On Fri, May 20, 2011 at 1:25 PM, Henry Story <[email protected]> wrote:
> In another thread I wrote:
>
>> think it would be interesting to add a method for getting the graph name,
>> btw, null if there is none
>
>
> Equality
> --------
>
> So as Reto pointed out this could bring up an issue of what the equality
> criterion of two graphs are.
>
> Currently the equality function is inherited from
> java.util.AbstractCollection which just verified that all members internally
> are the same. (Btw, that looks like something one should be careful about:
> calculating identity of large graphs seems a bit dangerous)
No, there are different identity criteria for Graphs and MGraphs, look
at their apis. And yes, graph is isomorphism is NP-hard so for big
graphs with many bnodes it may take forever.
>
> (I think that's why there is a TripleCollection which does not define
> equality.)
>
> So there are a few answers one could have.
> - two graphs are equal if their names are the same
currently a graph has all the names that were ever given to it, that
is any name that was ever given to an isomorphic triple collection
implementing graph.
> - two graphs are equal if their statements are the same
by that you mean isomorphic?
>
> With statements based equality it is already clear that that will depend on
> whether inferencing is enabled or not, and it could change as more
> information is available for inferencing.
>
> I wonder where graph equality is used at all, as it seems like a difficult to
> define matter.
It is defined very precisely and it is used.
>
> Use Cases
> ---------
>
> From a graph be able to find meta data about it easily: who said it, where
> when, is the current graph up to date?
>
> When fetching a graph from TcProvider with methods such as
>
> tc.getGraph("http://xmlns.com/foaf/knows";)
>
> have the returned graph indicate the proper name of the graph where that
> object is defined. That makes it easier to automate spiders.
>
> (not so sure but as a thought: It could allow for a very strict quasi
> identity criterion, that could be easy to calculate:
> two graphs are the same if
> the objects are the same
> or the names are the same and the hashes of all their statements are the same
> )
the names are the same if and only if the graphs are isomorphic,
Reto
>
> Henry
>
> Social Web Architect
> http://bblfish.net/
>
>
