UUIDBonker.java

import java.util.Random;
import java.util.UUID;

public class UUIDBonker {

    public static UUID randomUuid(Random random) {
        //fabric mappings seem to call m and l the other way round. This disagrees with UUID constructor.
        long m = random.nextLong() & -61441L | 16384L;
        long l = random.nextLong() & 4611686018427387903L | Long.MIN_VALUE;
        return new UUID(m,l);
    }

    public static long crackUUIDBits(long lsb, long msb) {
           long min1 = lsb >>> 32;
           //bitshift introduces potential rounding error. Also the constant in this line can maybe be pulled through most of the code
           long min2 = (msb - (11718085204285L >>> 16)) & 0xffff_ffffL;
           long max2 = min2 + 1;
           // generated by (((-11718085204285L << 48) >> 48) * -41139260) >> 16, it fixes rounding error
           long bugFixer = 14340612L;
           long a1 = (-28703124L * min1 + -41139260L * min2 + bugFixer) >> 32;
           long a2 = ( -4751153L * min1 +  2996749L * max2) >> 32;
           return (( a1 * -2996749L + a2 * -41139260L ) * 246154705703781L + 107048004364969L) & 0xffff_ffff_ffffL;
    }

    public static long crackUUIDString(String uuid) {
        uuid = uuid.replace("-","");
        return crackUUIDBits(Long.parseUnsignedLong(uuid.substring(0,16), 16), Long.parseUnsignedLong(uuid.substring(16,32), 16));
    }

    public static void main(String[] args) {
        int failcount = 0;
        //example UUID from a 1.16.1 villager
        String villagerUUID= "3355f18a-21c8-4f86-aeb6-f3191f681b6f";
        System.out.println(villagerUUID.equals(randomUuid(new Random(crackUUIDString(villagerUUID) ^ 0x5deece66dL)).toString()));
        for (int i = 0; i < 10000000; i++) {
            long s = new Random().nextLong() & 0xffff_ffff_ffffL;
            Random r = new Random(s ^ 0x5deece66dL);
            if (crackUUIDString(randomUuid(r).toString()) != s) {
                failcount += 1;
            }
        }
        System.out.println(failcount);
    }
}