[FLINK-2714] [Gelly] Copy triangle counting logic from EnumTrianglesOpt.java to Gelly library.

[ssaumitra]

Also reorganizing classes to use Gelly's Graph APIs.

[ssaumitra]

Just FYI, I am referring to vertices of Triad and edgesWithDegrees as V1 and V2. And vertices of edge as 0 and 1.
IMHO, org.apache.flink.graph.Edge can also have static variables V1, V2 so that group operators are more readable.

[vasia]

Hi @ssaumitra! Thanks a lot for the pull request, it looks good.
I was thinking, since this algorithm not only counts but actually enumerates all triangles, we could have 2 library methods, TriangleCounter and TriangleEnumerator. They would basically use the same algorithm internally, but the first would return the number of triangles, while the second would return a DataSet of Tuple3, containing the vertex IDs that form triangles. What do you think?

[ssaumitra]

It's good idea. In my opinion, if we provide TriangleEnumerator as a library function, TriangleCounter would be trivial extension to it (map 1, reduce a+b). So would it be required to add it in library?
Also if we want to expose triangle enumerator, I think we need to make Triad as a part of library too (right now it's private class)

[vasia]

Yes @ssaumitra, if we have the enumerator, then it's a matter of calling a count to get the number of triangles. So, let's just got for TriangleEnumerator in the library. As for exposing the Triad, there's not need to do so. Triad actually extends Tuple3, so I would say to simply return DataSet<Tuple3>.

[ssaumitra]

Fair enough. I will make this change in my pull request then. Can you please edit the JIRA too? So that updated pull request and JIRA title stay relevant?

[vasia]

Sure, will do! Thank you!

[ssaumitra]

I have converted it to TriangleEnumerator now.

[vasia]

unused import

[ssaumitra]

My bad, I was supposed to optimise them,

[vasia]

Hi @ssaumitra,
thanks a lot for updating the PR! I have left a few minor comments that should be easy to fix.

[ssaumitra]

@vasia I have updated it. Please have a look. Also should I be worried about Travis build failure? Looks like it's failing on master too,

[vasia]

Thanks a lot for the quick update @ssaumitra! It looks good now :)

[vasia]

I will add a description in the docs, squash the commits, and merge this.