[PPL-devel] Question about PPL: Only one polyhedron-representation

[Enea Zaffanella]

Il 25/10/2010 11:35, Sebastian Junges ha scritto:
> Dear PPL-developers,
> 
> We're currently considering to write a library making use of PPL.
> 
> The main problem is that for method investigations we would like to make
> use of only one representation a time for polyhedra. Is there an
> efficient way of using the existing PPL, in such a manner that we can
> choose whether a constraint- or generator-based representation of
> polyhedra is used.

No, currently there is no provision for such an option.

> We know that most functions are only implemented for only one of the
> representations, and we would add functions needed for the other
> representation ourselves. Thus, as question remains if we can
> efficiently stop the polyhedra from being updated to consistency after a
> changing function is run.

On the one hand, the PPL library does not force a polyhedron to have
both representations available.

On the other hand, most of the supported operations require that a
specific one of the two representations is available as input. Hence, if
that is not the case, the library will automatically perform the
conversion between representations. Moreover, on exit, if keeping both
representations is considered "cheap", then they will be kept. A few
operators can work on either one of the two representations, so they
never require a conversion.

I don't think there currently is a way to easily _and_ efficiently
"drop" one of the two representations. The simple (but inefficient) way
would be to construct a brand new polyhedron using one of the two
representations as input. If generally helpful, we might consider adding
such a "drop" operator ... but I would also like to hear the opinions of
other developers in this respect.

Cheers,
Enea.


> With kind regards,
> Sebastian Junges
> 
> Student Assistant
> RWTH Aachen
> Department of Computer Science
> Hybrid Systems Group
> 
> 
> 
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