Comb.cs

using System;
using System.Threading;

namespace developmentalmadness
{
    public class Comb
    {
        private static int counter;

        private static long GetTicks()
        {
            int i = Interlocked.Increment(ref counter);
            // use UTC now to prevent conflicts w/ daylight savings
            return DateTime.UtcNow.Ticks + i;
        }

        /// <summary>
        /// Creates a new sequential guid (aka comb) <see cref="http://www.informit.com/articles/article.aspx?p=25862&seqNum=7"/>.
        /// </summary>
        /// <remarks>A comb provides the benefits of a standard Guid w/o the database performance problems.</remarks>
        /// <returns>A new sequential guid (comb).</returns>
        public static Guid NewComb()
        {
            byte[] uid = Guid.NewGuid().ToByteArray();
            byte[] binDate = BitConverter.GetBytes(GetTicks());

            // create comb in SQL Server sort order
            return new Guid(new[]{
            // the first 7 bytes are random - if two combs
            // are generated at the same point in time
            // they are not guaranteed to be sequential.
            // But for every DateTime.Tick there are
            // 72,057,594,037,927,935 unique possibilities so
            // there shouldn't be any collisions                
            uid[0],
            uid[1],
            uid[2],
            uid[3],
            uid[4],
            uid[5],
            uid[6],
 
            // set the first 'nibble of the 7th byte to '1100' so 
            // later we can validate it was generated by us
            (byte)(0xc0 | (0xf & uid[7])),
            
            // the last 8 bytes are sequential,
            // these will reduce index fragmentation
            // to a degree as long as there are not a large
            // number of Combs generated per millisecond                
            binDate[1],
            binDate[0],
            binDate[7],
            binDate[6],
            binDate[5],
            binDate[4],
            binDate[3],
            binDate[2]
        });
        }

        /// <summary>
        /// Validates if comb was generated by this class
        /// </summary>
        /// <remarks>
        /// Guids generated by Guid.NewGuid() have a value of 
        /// 0100 for the first 4 bits of the 7th byte. Ours will
        /// have a value of 1100 for the 6th byte. We're checking that here.
        /// 
        /// We could do additional validation by verifying that
        /// the value of a new Guid is greater than the
        /// one being validated (or that the last 6 bytes
        /// resolve to a valid DateTime), but this should
        /// be enough for now.
        /// </remarks>
        public static bool IsComb(Guid value)
        {
            // get the 7th byte
            byte b = value.ToByteArray()[7];

            // make sure the first 'nibble' == 1100
            return (0xc0 & b) == 0xc0;
        }

        /// <summary>
        /// Validates Guid to determine the supplied
        /// value was generated by Comb.NewComb. If
        /// invalid an ArgumentException is thrown.
        /// </summary>
        /// <param name="value"></param>
        public static void ValidateComb(Guid value)
        {
            if (!Comb.IsComb(value))
                throw new ArgumentException("The supplied Id value was not generated by Comb.NewComb.");
        }
    }
}