Intrinsics.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.X86;

namespace Hellang.CompactGuid
{
    public static class Program
    {
        public static void Main()
        {
            var newGuid = Guid.NewGuid();

            Console.WriteLine(newGuid.ToString());
            Console.WriteLine(newGuid.ToCompactString());
        }
    }

    public static class CompactGuid
    {
        private static readonly string EmptyString = new string('0', Length);

        private const int Length = 26;

        public static string ToCompactString(this Guid value)
        {
            if (value == Guid.Empty)
            {
                return EmptyString;
            }

            if (BitConverter.IsLittleEndian)
            {
                value = value.ToUuid();
            }

            return value.ToString();
        }

        private static Guid ToUuid(this Guid value)
        {
            if (Ssse3.IsSupported)
            {
                var vector = Unsafe.As<Guid, Vector128<byte>>(ref value);

                var mask = Vector128.Create(434320308585955843L, 1084818905618843912L).AsByte();

                var result = Ssse3.Shuffle(vector, mask);

                return Unsafe.As<Vector128<byte>, Guid>(ref result);
            }

            throw new NotSupportedException();
        }
    }
}