package twiter.snowflake;

/**
 * twitter的snowflake演算法 -- java實現
 */
public class SnowFlake {

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

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

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

    /**
     * 每一部分向左的位移
     */
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private long datacenterId;
    private long machineId;
    /**
     * 序列號
     */
    private long sequence = 0L;
    /**
     * 上一次時間戳
     */
    private long lastStmp = -1L;

    /**
     * 構造器
     *
     * @param datacenterId 資料中心id (32之內,0~31)
     * @param machineId    機器標識id (32之內,0~31)
     */
    public SnowFlake(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_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");
        }
        /*if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            datacenterId = datacenterId & (MAX_DATACENTER_NUM - 1);
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            machineId = machineId & (MAX_MACHINE_NUM - 1);
        }*/
        this.datacenterId = datacenterId;
        this.machineId = machineId;
        this.lastStmp = System.currentTimeMillis();
    }

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

        if (currStmp == lastStmp) {
            //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) {
                currStmp = getNextMill();
            }
        } else {
            //不同毫秒內，序列號置為0
            sequence = 0L;
        }

        lastStmp = currStmp;

        return (currStmp - START_STMP) << TIMESTMP_LEFT //時間戳部分
                | datacenterId << DATACENTER_LEFT       //資料中心部分
                | machineId << MACHINE_LEFT             //機器標識部分
                | sequence;                             //序列號部分
    }

    private long getNextMill() {
        long mill = getNewstmp();
        while (mill <= lastStmp) {
            mill = getNewstmp();
        }
        return mill;
    }

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

    public static void main(String[] args) {


        /**
         *  datacenterId 資料中心id
         *  machineId    機器標識id
         */
        long datacenterId = 1L;
        long machineId = 4L;
        SnowFlake snowFlake = new SnowFlake(datacenterId, machineId);

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

    }
}

 

演算法思想