/** */
/**  
 * 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 constructor 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 (plus small modification by Anthony Mack)
 */

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

public class NtpMessage
	{
	//
	// Constants
	//
	private static final double SECONDS_BETWEEN_1900_1970 = 2208988800.0;
	private static final double MILLIS_PER_SECONDS 				= 1000.0;

	/** */
	/**
	 * 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];

		rootDelay = (array[4] * 256.0) 
		          +  unsignedByteToShort(array[5])
				      + (unsignedByteToShort(array[6]) / 256.0)
				      + (unsignedByteToShort(array[7]) / 65536.0);

		rootDispersion = (unsignedByteToShort(array[8]) * 256.0)
									 +  unsignedByteToShort(array[9])
				           + (unsignedByteToShort(array[10]) / 256.0)
				           + (unsignedByteToShort(array[11]) / 65536.0);

		referenceIdentifier[0] = array[12];
		referenceIdentifier[1] = array[13];
		referenceIdentifier[2] = array[14];
		referenceIdentifier[3] = array[15];

		referenceTimestamp = decodeTimestamp( array, 16 );
		originateTimestamp = decodeTimestamp( array, 24 );
		receiveTimestamp = decodeTimestamp( array, 32 );
		transmitTimestamp = decodeTimestamp( array, 40 );
		}

	/** */
	/**
	 * Constructs a new NtpMessage
	 */
	public NtpMessage( byte newLeapIndicator, byte newVersion, byte newMode,
			short newStratum, byte newPollInterval, byte newPrecision, double newRootDelay,
			double newRootDispersion, byte[] newReferenceIdentifier,
			double newReferenceTimestamp, double newOriginateTimestamp,
			double newReceiveTimestamp, double newTransmitTimestamp )
		{
		// ToDo: Validity checking
		leapIndicator = newLeapIndicator;
		version = newVersion;
		mode = newMode;
		stratum = newStratum;
		pollInterval = newPollInterval;
		precision = newPrecision;
		rootDelay = newRootDelay;
		rootDispersion = newRootDispersion;
		referenceIdentifier = newReferenceIdentifier;
		referenceTimestamp = newReferenceTimestamp;
		originateTimestamp = newOriginateTimestamp;
		receiveTimestamp = newReceiveTimestamp;
		transmitTimestamp = newTransmitTimestamp;
		}

	/** */
	/**
	 * Constructs a new NtpMessage in client -> server mode, and sets the transmit
	 * timestamp to the current time.
	 */
	public NtpMessage()
		{
		// Note that all the other member variables are already set with
		// appropriate default values.
		mode = 3;
		transmitTimestamp = convertFromMillis( System.currentTimeMillis() );
		}

	/** */
	/**
	 * This method constructs the data bytes of a raw NTP packet.
	 */
	public byte[] toByteArray()
		{
		// All bytes are automatically set to 0
		byte[] p = new byte[48];

		p[0] = (byte) (leapIndicator << 6 | version << 3 | mode);
		p[1] = (byte) stratum;
		p[2] = pollInterval;
		p[3] = precision;

		// root delay is a signed 16.16-bit FP, in Java an int is 32-bits
		int l = (int) (rootDelay * 65536.0);
		p[4] = (byte) ((l >> 24) & 0xFF);
		p[5] = (byte) ((l >> 16) & 0xFF);
		p[6] = (byte) ((l >> 8) & 0xFF);
		p[7] = (byte) (l & 0xFF);

		// root dispersion is an unsigned 16.16-bit FP, in Java there are no
		// unsigned primitive types, so we use a long which is 64-bits
		long ul = (long) (rootDispersion * 65536.0);
		p[8] = (byte) ((ul >> 24) & 0xFF);
		p[9] = (byte) ((ul >> 16) & 0xFF);
		p[10] = (byte) ((ul >> 8) & 0xFF);
		p[11] = (byte) (ul & 0xFF);

		p[12] = referenceIdentifier[0];
		p[13] = referenceIdentifier[1];
		p[14] = referenceIdentifier[2];
		p[15] = referenceIdentifier[3];

		encodeTimestamp( p, 16, referenceTimestamp );
		encodeTimestamp( p, 24, originateTimestamp );
		encodeTimestamp( p, 32, receiveTimestamp );
		encodeTimestamp( p, 40, transmitTimestamp );

		return p;
		}

	/** */
	/**
	 * Returns a string representation of a NtpMessage
	 */
	@Override
	public String toString()
		{
		String precisionStr = new DecimalFormat( "0.#E0" ).format( Math.pow( 2,
				precision ) );
		return "Leap indicator: " + leapIndicator + " " + "Version: " + version
				+ " " + "Mode: " + mode + " " + "Stratum: " + stratum + " " + "Poll: "
				+ pollInterval + " " + "Precision: " + precision + " (" + precisionStr
				+ " seconds) " + "Root delay: "
				+ new DecimalFormat( "0.00" ).format( rootDelay * 1000 ) + " ms "
				+ "Root dispersion: "
				+ new DecimalFormat( "0.00" ).format( rootDispersion * 1000 ) + " ms "
				+ "Reference identifier: "
				+ referenceIdentifierToString( referenceIdentifier, stratum, version )
				+ " " + "Reference timestamp: "
				+ timestampToString( referenceTimestamp ) + " "
				+ "Originate timestamp: " + timestampToString( originateTimestamp )
				+ " " + "Receive timestamp:   " + timestampToString( receiveTimestamp )
				+ " " + "Transmit timestamp:  " + timestampToString( transmitTimestamp );
		}

	/** */
	/**
	 * Converts an unsigned byte to a short. By default, Java assumes that a byte
	 * is signed.
	 */
	public static short unsignedByteToShort( byte b )
		{
		if( (b & 0x80) == 0x80 )
			return (short) (128 + (b & 0x7f));
		return b;
		}

	/** */
	/**
	 * Will read 8 bytes of a message beginning at <code>pointer</code> and return
	 * it as a double, according to the NTP 64-bit timestamp format.
	 */
	public static double decodeTimestamp( byte[] array, int pointer )
		{
		double r = 0.0;

		for( int i = 0; i < 8; i++ )
			{
			r += unsignedByteToShort( array[pointer + i] )
					* Math.pow( 2, (3 - i) * 8 );
			}

		return r;
		}

	/** */
	/**
	 * Encodes a timestamp in the specified position in the message
	 */
	public static void encodeTimestamp( byte[] array, int pointer,
			double timestamp )
		{
		// Converts a double into a 64-bit fixed point
		for( int i = 0; i < 8; i++ )
			{
			// 2^24, 2^16, 2^8, .. 2^-32
			double base = Math.pow( 2, (3 - i) * 8 );

			// Capture byte value
			array[pointer + i] = (byte) (timestamp / base);

			// Subtract captured value from remaining total
			timestamp = timestamp
					- (unsignedByteToShort( array[pointer + i] ) * base);
			}

		// From RFC 2030: It is advisable to fill the non-significant
		// low order bits of the timestamp with a random, unbiased
		// bitstring, both to avoid systematic roundoff errors and as
		// a means of loop detection and replay detection.
		array[7] = (byte) (Math.random() * 255.0);
		}

	/** */
	/**
	 * Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a
	 * formatted date/time string.
	 */
	public static String timestampToString( double timestamp )
		{
		if( timestamp == 0 )
			return "0";

		// timestamp is relative to 1900, utc is used by Java and is relative
		// to 1970
		double utc = timestamp - (2208988800.0);

		// milliseconds
		long ms = (long) (utc * 1000.0);

		// date/time
		String date = new SimpleDateFormat( "dd-MMM-yyyy HH:mm:ss" )
				.format( new Date( ms ) );

		// fraction
		double fraction = timestamp - ((long) timestamp);
		String fractionSting = new DecimalFormat( ".000000" ).format( fraction );

		return date + fractionSting;
		}

	/** */
	/**
	 * Returns a string representation of a reference identifier according to the
	 * rules set out in RFC 2030.
	 */
	public static String referenceIdentifierToString( byte[] ref, short stratum,
			byte version )
		{
		// From the RFC 2030:
		// 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.
		if( stratum == 0 || stratum == 1 )
			{
			return new String( ref );
			}

		// In NTP Version 3 secondary servers, this is the 32-bit IPv4
		// address of the reference source.
		else if( version == 3 )
			{
			return unsignedByteToShort( ref[0] ) + "." + unsignedByteToShort( ref[1] )
					+ "." + unsignedByteToShort( ref[2] ) + "."
					+ unsignedByteToShort( ref[3] );
			}

		// In NTP Version 4 secondary servers, this is the low order 32 bits
		// of the latest transmit timestamp of the reference source.
		else if( version == 4 )
			{
			return ""
					+ ((unsignedByteToShort( ref[0] ) / 256.0)
							+ (unsignedByteToShort( ref[1] ) / 65536.0)
							+ (unsignedByteToShort( ref[2] ) / 16777216.0) + (unsignedByteToShort( ref[3] ) / 4294967296.0));
			}

		return "";
		}
	
	static double convertFromMillis( long millis )
		{
		return ( millis / MILLIS_PER_SECONDS ) + SECONDS_BETWEEN_1900_1970;
		}
	}