[SciPy-dev] new sparsetools + umfpack

[Robert Cimrman]

New sparsetools implementation does not require CSR/CSC column/row
indices to be sorted in ascending order, while UMFPACK does. Currently,
every call to umfpack module (e.g. via linsolve.solve) implicitly calls
ensure_sorted_indices(inplace=True) method of CSR/CSC matrix, which is
rather fast, but in many cases is not necessary and the overhead can
become significant when solving lots of linear systems (e.g.
FE-discretized evolutionary nonlinear PDEs) - in this case, only one
call to ensure_sorted_indices() is necessary, then only values change,
not the structure.

Therefore, if there are no objections, I would like to remove the
implicit call to ensure_sorted_indices() from umfpack and require user
to call it explicitly (only if necessary) - an exception will be raised
by umfpack if the matrix is not in correct form, so it cannot lead to
hidden bugs. I will document this in linsolve, of course.

r.