using System.Reflection.PortableExecutable;

namespace ApiDemo
{
    /// <summary>
    /// 雪花算法生成id
    /// </summary>
    public class SnowFlake
    {
        /// <summary>
        /// 机器id所占的位数
        /// </summary>
        private const int workerIdBits = 5;

        /// <summary>
        /// 数据表述id所占位数
        /// </summary>
        private const int datacenterIdBits = 5;

        /// <summary>
        /// 支持的最大机器id，结果是31
        /// </summary>
        private const long maxWorkerId = -1L ^ (-1L << workerIdBits);

        /// <summary>
        /// 支持的最大数据表示id，结果是31
        /// </summary>
        private const long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

        /// <summary>
        /// 序列在id中所占的位数
        /// </summary>
        private const int sequenceBits = 12;

        /// <summary>
        /// 数据标识id向左移17位
        /// </summary>
        private const int datacenterIdShift = sequenceBits + workerIdBits;

        /// <summary>
        /// 机器ID向左移12位
        /// </summary>
        private const int workerIdShift = sequenceBits;

        private const int timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

        /// <summary>
        /// 生成序列的掩码
        /// </summary>
        private const long sequenceMask = -1L ^ (-1L << sequenceBits);

        /// <summary>
        /// 数据中心ID(0-31)
        /// </summary>
        public long DatacenterId { get; private set; }

        /// <summary>
        /// 工作机器ID(0-31)
        /// </summary>
        public long WorkerId { get; private set; }

        /// <summary>
        /// 毫秒内序列(0-4095)
        /// </summary>
        public long Sequence { get; private set; }

        /// <summary>
        /// 上次生成ID的时间戳
        /// </summary>
        public long LastTimestamp { get; private set; }

        /// <summary>
        /// 开始时间戳。首次使用前设置，否则无效，默认2010-1-1
        /// </summary>
        public DateTime StartTimestamp { get; set; } = new DateTime(2010,1,1);

        static object syncRoot = new object();
        static readonly Lazy<SnowFlake> snowflake = new(() => new SnowFlake(0L,0L));

        /// <summary>
        /// 默认金泰实例，WorkerId = 0,DatacenterId = 0
        /// </summary>
        public static SnowFlake Instance { get; } = snowflake.Value;

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="workerId">工作机器ID</param>
        /// <param name="datacenterId">数据中心ID</param>
        /// <exception cref="ArgumentOutOfRangeException"></exception>
        public SnowFlake(long workerId,long datacenterId)
        {
            if (workerId > maxWorkerId || workerId < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(workerId),$"不能大于{maxWorkerId}或小于0");
            }
            if (datacenterId > maxDatacenterId || datacenterId < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(datacenterId), $"不能大于{maxDatacenterId}或小于0");
            }
            WorkerId = workerId;
            DatacenterId = datacenterId;
            Sequence = 0L;
            LastTimestamp = -1L;
        }

        /// <summary>
        /// 获取下一个ID
        /// </summary>
        /// <returns></returns>
        public long NextId()
        {
            lock (syncRoot)
            {
                //获取当前时间戳
                long timestamp = GetCurrentTimestamp();
                if (timestamp > LastTimestamp)//时间戳改变，毫秒内序列重置
                {
                    Sequence = 0L;
                }
                else if (timestamp == LastTimestamp)//如果是同一时间生成的，则进行毫秒内序列
                {
                    Sequence = (Sequence + 1) & sequenceMask;
                    if (Sequence == 0)//毫秒内序列溢出
                    {
                        timestamp = GetNextTimestamp(LastTimestamp);//阻塞到下一个毫秒，获取新的时间戳
                    }
                }
                else//当前时间小于上一次ID生成的时间戳，证明系统时钟被回拨，此时需要做回拨处理
                {
                    Sequence = (Sequence + 1) & sequenceMask;
                    if (Sequence > 0)
                    {
                        timestamp = LastTimestamp;//停留再最后一次时间戳上，等待系统时间追上后即完全度过了时钟回拨问题
                    }
                    else {
                        timestamp = LastTimestamp + 1;//直接进位到下一个毫秒
                    }
                }
                LastTimestamp = timestamp;//上次生成ID的时间戳

                //移动并通过或运算拼到一起组成64位的ID
                var id = (timestamp << timestampLeftShift)
                        | (DatacenterId << datacenterIdShift)
                        | (WorkerId << workerIdShift)
                        | Sequence;
                return id;
            }
        }

        /// <summary>
        /// 阻塞到下一个毫秒，直到获得新的时间戳
        /// </summary>
        /// <param name="lastTimestamp"></param>
        /// <returns></returns>
        private long GetNextTimestamp(long lastTimestamp)
        {
            //获取当前时间戳
            long timestamp = GetCurrentTimestamp();
            while (timestamp <= lastTimestamp)
            { 
                timestamp = GetCurrentTimestamp();
            }
            return timestamp;
        }

        /// <summary>
        /// 获取当前时间戳
        /// </summary>
        /// <returns></returns>
        private long GetCurrentTimestamp()
        {
            return (long)(DateTime.Now - StartTimestamp).TotalMilliseconds;
        }
    }
}