracyTarget || a <= -_accuracyTarget)
                        {
                            Map extOut=(Map)message.getExt(true);
                            JSON.Literal timesync=new JSON.Literal("{\"tc\":" + tc + ",\"ts\":" + ts + ",\"p\":" + (System.currentTimeMillis() - ts)
                                    + ",\"a\":" + a + "}");

                            extOut.put("timesync",timesync);
                        }
                    }
                }
            }
        }
        return message;
    }
}