What about IFC64 GUID?

Question

I'm writing a C# application, where objects are to be identified by their Guid. According to the customer, there should be two types:

• Standard GUID (128-bit, 36 characters)
• IFC64 GUID (132-bit, 22 characters)

I believe that the System.Guid corresponds with the first one: System.Guid consists of one int (32-bit), two shorts (each 16-bit) and eight bytes (each 8 bits), which comes together to 32 + 2*16 + 8*8 = 128 bits.

But as far as the so-called IFC64 GUID, I don't find anything.

Does anybody know in which C# library this is defined?

Thanks in advance

Answer 1

The IFC GUIDs are 128 bit long (not 132 bit!), same as other UUIDs. When represented in text, IFC GUIDs are encoded using radix 64 and a 64-character alphabet (see below). This is used similar to the standard hexadecimal representation, but omits the dashes and uses a larger base, such that it is more compact than hexadecimal representation.

Note that contrary to the Base64 encoding method for arbitrary binary data to text aligned left and with right-side padding, the IFC radix 64 encoding is producing a base 64 number aligned right with left-side padding. Thus, since every digit is 6 bits and 128 has a reminder of 2 when divided by 6, the first (most significant) digit is always 0, 1, 2 or 3.

Here is some C# code to illustrate:

String charset = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$";
var hexUuid = Guid.NewGuid().ToString().Replace("-", "");
Console.WriteLine("Hex: " + hexUuid);
var numUuid = BigInteger.Parse("0" + hexUuid, NumberStyles.AllowHexSpecifier);
Console.WriteLine("Dec: " + numUuid);
var ifcUuid = Enumerable.Range(0,22).Select(i => charset[(int)((numUuid >> (i*6)) & 63)]).Reverse(); 
Console.WriteLine("IFC: " + string.Join("", ifcUuid));

Answer 2

You are looking for IfcOpenShell: https://github.com/IfcOpenShell/IfcOpenShell

Good implementation of expander-compressor for IFC GIDs here: https://github.com/IfcOpenShell/IfcOpenShell/blob/v0.7.0/src/ifcopenshell-python/ifcopenshell/guid.py

Answer 3

Xbim Toolkit has C# helper methods to translate between .net Guids and Ifc's encoded Guid. See https://github.com/xBimTeam/XbimEssentials/blob/master/Xbim.Ifc4/UtilityResource/IfcGloballyUniqueIdPartial.cs