/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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.apache.commons.net.examples.ntp;
  
  import java.io.IOException;
  import java.net.DatagramPacket;
  import java.net.DatagramSocket;
  
  import org.apache.commons.net.ntp.NtpUtils;
  import org.apache.commons.net.ntp.NtpV3Impl;
  import org.apache.commons.net.ntp.NtpV3Packet;
  import org.apache.commons.net.ntp.TimeStamp;
  
  /**
   * The SimpleNTPServer class is a UDP implementation of a server for the Network Time Protocol (NTP) version 3 as described in RFC 1305. It is a minimal NTP
   * server that doesn't actually adjust the time but only responds to NTP datagram requests with response sent back to originating host with info filled out
   * using the current clock time. To be used for debugging or testing.
   *
   * To prevent this from interfering with the actual NTP service it can be run from any local port.
   */
  public class SimpleNTPServer implements Runnable {
  
      public static void main(final String[] args) {
          int port = NtpV3Packet.NTP_PORT;
          if (args.length != 0) {
              try {
                  port = Integer.parseInt(args[0]);
              } catch (final NumberFormatException nfe) {
                  nfe.printStackTrace();
              }
          }
          final SimpleNTPServer timeServer = new SimpleNTPServer(port);
          try {
              timeServer.start();
          } catch (final IOException e) {
              e.printStackTrace();
          }
      }
  
      private int port;
      private volatile boolean running;
      private boolean started;
  
      private DatagramSocket socket;
  
      /**
       * Creates SimpleNTPServer listening on default NTP port.
       */
      public SimpleNTPServer() {
          this(NtpV3Packet.NTP_PORT);
      }
  
      /**
       * Creates SimpleNTPServer.
       *
       * @param port the local port the server socket is bound to, or <code>zero</code> for a system selected free port.
       * @throws IllegalArgumentException if port number less than 0
       */
      public SimpleNTPServer(final int port) {
          if (port < 0) {
              throw new IllegalArgumentException();
          }
          this.port = port;
      }
  
      /**
       * Connects to server socket and listen for client connections.
       *
       * @throws IOException if an I/O error occurs when creating the socket.
       */
      public void connect() throws IOException {
          if (socket == null) {
              socket = new DatagramSocket(port);
              // port = 0 is bound to available free port
              if (port == 0) {
                  port = socket.getLocalPort();
              }
              System.out.println("Running NTP service on port " + port + "/UDP");
          }
      }
  
      public int getPort() {
          return port;
      }
  
     /**
      * Handles incoming packet. If NTP packet is client-mode then respond to that host with a NTP response packet otherwise ignore.
      *
      * @param request incoming DatagramPacket
      * @param rcvTime time packet received
      *
      * @throws IOException if an I/O error occurs.
      */
     protected void handlePacket(final DatagramPacket request, final long rcvTime) throws IOException {
         final NtpV3Packet message = new NtpV3Impl();
         message.setDatagramPacket(request);
         System.out.printf("NTP packet from %s mode=%s%n", request.getAddress().getHostAddress(), NtpUtils.getModeName(message.getMode()));
         if (message.getMode() == NtpV3Packet.MODE_CLIENT) {
             final NtpV3Packet response = new NtpV3Impl();
 
             response.setStratum(1);
             response.setMode(NtpV3Packet.MODE_SERVER);
             response.setVersion(NtpV3Packet.VERSION_3);
             response.setPrecision(-20);
             response.setPoll(0);
             response.setRootDelay(62);
             response.setRootDispersion((int) (16.51 * 65.536));
 
             // originate time as defined in RFC-1305 (t1)
             response.setOriginateTimeStamp(message.getTransmitTimeStamp());
             // Receive Time is time request received by server (t2)
             response.setReceiveTimeStamp(TimeStamp.getNtpTime(rcvTime));
             response.setReferenceTime(response.getReceiveTimeStamp());
             response.setReferenceId(0x4C434C00); // LCL (Undisciplined Local Clock)
 
             // Transmit time is time reply sent by server (t3)
             response.setTransmitTime(TimeStamp.getNtpTime(System.currentTimeMillis()));
 
             final DatagramPacket dp = response.getDatagramPacket();
             dp.setPort(request.getPort());
             dp.setAddress(request.getAddress());
             socket.send(dp);
         }
         // otherwise if received packet is other than CLIENT mode then ignore it
     }
 
     /**
      * Returns state of whether time service is running.
      *
      * @return true if time service is running
      */
     public boolean isRunning() {
         return running;
     }
 
     /**
      * Returns state of whether time service is running.
      *
      * @return true if time service is running
      */
     public boolean isStarted() {
         return started;
     }
 
     /**
      * Main method to service client connections.
      */
     @Override
     public void run() {
         running = true;
         final byte[] buffer = new byte[48];
         final DatagramPacket request = new DatagramPacket(buffer, buffer.length);
         do {
             try {
                 socket.receive(request);
                 final long rcvTime = System.currentTimeMillis();
                 handlePacket(request, rcvTime);
             } catch (final IOException e) {
                 if (running) {
                     e.printStackTrace();
                 }
                 // otherwise socket thrown exception during shutdown
             }
         } while (running);
     }
 
     /**
      * Starts time service and provide time to client connections.
      *
      * @throws IOException if an I/O error occurs when creating the socket.
      */
     public void start() throws IOException {
         if (socket == null) {
             connect();
         }
         if (!started) {
             started = true;
             new Thread(this).start();
         }
     }
 
     /**
      * Closes server socket and stop listening.
      */
     public void stop() {
         running = false;
         if (socket != null) {
             socket.close(); // force closing of the socket
             socket = null;
         }
         started = false;
     }
 
 }