import java.security.SecureRandom;

/**
 * 自定义 ID 生成器
 * ID 生成规则: ID长达 64 bits
 * 
 * | 41 bits: Timestamp (毫秒) | 3 bits: 区域（机房） | 10 bits: 机器编号 | 10 bits: 序列号 |
 */
public class CustomUUID {
    // 基准时间
    private long twepoch = 1288834974657L; //Thu, 04 Nov 2010 01:42:54 GMT
    // 区域标志位数
    private final static long regionIdBits = 3L;
    // 机器标识位数
    private final static long workerIdBits = 10L;
    // 序列号识位数
    private final static long sequenceBits = 10L;

    // 区域标志ID最大值
    private final static long maxRegionId = -1L ^ (-1L << regionIdBits);
    // 机器ID最大值
    private final static long maxWorkerId = -1L ^ (-1L << workerIdBits);
    // 序列号ID最大值
    private final static long sequenceMask = -1L ^ (-1L << sequenceBits);

    // 机器ID偏左移10位
    private final static long workerIdShift = sequenceBits;
    // 业务ID偏左移20位
    private final static long regionIdShift = sequenceBits + workerIdBits;
    // 时间毫秒左移23位
    private final static long timestampLeftShift = sequenceBits + workerIdBits + regionIdBits;

    private static long lastTimestamp = -1L;

    private long sequence = 0L;
    private final long workerId;
    private final long regionId;

    public CustomUUID(long workerId, long regionId) {

        // 如果超出范围就抛出异常
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException("worker Id can‘t be greater than %d or less than 0");
        }
        if (regionId > maxRegionId || regionId < 0) {
            throw new IllegalArgumentException("datacenter Id can‘t be greater than %d or less than 0");
        }

        this.workerId = workerId;
        this.regionId = regionId;
    }

    public CustomUUID(long workerId) {
        // 如果超出范围就抛出异常
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException("worker Id can‘t be greater than %d or less than 0");
        }
        this.workerId = workerId;
        this.regionId = 0;
    }

    public long generate() {
        return this.nextId(false, 0);
    }

    /**
     * 实际产生代码的
     *
     * @param isPadding
     * @param busId
     * @return
     */
    private synchronized long nextId(boolean isPadding, long busId) {

        long timestamp = timeGen();
        long paddingnum = regionId;

        if (isPadding) {
            paddingnum = busId;
        }

        if (timestamp < lastTimestamp) {
            try {
                throw new Exception("Clock moved backwards.  Refusing to generate id for " + (lastTimestamp - timestamp) + " milliseconds");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        //如果上次生成时间和当前时间相同,在同一毫秒内
        if (lastTimestamp == timestamp) {
            //sequence自增，因为sequence只有10bit，所以和sequenceMask相与一下，去掉高位
            sequence = (sequence + 1) & sequenceMask;
            //判断是否溢出,也就是每毫秒内超过1024，当为1024时，与sequenceMask相与，sequence就等于0
            if (sequence == 0) {
                //自旋等待到下一毫秒
                timestamp = tailNextMillis(lastTimestamp);
            }
        } else {
            // 如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加,
            // 为了保证尾数随机性更大一些,最后一位设置一个随机数
            sequence = new SecureRandom().nextInt(10);
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) | (paddingnum << regionIdShift) | (workerId << workerIdShift) | sequence;
    }

    // 防止产生的时间比之前的时间还要小（由于NTP回拨等问题）,保持增量的趋势.
    private long tailNextMillis(final long lastTimestamp) {
        long timestamp = this.timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = this.timeGen();
        }
        return timestamp;
    }

    // 获取当前的时间戳
    protected long timeGen() {
        return System.currentTimeMillis();
    }
}