Special-ness of root set: Impl. vs. data model

[Mark Beall]

It certainly seems that the root set is special in some ways. I guess  
the question is, in which ways is it special?

For example, calling iMesh_addEntToSet or iMesh_rmvEntFromSet on the  
root set doesn't make sense and should probably be an error, right?

So, clearly there are case where the root set is specified to behave  
differently, but at the current time I doubt it's clear all the cases  
that it should.

mark

On Aug 27, 2010, at 11:45 AM, Miller, Mark C. wrote:

> Hi Folks,
>
> I am gaining some experience with the implementations that suggests  
> that root set is
> treated very differently than other sets. While I understand there  
> are probably good
> reasons from an implementation standpoint to do this, consider a  
> very simple tool like
> eyeMesh browser. What that tool basically does is traverse the set  
> containment
> hierarchy (the hierarchy defined by getEntSets() and then descending  
> into each of those
> sets). As eyeMesh traverses the hierarchy it gathers all the  
> information it can regarding
> each set; getNumTopo, getNumOfTopoType, isList, getNumPrnt,  
> getNumChld, etc. eyeMesh
> then presents this data in a textual oriented gui that allows you to  
> browse what is there.
>
> It is coded using a simple recursion starting from the root set.  
> Because of that, it queries
> everything it ordinarily would, even on root set; isList, getNumPrnt  
> for example. These calls
> have shown to be problematic on a few implementations as no one  
> expected to have to
> implement them with the possibility that the given set might be the  
> root set.
>
> In further investigating, I've learned that, for example,  
> addPrntChld is not necessarily
> expected to work with root set as well there might be other calls in  
> iMesh that are
> problematic to support on the root set.
>
> But, these are all impelementation issues and are not part of the  
> data model. From the
> point of view of the data model, the only thing that makes the root  
> set 'special' is that
> all other things are contained within it. It is effectively the  
> 'universe' set. It cannot have
> 'parents' or be 'contained in' another set.
>
> So, in my view, with my data model hat on and working only from the  
> iMesh.h header
> file (not from an implementation), I think that the root set ought  
> to behave 'just like any
> other set' and that there should be no implementation-driven  
> restrictions upon its use
> in the API other than those that can be derived directly from the  
> data model. In particular,
> implementations should be coded to operate 'reasonably' in all cases  
> where an entity set
> handle is required and caller passes root set for such. If the call  
> does not make any sense
> in the data model, then it is ok for the call to error out. However,  
> if there is no legitimate
> argument that the call does NOT make sense from the perspective of  
> the data model
> when 'entity set handle' is replaced with 'root set', then the call  
> should return normally
> and with reasonable results.
>
> If this is not consistent with others understanding of the data  
> model and implementations,
> please speak up loudly now so we can identify all the issues.
>
>
> Mark
>

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