using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SomeNameSpace
{
  class CombGuid
  {

    /*
     * "COMB" Guids are a little better then standard .Net GUID's
     * for use as primary keys.  The are generated in sequence,
     * which is better for indexing and scanning.  This is similar to
     * the NewSequentialID() functionality in SQL 2005+, but Comb Guid's
     * can be generated outside of the SQL database and passed in as
     * a parameter, so you know in code what your primary key
     * is without having to pull it from the database after insert.
     */


    #region CombGuid Explaination
    //http://www.ideablade.com/techtip_improve_your_guid.htm

    /*********************************
     *
    /// <summary>Create a new Guid.Comb.</summary>
    /// <remarks>
    /// Version by jtmueller, Jul 9, 2002.
    /// <![CDATA[
    /// http://www.informit.com/discussion/index.asp?postid=a8275a70-0698-46f0-8c8f-bf687464628c&rl=1
    /// ]]>
    /// The author wonders:
    /// "One issue is that while the resolution for a DATETIME in SQL Server is 1/300th of a second,
    /// the resolution for a DateTime in .NET is 1/10000th of a second (100 nanoseconds).
    /// This means that the sequential part of the COMB will change more frequently
    /// than in the SQL Server version, but I'm not sure if this is a good or a bad thing."
    /// On the chance that this is a problem, we've gone with the version below.
    /// </remarks>
    public static Guid NewComb() {
      byte[] dateBytes = BitConverter.GetBytes(DateTime.UtcNow.Ticks);
      byte[] guidBytes = Guid.NewGuid().ToByteArray();
      // copy the last six bytes from the date to the last six bytes of the GUID
      Array.Copy(dateBytes, dateBytes.Length - 7, guidBytes, guidBytes.Length - 7, 6);
      return new Guid(guidBytes);
    }
    *
    *********************************/

    #endregion

    /// <summary>Create a new Guid.Comb.</summary>
    /// <remarks>
    /// Version by glapointe, Aug 16, 2002
    /// <![CDATA[
    /// http://www.informit.com/discussion/index.asp?postid=a8275a70-0698-46f0-8c8f-bf687464628c&rl=1
    /// ]]>
    /// This one purports to "match exactly (or pretty darn close) the combs created in SQL Server."
    /// However, I modified slightly to use UtcNow and static BaseDate values.
    /// UtcNow gives cross-timezone ordering of records inserted with Guid.Comb keys.
    /// </remarks>
    public static Guid NewComb() {
      byte[] guidArray = System.Guid.NewGuid().ToByteArray();

      DateTime now = DateTime.UtcNow; // was: DateTime now = DateTime.Now;

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

      // Convert days and msecs to byte arrays
      // SQL Server is accurate to 1/300th of a millisecond
      // .NET DateTime ticks are in milliseconds
      // so we divide .NET ticks 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 System.Guid(guidArray);
    }

    /// <summary>Extract datetime part of a Guid.Comb.</summary>
    /// <remarks>
    /// Please note: Because I modified <see cref="NewComb"/> to use Utc datetime,
    /// this function returns the date in Universal Time;
    /// you may want to convert back to local time with
    /// <code>GetDateFromComb(aGuidComb).ToLocalTime();</code>
    /// </remarks>
    public static DateTime GetDateFromComb(System.Guid guid) {
      byte[] daysArray = new byte[4];
      byte[] msecsArray = new byte[4];
      byte[] guidArray = guid.ToByteArray();

      // Copy the date parts of the guid to the respective byte arrays.
      Array.Copy(guidArray, guidArray.Length - 6, daysArray, 2, 2);
      Array.Copy(guidArray, guidArray.Length - 4, msecsArray, 0, 4);

      // Reverse the arrays to put them into the appropriate order
      Array.Reverse(daysArray);
      Array.Reverse(msecsArray);

      // Convert the bytes to ints
      int days = BitConverter.ToInt32(daysArray, 0);
      int msecs = BitConverter.ToInt32(msecsArray, 0);

      DateTime date = msBaseDate.AddDays(days);
      date = date.AddMilliseconds(msecs * 3.333333);

      return date;
    }
    private static DateTime msBaseDate = new DateTime(1900, 1, 1);
    private static long msBaseDateTicks = msBaseDate.Ticks;

  }
}