package com.codingapi.txlcn.tm.id;


import java.io.Serializable;
import java.util.Date;

/**
 * Twitter的Snowflake 算法<br>
 * 分布式系统中，有一些需要使用全局唯一ID的场景，有些时候我们希望能使用一种简单一些的ID，并且希望ID能够按照时间有序生成。
 *
 * <p>
 * snowflake的结构如下(每部分用-分开):<br>
 *
 * <pre>
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000
 * </pre>
 * <p>
 * 第一位为未使用(符号位表示正数)，接下来的41位为毫秒级时间(41位的长度可以使用69年)<br>
 * 然后是5位dataCenterId和5位workerId(10位的长度最多支持部署1024个节点）<br>
 * 最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）
 * <p>
 * 并且可以通过生成的id反推出生成时间,dataCenterId和workerId
 * <p>
 * 参考：http://www.cnblogs.com/relucent/p/4955340.html
 *
 * @author Looly
 * @since 3.0.1
 */
public class Snowflake implements Serializable {
    private static final long serialVersionUID = 1L;

    @SuppressWarnings("SpellCheckingInspection")
    private final long twepoch;
    private final long workerIdBits = 5L;
    private final long dataCenterIdBits = 5L;
    /**
     * 最大支持机器节点数0~31，一共32个
     */
    @SuppressWarnings({"PointlessBitwiseExpression", "FieldCanBeLocal"})
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

    /**
     * 最大支持数据中心节点数0~31，一共32个
     */
    @SuppressWarnings({"PointlessBitwiseExpression", "FieldCanBeLocal"})
    private final long maxDataCenterId = -1L ^ (-1L << dataCenterIdBits);
    /**
     * 序列号12位
     */
    private final long sequenceBits = 12L;
    /**
     * 机器节点左移12位
     */
    private final long workerIdShift = sequenceBits;
    /**
     * 数据中心节点左移17位
     */
    private final long dataCenterIdShift = sequenceBits + workerIdBits;
    /**
     * 时间毫秒数左移22位
     */
    private final long timestampLeftShift = sequenceBits + workerIdBits + dataCenterIdBits;
    @SuppressWarnings({"PointlessBitwiseExpression", "FieldCanBeLocal"})
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    private final long workerId;
    private final long dataCenterId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;

    /**
     * 构造
     *
     * @param workerId 终端ID
     */
    public Snowflake(long workerId, long dataCenterId) {
        this(null, workerId, dataCenterId);
    }

    /**
     * @param epochDate    初始化时间起点（null表示默认起始日期）,后期修改会导致id重复,如果要修改连workerId dataCenterId，慎用
     * @param workerId     工作机器节点id
     * @param dataCenterId 数据中心id
     * @since 5.1.3
     */
    public Snowflake(Date epochDate, long workerId, long dataCenterId) {
        if (null != epochDate) {
            this.twepoch = epochDate.getTime();
        } else {
            // Thu, 04 Nov 2010 01:42:54 GMT
            this.twepoch = 1288834974657L;
        }
        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("dataCenterId Id can't be greater than %d or less than 0", maxDataCenterId));
        }
        this.workerId = workerId;
        this.dataCenterId = dataCenterId;
    }

    /**
     * 根据Snowflake的ID，获取机器id
     *
     * @param id snowflake算法生成的id
     * @return 所属机器的id
     */
    public long getWorkerId(long id) {
        return id >> workerIdShift & ~(-1L << workerIdBits);
    }

    /**
     * 根据Snowflake的ID，获取数据中心id
     *
     * @param id snowflake算法生成的id
     * @return 所属数据中心
     */
    public long getDataCenterId(long id) {
        return id >> dataCenterIdShift & ~(-1L << dataCenterIdBits);
    }

    /**
     * 根据Snowflake的ID，获取生成时间
     *
     * @param id snowflake算法生成的id
     * @return 生成的时间
     */
    public long getGenerateDateTime(long id) {
        return (id >> timestampLeftShift & ~(-1L << 41L)) + twepoch;
    }

    /**
     * 下一个ID
     *
     * @return ID
     */
    public synchronized long nextId() {
        long timestamp = genTime();
        if (timestamp < lastTimestamp) {
            if (lastTimestamp - timestamp < 2000) {
                // 容忍2秒内的回拨，避免NTP校时造成的异常
                timestamp = lastTimestamp;
            } else {
                // 如果服务器时间有问题(时钟后退) 报错。
                throw new IllegalStateException(String.format("Clock moved backwards. Refusing to generate id for %d ms", lastTimestamp - timestamp));
            }
        }

        if (timestamp == lastTimestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) | (dataCenterId << dataCenterIdShift) | (workerId << workerIdShift) | sequence;
    }

    /**
     * 下一个ID（字符串形式）
     *
     * @return ID 字符串形式
     */
    public String nextIdStr() {
        return Long.toString(nextId());
    }

    // ------------------------------------------------------------------------------------------------------------------------------------ Private method start

    /**
     * 循环等待下一个时间
     *
     * @param lastTimestamp 上次记录的时间
     * @return 下一个时间
     */
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = genTime();
        // 循环直到操作系统时间戳变化
        while (timestamp == lastTimestamp) {
            timestamp = genTime();
        }
        if (timestamp < lastTimestamp) {
            // 如果发现新的时间戳比上次记录的时间戳数值小，说明操作系统时间发生了倒退，报错
            throw new IllegalStateException(
                    String.format("Clock moved backwards. Refusing to generate id for %d ms", lastTimestamp - timestamp));
        }
        return timestamp;
    }

    /**
     * 生成时间戳
     *
     * @return 时间戳
     */
    private long genTime() {
        return System.currentTimeMillis();
    }
    // ------------------------------------------------------------------------------------------------------------------------------------ Private method end
}
