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package com.pippsford.util;
import java.security.SecureRandom;
import java.util.UUID;
/**
* Creates Version 7 UUIDs as defined in the Peabody proposal.
*
* See: https://web.archive.org/web/20220620082741/https://datatracker.ietf.org/doc/html/draft-peabody-dispatch-new-uuid-format
*
* @author Simon Greatrix on 27/06/2022.
*/
public class Uuid7 {
private static final Object LOCK = new Object();
/** Random number generator. */
private static final SecureRandom RANDOM = new SecureRandom();
private static int lastRand = 0;
private static long lastTime = Long.MIN_VALUE;
/**
* Create a type 7 UUID. These UUIDs can be ordered by their natural bit ordering into time order, include a millisecond precision timestamp, and at least
* 30 random bits. At most 4096 UUIDs can be generated in any given millisecond. If more than that are generated then the quota available to future
* milliseconds will be used.
*
* @return A new type 7 UUID
*/
public static UUID create() {
// 48-bit timestamp
long timeStamp;
int rand1;
synchronized (LOCK) {
timeStamp = System.currentTimeMillis() & 0xffff_ffff_ffffL;
if (timeStamp > lastTime) {
// In a new millisecond.
lastTime = timeStamp;
lastRand = rand1 = RANDOM.nextInt(0x1000);
} else {
// In the same (or older) millisecond, so just increment the count.
lastRand = rand1 = (lastRand + 1) & 0xfff;
if (lastRand == 0) {
// looped over into a new millisecond
lastTime = (lastTime + 1) & 0xffff_ffff_ffffL;
}
timeStamp = lastTime;
}
}
long uuid1 = (timeStamp << 16) | 0x7000 | rand1;
long uuid2 = 0x8000_0000_0000_0000L | RANDOM.nextLong() & 0x3fff_ffff_ffff_ffffL;
return new UUID(uuid1, uuid2);
}
}