public class Snowflake {

    /** 開始時間戳 (2021-01-01) */
    private final long START_TIMESTAMP = 1609430400000L;

    /** 機器ID所占的位數(shù) */
    private final long WORKER_ID_BITS = 5L;

    /** 數(shù)據(jù)標(biāo)識ID所占的位數(shù) */
    private final long DATA_CENTER_ID_BITS = 5L;

    /** 支持的最大機器ID，結(jié)果是31 (0B11111) */
    private final long MAX_WORKER_ID = ~(-1L << WORKER_ID_BITS);

    /** 支持的最大數(shù)據(jù)標(biāo)識ID，結(jié)果是31 (0B11111) */
    private final long MAX_DATA_CENTER_ID = ~(-1L << DATA_CENTER_ID_BITS);

    /** 序列在ID中占的位數(shù) */
    private final long SEQUENCE_BITS = 12L;

    /** 機器ID向左移12位 */
    private final long WORKER_ID_SHIFT = SEQUENCE_BITS;

    /** 數(shù)據(jù)標(biāo)識ID向左移17位(12+5) */
    private final long DATA_CENTER_ID_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS;

    /** 時間戳向左移22位(5+5+12) */
    private final long TIMESTAMP_LEFT_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS + DATA_CENTER_ID_BITS;

    /** 支持的最大序列號，結(jié)果是4095 (0B111111111111) */
    private final long MAX_SEQUENCE = ~(-1L << SEQUENCE_BITS);

    /** 工作機器ID */
    private final long workerId;

    /** 數(shù)據(jù)中心ID */
    private final long dataCenterId;

    /** 毫秒內(nèi)序列號 */
    private long sequence = 0L;

    /** 上次生成ID的時間戳 */
    private long lastTimestamp = -1L;

    /**
     * 構(gòu)造函數(shù)
     * @param workerId 工作機器ID
     * @param dataCenterId 數(shù)據(jù)中心ID
     */
    public Snowflake(long workerId, long dataCenterId) {
        if (workerId > MAX_WORKER_ID || workerId < 0) {
            throw new IllegalArgumentException(String.format("WorkerID不能超過%d且不能小于0", MAX_WORKER_ID));
        }
        if (dataCenterId > MAX_DATA_CENTER_ID || dataCenterId < 0) {
            throw new IllegalArgumentException(String.format("DataCenterID不能超過%d且不能小于0", MAX_DATA_CENTER_ID));
        }
        this.workerId = workerId;
        this.dataCenterId = dataCenterId;
    }

    /**
     * 生成ID
     * @return long類型的ID
     */
    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();

        // 如果當(dāng)前時間小于上次生成ID的時間戳，說明系統(tǒng)時鐘回退過，拋出異常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("系統(tǒng)時鐘回退，拒絕生成ID，上次生成ID的時間戳：%d，當(dāng)前時間戳：%d",
                    lastTimestamp, timestamp));
        }

        // 如果當(dāng)前時間等于上次生成ID的時間戳（同一毫秒內(nèi)），則序列號加1
        if (timestamp == lastTimestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0) {
                // 如果序列號已經(jīng)超過最大值，需要等待到下一毫秒再繼續(xù)生成ID
                timestamp = waitNextMillis(timestamp);
            }
        } else {
            sequence = 0L;
        }

        // 更新上次生成ID的時間戳
        lastTimestamp = timestamp;

        // 生成ID
        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_LEFT_SHIFT) |
                (dataCenterId << DATA_CENTER_ID_SHIFT) |
                (workerId << WORKER_ID_SHIFT) |
                sequence;
    }

    /**
     * 等待下一毫秒
     * @param timestamp 上次生成ID的時間戳
     * @return 下一毫秒的時間戳
     */
    private long waitNextMillis(long timestamp) {
        long nextTimestamp = System.currentTimeMillis();
        while (nextTimestamp <= timestamp) {
            nextTimestamp = System.currentTimeMillis();
        }
        return nextTimestamp;
    }

    // 示例
    public static void main(String[] args) {
        Snowflake snowflake = new Snowflake(1, 1);
        System.out.println(snowflake.nextId());
    }
}