using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
public class IdWorker
{
    //機器ID
    private static long workerId;
    private static long twepoch = 687888001020L; //唯一時間，這是一個避免重復的隨機量，自行設定不要大于當前時間戳
    private static long sequence = 0L;
    private static int workerIdBits = 4; //機器碼字節(jié)數(shù)。4個字節(jié)用來保存機器碼(定義為Long類型會出現(xiàn)，最大偏移64位，所以左移64位沒有意義)
    public static long maxWorkerId = -1L ^ -1L << workerIdBits; //最大機器ID
    private static int sequenceBits = 10; //計數(shù)器字節(jié)數(shù)，10個字節(jié)用來保存計數(shù)碼
    private static int workerIdShift = sequenceBits; //機器碼數(shù)據(jù)左移位數(shù)，就是后面計數(shù)器占用的位數(shù)
    private static int timestampLeftShift = sequenceBits + workerIdBits; //時間戳左移動位數(shù)就是機器碼和計數(shù)器總字節(jié)數(shù)
    public static long sequenceMask = -1L ^ -1L << sequenceBits; //一微秒內(nèi)可以產(chǎn)生計數(shù)，如果達到該值則等到下一微妙在進行生成
    private long lastTimestamp = -1L;
 
    /// <summary>
    /// 機器碼
    /// </summary>
    /// <param name="workerId"></param>
    public IdWorker(long workerId)
    {
        if (workerId > maxWorkerId || workerId < 0)
            throw new Exception(string.Format("worker Id can't be greater than {0} or less than 0 ", workerId));
        IdWorker.workerId = workerId;
    }
 
    public long nextId()
    {
        lock (this)
        {
            long timestamp = timeGen();
            if (this.lastTimestamp == timestamp)
            { //同一微妙中生成ID
                IdWorker.sequence = (IdWorker.sequence + 1) & IdWorker.sequenceMask; //用&運算計算該微秒內(nèi)產(chǎn)生的計數(shù)是否已經(jīng)到達上限
                if (IdWorker.sequence == 0)
                {
                    //一微妙內(nèi)產(chǎn)生的ID計數(shù)已達上限，等待下一微妙
                    timestamp = tillNextMillis(this.lastTimestamp);
                }
            }
            else
            { //不同微秒生成ID
                IdWorker.sequence = 0; //計數(shù)清0
            }
            if (timestamp < lastTimestamp)
            { //如果當前時間戳比上一次生成ID時時間戳還小，拋出異常，因為不能保證現(xiàn)在生成的ID之前沒有生成過
                throw new Exception(string.Format("Clock moved backwards.  Refusing to generate id for {0} milliseconds",
                    this.lastTimestamp - timestamp));
            }
            this.lastTimestamp = timestamp; //把當前時間戳保存為最后生成ID的時間戳
            long nextId = (timestamp - twepoch << timestampLeftShift) | IdWorker.workerId << IdWorker.workerIdShift | IdWorker.sequence;
            return nextId;
        }
    }
 
    /// <summary>
    /// 獲取下一微秒時間戳
    /// </summary>
    /// <param name="lastTimestamp"></param>
    /// <returns></returns>
    private long tillNextMillis(long lastTimestamp)
    {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp)
        {
            timestamp = timeGen();
        }
        return timestamp;
    }
 
    /// <summary>
    /// 生成當前時間戳
    /// </summary>
    /// <returns></returns>
    private long timeGen()
    {
        return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
    }
}
 
/// <summary>
/// 生成雪花ID
/// </summary>
public static class SnowFlake
{
    private static long _workerId = 9;
    private static IdWorker _idWorker = null;
 
    public static string NewId()
    {
        if (_idWorker == null)
            _idWorker = new IdWorker(_workerId);
        return _idWorker.nextId().ToString();
    }
}