/** *//**
 * This class represents a NTP message, as specified in RFC 2030.  The message
 * format is compatible with all versions of NTP and SNTP.  ToDo: Is this true?
 *
 * This class does not support the optional authentication protocol, and
 * ignores the key ID and message digest fields.
 * 
 * For convenience, this class exposes message values as native Java types, not
 * the NTP-specified data formats.  For example, timestamps are
 * stored as doubles (as opposed to the NTP unsigned 64-bit fixed point
 * format).
 * 
 * However, the contructor NtpMessage(byte[]) and the method toByteArray()
 * allow the import and export of the raw NTP message format.
 * 
 * 
 * Usage example
 * 
 * // Send message
 * DatagramSocket socket = new DatagramSocket();
 * InetAddress address = InetAddress.getByName("ntp.cais.rnp.br");
 * byte[] buf = new NtpMessage().toByteArray();
 * DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 123);
 * socket.send(packet);
 * 
 * // Get response
 * socket.receive(packet);
 * System.out.println(msg.toString());
 * 
 *  
 * This code is copyright (c) Adam Buckley 2004
 *
 * This program is free software; you can redistribute it and/or modify it 
 * under the terms of the GNU General Public License as published by the Free 
 * Software Foundation; either version 2 of the License, or (at your option) 
 * any later version.  A HTML version of the GNU General Public License can be
 * seen at http://www.gnu.org/licenses/gpl.html
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT 
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for 
 * more details.
 * 
 * 
 * Comments for member variables are taken from RFC2030 by David Mills,
 * University of Delaware.
 * 
 * Number format conversion code in NtpMessage(byte[] array) and toByteArray()
 * inspired by http://www.pps.jussieu.fr/~jch/enseignement/reseaux/
 * NTPMessage.java which is copyright (c) 2003 by Juliusz Chroboczek
 * 
 * @author Adam Buckley
 */


import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

public class NtpMessage {
    /** *//**
     * This is a two-bit code warning of an impending leap second to be
     * inserted/deleted in the last minute of the current day. It's values may
     * be as follows:
     * 
     * Value Meaning ----- ------- 0 no warning 1 last minute has 61 seconds 2
     * last minute has 59 seconds) 3 alarm condition (clock not synchronized)
     */
    public byte leapIndicator = 0;

    /** *//**
     * This value indicates the NTP/SNTP version number. The version number is 3
     * for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If
     * necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating
     * context must be inspected.
     */
    public byte version = 3;

    /** *//**
     * This value indicates the mode, with values defined as follows:
     * 
     * Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric
     * passive 3 client 4 server 5 broadcast 6 reserved for NTP control message
     * 7 reserved for private use
     * 
     * In unicast and anycast modes, the client sets this field to 3 (client) in
     * the request and the server sets it to 4 (server) in the reply. In
     * multicast mode, the server sets this field to 5 (broadcast).
     */
    public byte mode = 0;

    /** *//**
     * This value indicates the stratum level of the local clock, with values
     * defined as follows:
     * 
     * Stratum Meaning ---------------------------------------------- 0
     * unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15
     * secondary reference (via NTP or SNTP) 16-255 reserved
     */
    public short stratum = 0;

    /** *//**
     * This value indicates the maximum interval between successive messages, in
     * seconds to the nearest power of two. The values that can appear in this
     * field presently range from 4 (16 s) to 14 (16284 s); however, most
     * applications use only the sub-range 6 (64 s) to 10 (1024 s).
     */
    public byte pollInterval = 0;

    /** *//**
     * This value indicates the precision of the local clock, in seconds to the
     * nearest power of two. The values that normally appear in this field
     * range from -6 for mains-frequency clocks to -20 for microsecond clocks
     * found in some workstations.
     */
    public byte precision = 0;

    /** *//**
     * This value indicates the total roundtrip delay to the primary reference
     * source, in seconds. Note that this variable can take on both positive and
     * negative values, depending on the relative time and frequency offsets.
     * The values that normally appear in this field range from negative values
     * of a few milliseconds to positive values of several hundred milliseconds.
     */
    public double rootDelay = 0;

    /** *//**
     * This value indicates the nominal error relative to the primary reference
     * source, in seconds. The values that normally appear in this field range
     * from 0 to several hundred milliseconds.
     */
    public double rootDispersion = 0;

    /** *//**
     * This is a 4-byte array identifying the particular reference source. In
     * the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or
     * stratum-1 (primary) servers, this is a four-character ASCII string, left
     * justified and zero padded to 32 bits. In NTP Version 3 secondary servers,
     * this is the 32-bit IPv4 address of the reference source. In NTP Version 4
     * secondary servers, this is the low order 32 bits of the latest transmit
     * timestamp of the reference source. NTP primary (stratum 1) servers should
     * set this field to a code identifying the external reference source
     * according to the following list. If the external reference is one of
     * those listed, the associated code should be used. Codes for sources not
     * listed can be contrived as appropriate.
     * 
     * Code External Reference Source ---- ------------------------- LOCL
     * uncalibrated local clock used as a primary reference for a subnet without
     * external means of synchronization PPS atomic clock or other
     * pulse-per-second source individually calibrated to national standards
     * ACTS NIST dialup modem service USNO USNO modem service PTB PTB (Germany)
     * modem service TDF Allouis (France) Radio 164 kHz DCF Mainflingen
     * (Germany) Radio 77.5 kHz MSF Rugby (UK) Radio 60 kHz WWV Ft. Collins (US)
     * Radio 2.5, 5, 10, 15, 20 MHz WWVB Boulder (US) Radio 60 kHz WWVH Kaui
     * Hawaii (US) Radio 2.5, 5, 10, 15 MHz CHU Ottawa (Canada) Radio 3330,
     * 7335, 14670 kHz LORC LORAN-C radionavigation system OMEG OMEGA
     * radionavigation system GPS Global Positioning Service GOES Geostationary
     * Orbit Environment Satellite
     */
    public byte[] referenceIdentifier = { 0, 0, 0, 0 };

    /** *//**
     * This is the time at which the local clock was last set or corrected, in
     * seconds since 00:00 1-Jan-1900.
     */
    public double referenceTimestamp = 0;

    /** *//**
     * This is the time at which the request departed the client for the server,
     * in seconds since 00:00 1-Jan-1900.
     */
    public double originateTimestamp = 0;

    /** *//**
     * This is the time at which the request arrived at the server, in seconds
     * since 00:00 1-Jan-1900.
     */
    public double receiveTimestamp = 0;

    /** *//**
     * This is the time at which the reply departed the server for the client,
     * in seconds since 00:00 1-Jan-1900.
     */
    public double transmitTimestamp = 0;

    /** *//**
     * Constructs a new NtpMessage from an array of bytes.
     */
    public NtpMessage(byte[] array) {
        // See the packet format diagram in RFC 2030 for details
        leapIndicator = (byte) ((array[0] >> 6) & 0x3);
        version = (byte) ((array[0] >> 3) & 0x7);
        mode = (byte) (array[0] & 0x7);
        stratum = unsignedByteToShort(array[1]);
        pollInterval = array[2];
        precision = array[3];