Convert GUID to ObjectID

ObjectIdGuidExtensions.cs

public static class ObjectIdGuidExtensions
{
    private const byte TimeBasedGuidVersion = 1;
    private static readonly DateTimeOffset UuidEpoch = new(1582, 10, 15, 0, 0, 0, TimeSpan.Zero);

    public static Guid ToGuid(this string objId)
    {
        var oIdSpan = new ReadOnlySpan<byte>(Convert.FromHexString(objId));

        var timestamp = BinaryPrimitives.ReadUInt32BigEndian(oIdSpan[..4]);
        var guidTimestamp = (ulong)(DateTimeOffset.FromUnixTimeSeconds(timestamp) - UuidEpoch).Ticks + oIdSpan[9];
        var guidTimestampAndVersion = guidTimestamp + ((ulong)TimeBasedGuidVersion << 60);

        var guidBytes = new byte[16];
        var guidSpan = guidBytes.AsSpan();

        BinaryPrimitives.WriteUInt64LittleEndian(guidSpan[..8], guidTimestampAndVersion);
        guidSpan[8] = 1 << 7;
        oIdSpan[10..12].CopyTo(guidSpan[9..11]);
        oIdSpan[4..9].CopyTo(guidSpan[11..16]);

        return new Guid(guidBytes);
    }

    public static string ToObjectId(this Guid g)
    {
        var guidSpan = new ReadOnlySpan<byte>(g.ToByteArray());

        if (guidSpan[7] >> 4 != TimeBasedGuidVersion)
        {
            throw new ArgumentOutOfRangeException(nameof(g), "Поддерживаются только GUID версии 1.");
        }

        var guidTimestampAndVersion = BinaryPrimitives.ReadUInt64LittleEndian(guidSpan[..8]);
        var guidTimestamp = guidTimestampAndVersion - ((ulong)TimeBasedGuidVersion << 60);
        var ninthByte = (byte)(guidTimestamp % 1000);
        var timestamp = (uint)(UuidEpoch + TimeSpan.FromTicks((long)(guidTimestamp - ninthByte))).ToUnixTimeSeconds();

        var oIdBytes = new byte[12];
        var oIdSpan = oIdBytes.AsSpan();

        BinaryPrimitives.WriteUInt32BigEndian(oIdSpan[..4], timestamp);
        guidSpan[11..16].CopyTo(oIdSpan[4..9]);
        oIdSpan[9] = ninthByte;
        guidSpan[9..11].CopyTo(oIdSpan[10..12]);

        return Convert.ToHexString(oIdBytes).ToLower();
    }
}