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package com.dahuatech.hutool.core.lang;

import com.dahuatech.hutool.core.date.SystemClock;
import com.dahuatech.hutool.core.util.StrUtil;

import java.io.Serializable;

/**
 * Twitter的Snowflake 算法

 * 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。
 *
 * snowflake的结构如下(每部分用-分开):

 *
 * 
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000
 * 
 *
 * 第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)

 * 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）

 * 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）
 *
 * 并且可以通过生成的id反推出生成时间,datacenterId和workerId
 *
 * 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;

  // Thu, 04 Nov 2010 01:42:54 GMT
  private final long twepoch = 1288834974657L;
  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
  private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
  // 最大支持数据中心节点数0~31，一共32个
  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;
  private final long sequenceMask = -1L ^ (-1L << sequenceBits); // 4095

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

  /**
   * 构造
   *
   * @param workerId 终端ID
   * @param datacenterId 数据中心ID
   */
  public Snowflake(long workerId, long datacenterId) {
    this(workerId, datacenterId, false);
  }

  /**
   * 构造
   *
   * @param workerId 终端ID
   * @param datacenterId 数据中心ID
   * @param isUseSystemClock 是否使用{@link SystemClock} 获取当前时间戳
   */
  public Snowflake(long workerId, long datacenterId, boolean isUseSystemClock) {
    if (workerId > maxWorkerId || workerId < 0) {
      throw new IllegalArgumentException(
          StrUtil.format("worker Id can't be greater than {} or less than 0", maxWorkerId));
    }
    if (datacenterId > maxDatacenterId || datacenterId < 0) {
      throw new IllegalArgumentException(
          StrUtil.format("datacenter Id can't be greater than {} or less than 0", maxDatacenterId));
    }
    this.workerId = workerId;
    this.datacenterId = datacenterId;
    this.useSystemClock = isUseSystemClock;
  }

  /**
   * 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) {
      // 如果服务器时间有问题(时钟后退) 报错。
      throw new IllegalStateException(
          StrUtil.format(
              "Clock moved backwards. Refusing to generate id for {}ms",
              lastTimestamp - timestamp));
    }
    if (lastTimestamp == timestamp) {
      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();
    while (timestamp <= lastTimestamp) {
      timestamp = genTime();
    }
    return timestamp;
  }

  /**
   * 生成时间戳
   *
   * @return 时间戳
   */
  private long genTime() {
    return this.useSystemClock ? SystemClock.now() : System.currentTimeMillis();
  }
  // ------------------------------------------------------------------------------------------------------------------------------------ Private method end
}