雪花算法，在分布式环境下实现高效的ID生成

其实雪花算法比较简单，可能称不上什么算法就是一种构造UID的方法。

点1：UID是一个long类型的41位时间戳，10位存储机器码，12位存储序列号。

点2：时间戳的单位是毫秒，可以同时链接1024台机器，每台机器每毫秒可以使用4096个序列好，我们会给生成id上一个同步锁，阻塞住其他线程的访问。

点3：利用掩码我们可以检测序列是否溢出，如果溢出的话，就强制等待到下一毫秒。

/**
 * @author hardstone
 * @since 29 July 2023(1690603385473)
 */
public class SnowFlakes {
    //开始的时间戳
    private final  long start = 1690603385473L;
    //机器标识长度5位
    private final long machineIdBits = 5L;
    //机器集群标识长度5位
    private final long centerIdBits = 5L;
    //序列标识所占位数12位
    private final long sequenceBits = 12L;
    //机器标识最大值
    private final long maxMachineId = -1L ^ (-1L << machineIdBits);
    //机器集群标识最大值
    private final long maxCenterId = -1L ^ (-1L << centerIdBits);
    //序列标识的最大值
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);
    //机器标识左移长度
    private final long machineIdShift = sequenceBits;
    //机器集群标识左移长度
    private final long centerIdShift = sequenceBits + machineIdBits;
    //时间戳左移长度
    private final long timeStampIdShift = sequenceBits + machineIdBits + centerIdBits;
    //序列Id
    private long sequence = 0L;

    //机器Id
    private long machineId;

    //机器集群Id
    private long centerId;

    //时间戳
    private long lastTimeStamp = -1L;

    public SnowFlakes(long machineId, long centerId) {
        if (machineId > maxMachineId || machineId < 0) {
            throw new IllegalArgumentException(String.format("WorkerId should be between 0 and 31"));
        }
        if (centerId > maxCenterId || centerId < 0) {
            throw new IllegalArgumentException(String.format("CenterId should be between 0 and 31"));
        }
    }
    public synchronized long nextId() {
        long timeStamp = System.currentTimeMillis();
        //时间回滚现象
        if (timeStamp < lastTimeStamp) {
            throw new RuntimeException(
                    String.format("Time gone backwards!")
            );
        }
        if (lastTimeStamp == timeStamp) {
            sequence = (sequence + 1) & sequenceMask;
            //如果序列分配完了
            if (sequence == 0) {
                timeStamp = getNextMillis(lastTimeStamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimeStamp = timeStamp;
        return ((timeStamp - start) << timeStampIdShift)
                | (centerId << centerIdShift)
                | (machineId << machineIdShift)
                | sequence;
    }
    protected long getNextMillis(long lastTimeStamp) {
        long timeStamp = System.currentTimeMillis();
        while (timeStamp <= lastTimeStamp) {
            timeStamp = System.currentTimeMillis();
        }
        return timeStamp;
    }
    public static void main(String[] args) {
        System.out.println(new SnowFlakes(0, 0).nextId());
    }
}