On Fri, Nov 8, 2013 at 11:47 AM, Luc Maisonobe <l...@spaceroots.org> wrote:
> > is there still consensus that we are going to remove the sparse > > implementations with 4.0? > > Well, I really think it is a pity, we should support this. But lets face > it: up to now we have been unable to do it properly. Sébastien who tried > to do something in this direction has left the project and nobody > replaced him. > I have done a fair bit of noodling and was unable to come up with a solution that is performant. The issue is that you essentially have to maintain a additional bitmask of exceptional values in addition to the implicit bitmask of non-zero elements. I don't see any way of determining that exceptional value bitmask short of a full scan. Moreover, the cost of propagating the exceptional value bitmask significantly changes the cost of various operations because exceptions require an OR while multiplication allows use of an AND. Furthermore, even after the operation itself and the operation on the exception bitmask are done, there needs to be another scan of the results to find new exceptional values. So the upshot is that dealing with this will cost at least a significant integer degradation in performance at no benefit relative to the normal user's expectations with regard to sparse vector operations. I say no benefit because no other package handles this sort of issue so users are very used to imprecise handling of exceptional values.
