* 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 - (double) (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 "";
}
}