Re: Ignite ML, next steps (IGNITE-5029)

[ArtemM <ar...@gmail.com>]

In the initial implementation of foldMap/map for sparse we considered only
nonzero elements. This is problem because it conflicts with behaviour  of
other matrix implementation. For example, if we try to run map (+1) over
sparse identity matrix, we'll get not matrix with 2's on diagonal and other
elements equal to one, but matrix with 2s on diagonal and 0s in other
positions. Also if we try fold (by fold(f) i mean foldMap(f, identity))
sparse identity matrix with min, we'll get 1 instead of 0. I've started
fixing this behaviour in IGNITE-5102. Basic idea for fixing map is to map
default element along with nondefault elements (we'll now have not necessary
zero as default element). For fold the idea is in performing folding n times
with default element (n is count of default element) after folding on
nondefault elements.  Actually this approach requires accumulator function
to be commutative, but it seems that it does not conflict with javadoc for
foldMap (it does not specify any order of folding explicitly, so there is no
order guaranteed and therefore for operation to be well defined, accumulator
should be commutative). Any objections for this?



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