snow flake, distributed (basicly) time-sortable ID generator in C#. Sleeps to avoid seq exhaustion and time reversal.

Snowflake.cs

using System;
using System.Threading;

namespace Myl7Snowflake
{
    public class Snowflake
    {
        private readonly ushort _machineId;

        public Snowflake(ushort machineId)
        {
            _machineId = (ushort)(machineId & 0x3ff);
        }

        private readonly Mutex _mutex = new Mutex();
        private long _pre = DateTimeOffset.Now.ToUnixTimeMilliseconds();
        private ushort _seq;

        public long Next()
        {
            _mutex.WaitOne();
            var now = DateTimeOffset.Now.ToUnixTimeMilliseconds();
            if (_pre < now)
            {
                _pre = now;
                _seq = 0;
            }
            else
            {
                _seq = (ushort)((_seq + 1) & 0xfff);
                if (_seq == 0)
                {
                    _pre++;
                    if (now < _pre)
                    {
                        Thread.Sleep((int)(_pre - now));
                    }
                }
            }

            var res = Gen();
            _mutex.ReleaseMutex();
            return res;
        }

        private long Gen()
        {
            var h = ((ulong)_pre & 0x1ffffffffff) << 22;
            var m = (ulong)_machineId << 12;
            var l = (ulong)_seq & 0xfff;
            return (long)(h | m | l);
        }
    }
}