public class IdWorker {

   private long workerId; // 這個就是代表了機器id
   private long datacenterId; // 這個就是代表了機房id
   private long sequence; // 這個就是代表了一毫秒內生成的多個id的最新序號

   public IdWorker(long workerId, long datacenterId, long sequence){

       // sanity check for workerId
       // 這兒不就檢查了一下，要求就是你傳遞進來的機房id和機器id不能超過32，不能小于0
       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));
       }

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

   private long twepoch = 1288834974657L;

   private long workerIdBits = 5L;
   private long datacenterIdBits = 5L;
   
   // 這個是二進制運算，就是5 bit最多只能有31個數字，也就是說機器id最多只能是32以內
   private long maxWorkerId = -1L ^ (-1L << workerIdBits);

   // 這個是一個意思，就是5 bit最多只能有31個數字，機房id最多只能是32以內
   private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
   private long sequenceBits = 12L;

   private long workerIdShift = sequenceBits;
   private long datacenterIdShift = sequenceBits + workerIdBits;
   private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
   private long sequenceMask = -1L ^ (-1L << sequenceBits);

   private long lastTimestamp = -1L;

   public long getWorkerId(){
       return workerId;
   }

   public long getDatacenterId(){
       return datacenterId;
   }

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

   // 這個是核心方法，通過調用nextId()方法，讓當前這臺機器上的snowflake算法程序生成一個全局唯一的id
   public synchronized long nextId(){

       // 這兒就是獲取當前時間戳，單位是毫秒
       long timestamp = timeGen();

       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));
       }      
       // 下面是說假設在同一個毫秒內，又發送了一個請求生成一個id
       // 這個時候就得把seqence序號給遞增1，最多就是4096
       if (lastTimestamp == timestamp) {
       
           // 這個意思是說一個毫秒內最多只能有4096個數字，無論你傳遞多少進來，
           //這個位運算保證始終就是在4096這個范圍內，避免你自己傳遞個sequence超過了4096這個范圍
           sequence = (sequence + 1) & sequenceMask;

           if (sequence == 0) {
               timestamp = tilNextMillis(lastTimestamp);
           }
       
       } else {
           sequence = 0;
       }

       // 這兒記錄一下最近一次生成id的時間戳，單位是毫秒
       lastTimestamp = timestamp;

       // 這兒就是最核心的二進制位運算操作，生成一個64bit的id
       // 先將當前時間戳左移，放到41 bit那兒；將機房id左移放到5 bit那兒；將機器id左移放到5 bit那兒；將序號放最后12 bit
       // 最后拼接起來成一個64 bit的二進制數字，轉換成10進制就是個long型
       return ((timestamp - twepoch) << timestampLeftShift) |
               (datacenterId << datacenterIdShift) |
               (workerId << workerIdShift) | sequence;
   }

   private long tilNextMillis(long lastTimestamp){
       
       long timestamp = timeGen();
       
       while (timestamp <= lastTimestamp) {
           timestamp = timeGen();
       }
       return timestamp;
   }

   private long timeGen(){
       return System.currentTimeMillis();
   }
   //---------------測試---------------
   public static void main(String[] args){
       
       IdWorker worker = new IdWorker(1,1,1);
       
       for (int i = 0; i < 30; i++) {
           System.out.println(worker.nextId());
       }
   }
}