/**
     * Total 64 bits
     */
    public static final int TOTAL_BITS = 1 << 6;

    /**
     * Bits for [sign-> second-> workId-> sequence]
     */
    private int signBits = 1;
    private final int timestampBits;
    private final int workerIdBits;
    private final int sequenceBits;

    /**
     * Max value for workId & sequence
     */
    private final long maxDeltaSeconds;
    private final long maxWorkerId;
    private final long maxSequence;

    /**
     * Shift for timestamp & workerId
     */
    /**
     * timestamp需要位移多少位
     */
    private final int timestampShift;
    /**
     * workerId需要位移多少位
     */
    private final int workerIdShift;

    /**
     * Constructor with timestampBits, workerIdBits, sequenceBits<br>
     * The highest bit used for sign, so <code>63</code> bits for timestampBits, workerIdBits, sequenceBits
     */
    public BitsAllocator(int timestampBits, int workerIdBits, int sequenceBits) {
        // make sure allocated 64 bits
        int allocateTotalBits = signBits + timestampBits + workerIdBits + sequenceBits;
        Assert.isTrue(allocateTotalBits == TOTAL_BITS, "allocate not enough 64 bits");

        // initialize bits
        this.timestampBits = timestampBits;
        this.workerIdBits = workerIdBits;
        this.sequenceBits = sequenceBits;

        // initialize max value
        //先将-1左移timestampBits位，得到-10000...（timestampBits-1个零)
        //然后取反，得到1111（timestampBits-1）个1
        //等价于2的timestampBits次方-1
        this.maxDeltaSeconds = ~(-1L << timestampBits);
        this.maxWorkerId = ~(-1L << workerIdBits);
        this.maxSequence = ~(-1L << sequenceBits);

        // initialize shift
        this.timestampShift = workerIdBits + sequenceBits;
        this.workerIdShift = sequenceBits;
    }

    /**
     * Allocate bits for UID according to delta seconds & workerId & sequence<br>
     * <b>Note that: </b>The highest bit will always be 0 for sign
     *
     * 这里就是把不同的字段放到相应的位上
     * id的总体结构是：
     * sign (fixed 1bit) -> deltaSecond -> workerId -> sequence(within the same second)
     * deltaSecond 左移（workerIdBits + sequenceBits）位，workerId左移sequenceBits位，此时就完成了字节的分配
     * @param deltaSeconds
     * @param workerId
     * @param sequence
     * @return
     */
    public long allocate(long deltaSeconds, long workerId, long sequence) {
        return (deltaSeconds << timestampShift) | (workerId << workerIdShift) | sequence;
    }

@Value("${snowflake.timeBits}")
    protected int timeBits = 28;

    @Value("${snowflake.workerBits}")
    protected int workerBits = 22;

    @Value("${snowflake.seqBits}")
    protected int seqBits = 13;

    @Value("${snowflake.epochStr}")
    /** Customer epoch, unit as second. For example 2016-05-20 (ms: 1463673600000)*/
    protected String epochStr = "2016-05-20";
    protected long epochSeconds = TimeUnit.MILLISECONDS.toSeconds(1463673600000L);

    @Autowired
    @Qualifier(value = "dbWorkerIdAssigner")
    protected WorkerIdAssigner workerIdAssigner;

    /** Stable fields after spring bean initializing */
    protected BitsAllocator bitsAllocator;


    protected long workerId;


    /** Volatile fields caused by nextId() */
    protected long sequence = 0L;
    protected long lastSecond = -1L;


    @PostConstruct
    public void afterPropertiesSet() throws Exception {
        bitsAllocator = new BitsAllocator(timeBits,workerBits,seqBits);
        // initialize worker id
        workerId = workerIdAssigner.assignWorkerId();

        if(workerId > bitsAllocator.getMaxWorkerId()){
            throw new RuntimeException("Worker id " + workerId + " exceeds the max " + bitsAllocator.getMaxWorkerId());
        }

        if (StringUtils.isNotBlank(epochStr)) {
            this.epochSeconds = TimeUnit.MILLISECONDS.toSeconds(DateUtils.parseByDayPattern(epochStr).getTime());
        }

        log.info("Initialized bits(1, {}, {}, {}) for workerID:{}", timeBits, workerBits, seqBits, workerId);
    }

/**
     * Get UID
     *
     * @return UID
     * @throws UidGenerateException in the case: Clock moved backwards; Exceeds the max timestamp
     */
    protected synchronized long nextId() {
        long currentSecond = getCurrentSecond();

        // Clock moved backwards, refuse to generate uid
        //todo 时钟回拨问题待解决
        if (currentSecond < lastSecond) {
            long refusedSeconds = lastSecond - currentSecond;
            throw new UidGenerateException("Clock moved backwards. Refusing for %d seconds", refusedSeconds);
        }

        // At the same second, increase sequence
        //同一秒内的，seq加一
        if (currentSecond == lastSecond) {
            //seq 加一，如果大于MaxSequence，就变成0
            //如果大于MaxSequence 就是seq能取到的最大值，二进制（seqBits -1）位全是1
            sequence = (sequence + 1) & bitsAllocator.getMaxSequence();
            // Exceed the max sequence, we wait the next second to generate uid
            //号发完了，等到下一秒
            if (sequence == 0) {
                currentSecond = getNextSecond(lastSecond);
            }

            // At the different second, sequence restart from zero
        } else {
            //新的一秒，重新开始发号
            sequence = 0L;
        }
        lastSecond = currentSecond;
        // Allocate bits for UID
        return bitsAllocator.allocate(currentSecond - epochSeconds, workerId, sequence);
    }