﻿using System;

namespace HelloEsent
{
    public class SequentialGuid
    {
        public static Guid New()
        {
            byte[] guidArray = Guid.NewGuid().ToByteArray();

            DateTime baseDate = new DateTime(1900, 1, 1);
            DateTime now = DateTime.UtcNow;

            // Get the days and milliseconds which will be used to build the byte string 
            TimeSpan days = new TimeSpan(now.Ticks - baseDate.Ticks);
            TimeSpan msecs = new TimeSpan(now.Ticks - (new DateTime(now.Year, now.Month, now.Day).Ticks));

            // Convert to a byte array 
            // Note that SQL Server is accurate to 1/300th of a millisecond so we divide by 3.333333 
            byte[] daysArray = BitConverter.GetBytes(days.Days);
            byte[] msecsArray = BitConverter.GetBytes((long)(msecs.TotalMilliseconds / 3.333333));

            // Reverse the bytes to match SQL Servers ordering 
            Array.Reverse(daysArray);
            Array.Reverse(msecsArray);

            // Copy the bytes into the guid 
            Array.Copy(daysArray, daysArray.Length - 2, guidArray, guidArray.Length - 6, 2);
            Array.Copy(msecsArray, msecsArray.Length - 4, guidArray, guidArray.Length - 4, 4);

            return new Guid(guidArray);
        }
    }
}