Re: [eigen] Specializing max_coeff_visitor for some number types

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Wed, 20 Aug 2014, Christoph Hertzberg wrote:

> On 19.08.2014 15:53, Christoph Hertzberg wrote:
> > On 17.08.2014 19:34, Marc Glisse wrote:
> > > Any comment on this patch?
> > > http://listengine.tuxfamily.org/lists.tuxfamily.org/eigen/2014/06/msg00012.html
> >
> > Hi Marc,
> >
> > sorry I forgot about this thread, so thanks for the reminder.
> >
> > As no further objections arose to your patch, I'll commit it soon.
>
> One thing that is missing is that only LU decompositions are considered by 
> your patch.
> Actually, a clean solution should somehow share the pivoting code for all 
> pivoting decompositions. At least for QR and LU the code is very similar at 
> the moment. Any thoughts?

The codes are similar, but not the same. QR has even more threshold, 
precision, isMuchSmallerThan and stuff. Merging those codes requires a 
better global understanding of what they do than I have (finding the line 
that says "max" and replacing it with "best" is about as far as I looked 
at the pivoting code...).

Introducing the score in QR shouldn't be much harder than in the full LU, 
as long as we basically ignore the effect on biggest_pivot, the case of 
non-zero epsilon, etc.

Maybe I should mention in the doc that when we specialize scores, non-zero 
values of epsilon are not supported and biggest_pivot -type functions may 
give unexpected results. It may be possible to define a sensible 
interaction between scores and non-0 thresholds, but that requires a lot 
more thought.

So no, I don't particularly have any idea.
(and I am currently only using LU, somehow every time I tried to use 
something else it failed because it required sqrt or some other thing not 
available for rationals)

> > I think adding some documentation somewhere here would be nice:
> > http://eigen.tuxfamily.org/dox-devel/TopicCustomizingEigen.html#CustomScalarType

I'll write something as soon as the patch is in, so I know what to 
document. An example with rationals (minimize the complexity of the pivot) 
seems easier than intervals.

--
Marc Glisse