記錄下 java版本的雪花算法

/**
 * Twitter的SnowFlake算法,使用SnowFlake算法生成一個整數，然後轉化爲62進制變成一個短地址URL
 *
 * https://github.com/beyondfengyu/SnowFlake
 */
public class SnowFlakeShortUrl {

    /**
     * 起始的時間戳
     */
    private final static long START_TIMESTAMP = 1480166465631L;

    /**
     * 每一部分佔用的位數
     */
    private final static long SEQUENCE_BIT = 12;   //序列號佔用的位數
    private final static long MACHINE_BIT = 5;     //機器標識佔用的位數
    private final static long DATA_CENTER_BIT = 5; //數據中心佔用的位數

    /**
     * 每一部分的最大值
     */
    private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
    private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
    private final static long MAX_DATA_CENTER_NUM = -1L ^ (-1L << DATA_CENTER_BIT);

    /**
     * 每一部分向左的位移
     */
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATA_CENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATA_CENTER_LEFT + DATA_CENTER_BIT;

    private long dataCenterId;  //數據中心
    private long machineId;     //機器標識
    private long sequence = 0L; //序列號
    private long lastTimeStamp = -1L;  //上一次時間戳

    private long getNextMill() {
        long mill = getNewTimeStamp();
        while (mill <= lastTimeStamp) {
            mill = getNewTimeStamp();
        }
        return mill;
    }

    private long getNewTimeStamp() {
        return System.currentTimeMillis();
    }

    /**
     * 根據指定的數據中心ID和機器標誌ID生成指定的序列號
     *
     * @param dataCenterId 數據中心ID
     * @param machineId    機器標誌ID
     */
    public SnowFlakeShortUrl(long dataCenterId, long machineId) {
        if (dataCenterId > MAX_DATA_CENTER_NUM || dataCenterId < 0) {
            throw new IllegalArgumentException("DtaCenterId can't be greater than MAX_DATA_CENTER_NUM or less than 0！");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("MachineId can't be greater than MAX_MACHINE_NUM or less than 0！");
        }
        this.dataCenterId = dataCenterId;
        this.machineId = machineId;
    }

    /**
     * 產生下一個ID
     *
     * @return
     */
    public synchronized long nextId() {
        long currTimeStamp = getNewTimeStamp();
        if (currTimeStamp < lastTimeStamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (currTimeStamp == lastTimeStamp) {
            //相同毫秒內，序列號自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列數已經達到最大
            if (sequence == 0L) {
                currTimeStamp = getNextMill();
            }
        } else {
            //不同毫秒內，序列號置爲0
            sequence = 0L;
        }

        lastTimeStamp = currTimeStamp;

        return (currTimeStamp - START_TIMESTAMP) << TIMESTAMP_LEFT //時間戳部分
                | dataCenterId << DATA_CENTER_LEFT       //數據中心部分
                | machineId << MACHINE_LEFT             //機器標識部分
                | sequence;                             //序列號部分
    }

    public static void main(String[] args) {
        SnowFlakeShortUrl snowFlake = new SnowFlakeShortUrl(2, 3);

        for (int i = 0; i < (1 << 4); i++) {
            //10進制
            System.out.println(snowFlake.nextId());
        }
    }
}