using System;

namespace ZC
{
    /// <summary>
    /// id生成器，当前使用的是时间戳作为id生成器的唯一id
    /// </summary>
    public static class IDGenerator
    {
        // 
        private static long lastTimestamp = -1L;
        private static long sequence = 0L;
        private static readonly long twepoch = 1288834974657L; // Twitter Snowflake算法中的自定义起始时间
        private static readonly long machineId = 1L; // 机器码，可以自定义

        public static long Generate()
        {
            long timestamp = TimeGen();

            if (lastTimestamp == timestamp)
            {
                sequence = (sequence + 1) & 4095L;
                if (sequence == 0)
                {
                    timestamp = TilNextMillis(lastTimestamp);
                }
            }
            else
            {
                sequence = 0L;
            }

            lastTimestamp = timestamp;

            return (timestamp - twepoch) << 22 | machineId << 12 | sequence;
        }

        private static long TilNextMillis(long lastTimestamp)
        {
            long timestamp = TimeGen();
            while (timestamp <= lastTimestamp)
            {
                timestamp = TimeGen();
            }

            return timestamp;
        }

        private static long TimeGen()
        {
            return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
        }
    }
}