SnowFlake 雪花演算法

雪花演算法是Twitter推出的⼀個⽤於⽣成分佈式ID的策略。
雪花演算法是⼀個演算法，基於這個演算法可以⽣成ID，⽣成的ID是⼀個long型，那麼在Java中⼀個long
型是8個位元組，算下來是64bit，如下是使⽤雪花演算法⽣成的⼀個ID的⼆進位制形式示意：

/**
 * 官方推出，Scala程式語言來實現的
 * Java前輩用Java語言實現了雪花演算法
 */
public class IdWorker{

    //下面 下麪兩個每個5位，加起來就是10位的工作機器id
    private long workerId;    //工作id
    private long datacenterId;   //數據id
    //12位元的序列號
    private long sequence;

    public IdWorker(long workerId, long datacenterId, long sequence){
        // sanity check for workerId
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0",maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0",maxDatacenterId));
        }
        System.out.printf("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",
                timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId);

        this.workerId = workerId;
        this.datacenterId = datacenterId;
        this.sequence = sequence;
    }

    //初始時間戳
    private long twepoch = 1288834974657L;

    //長度爲5位
    private long workerIdBits = 5L;
    private long datacenterIdBits = 5L;
    //最大值
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    //序列號id長度
    private long sequenceBits = 12L;
    //序列號最大值
    private long sequenceMask = -1L ^ (-1L << sequenceBits);
    
    //工作id需要左移的位數，12位元
    private long workerIdShift = sequenceBits;
   //數據id需要左移位數 12+5=17位
    private long datacenterIdShift = sequenceBits + workerIdBits;
    //時間戳需要左移位數 12+5+5=22位
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    
    //上次時間戳，初始值爲負數
    private long lastTimestamp = -1L;

    public long getWorkerId(){
        return workerId;
    }

    public long getDatacenterId(){
        return datacenterId;
    }

    public long getTimestamp(){
        return System.currentTimeMillis();
    }

     //下一個ID生成演算法
    public synchronized long nextId() {
        long timestamp = timeGen();

        //獲取當前時間戳如果小於上次時間戳，則表示時間戳獲取出現異常
        if (timestamp < lastTimestamp) {
            System.err.printf("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds",
                    lastTimestamp - timestamp));
        }

        //獲取當前時間戳如果等於上次時間戳
        //說明：還處在同一毫秒內，則在序列號加1；否則序列號賦值爲0，從0開始。
        if (lastTimestamp == timestamp) {  // 0  - 4095
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }
        
        //將上次時間戳值重新整理
        lastTimestamp = timestamp;

        /**
          * 返回結果：
          * (timestamp - twepoch) << timestampLeftShift) 表示將時間戳減去初始時間戳，再左移相應位數
          * (datacenterId << datacenterIdShift) 表示將數據id左移相應位數
          * (workerId << workerIdShift) 表示將工作id左移相應位數
          * | 是按位元或運算子，例如：x | y，只有當x，y都爲0的時候結果才爲0，其它情況結果都爲1。
          * 因爲個部分只有相應位上的值有意義，其它位上都是0，所以將各部分的值進行 | 運算就能得到最終拼接好的id
        */
        return ((timestamp - twepoch) << timestampLeftShift) |
                (datacenterId << datacenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    //獲取時間戳，並與上次時間戳比較
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    //獲取系統時間戳
    private long timeGen(){
        return System.currentTimeMillis();
    }




    public static void main(String[] args) {
        IdWorker worker = new IdWorker(21,10,0);
        for (int i = 0; i < 100; i++) {
            System.out.println(worker.nextId());
        }
    }

}