Twitter-Snowflake生成

snowflake，64位自增ID算法：

第一位为0（不用），41位时间戳，10位工作机器id，12位序列号组合在一起

时间戳生成：

uint64_t generateStamp()
{
timeval tv;
gettimeofday(&tv, 0);
return (uint64_t)tv.tv_sec * 1000 + (uint64_t)tv.tv_usec / 1000;
}

序列号生成：

uint64_t waitNextMs( uint64_t lastStamp){ 

  uint64_t cur = 0; do { 

 cur = generateStamp(); 

 } while (cur <= lastStamp); return cur;

}

算法实现：

public class IdWorker {
    private final long twepoch = 1288834974657L;
    // 机器标识位数
    private final long workerIdBits = 5L;
    // 数据中心标识位数
    private final long datacenterIdBits = 5L;
    // 机器ID最大值 31
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
    // 数据中心ID最大值 31
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    // 毫秒内自增位
    private final long sequenceBits = 12L;
    // 机器ID偏左移12位
    private final long workerIdShift = sequenceBits;
    private final long datacenterIdShift = sequenceBits + workerIdBits;
    // 时间毫秒左移22位
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;

    public IdWorker(long workerId, long datacenterId) {
        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;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        if (lastTimestamp == timestamp) {
            // 当前毫秒内，则+1
            sequence = (sequence + 1) & sequenceMask;
            // 当前毫秒内计数满了，则等待下一秒
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;
        // ID偏移组合生成最终的ID，并返回ID
        return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
    }

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

    protected long timeGen() {
        return System.currentTimeMillis();
    }
}