Diffing_Engine: review the structure/organisation of the identifiers

[alelom]

Category
Persistent	The id is a GUID generated upon object creation. If the object is modified, this id is transferred to the new instance of the object.
Instance	if any property of the object changes, the id changes.

Name	Type (Persistent or instance)	Example	Description
External Id (or "Adapter Id")	(Depends on software)	A number	The id assigned to the object by the external software. This may or may not be unique per se (e.g. could be unique within a certain category, but not in absolute sense). Example: in robot, a bar is assigned a progressive number starting from 1; nodes too.
Instance External Id	Instance	a guid or hash	An id assigned to the object by the external software that is unique for that object and its defining properties. The object, if modified, will get a different id of this type. Example: in Robot, this can be obtained by the combination of the object Type (e.g. Robot.Bar) with its External Id (e.g. `1).
Persistent External Id	Persistent	a guid	An id assigned to the object by the external software that is unique for that object. Persistent = remains the same if the object is modified; it is generated only once upon object creation. Example: in Revit, the UniqueId.
BHoM Guid	Persistent	a guid	The BHoM Guid is generated once upon object instantiation. If the object is modified, the Guid is transferred to the new object.
BHoM Hash	Instance	a hash	An id that uniquely identifies an object with all its properties set to a certain value. If any property is modified, this id changes.

[alelom]

@al-fisher

[al-fisher]

Thanks @alelom - looks like a good summary - and consistent with current expected use of BHoM Guid and Hash.

From descriptions - new external IDs are clear except I think your example for Robot.Bar (where combining Type and Number is I believe a Persistent ID not an Instance ID
i.e. if you change the cross-section is Robot - the example ID given above would not change (would need to be a hash encompassing Section itself)

Would also be good to include notes on implementation of each of the above.
That is External IDs being Fragments I presume.
BHoM Persistent Guid naturally being a Property on the parent defining object
And BHoM Instance ID or Hash being a lazily computed Query Method.

[alelom]

• "Instance" --> "State" or "Version". The naming is to be revised - "state" could be confusing (@adecler) - "version" has to many implications (@alelom @al-fisher ) -- just use hash (@adecler , @alelom, @al-fisher )
• DiffingHash --> excludes some properties by default // Hash --> includes everything