[dune-pdelab] Interface changes for containers in LocalOperator

[Steffen Müthing]

Dear PDELab developers,

in order to simplify the implementation of the new GridOperator assemblers and to
clarify the purpose of the residual / jacobian containers, we already changed the interface
of the residual and jacobian containers in the trunk of PDELab. As these changes will
require some minor changes to all local operators anyway (to silence the torrent of
deprecation warnings), Christian and I yesterday thought about taking the change a
little further.

Right now, you always have to remember to call localIndex() on the correct LFS and
to only use the translated index to access the container. In our opinion, there are
three problems with that:

1) A user can easily forget the call, causing his local operator to assemble nonsense.
2) It is possible (and not exactly rare) to use the localIndex() method from the wrong
   function space to access a container. As most of our schemes are of Galerkin type,
   this usually doesn't matter, but if the two function spaces really differ, it is another
   source of subtle bugs.
3) All those localIndex() calls really clutter up the code - I especially noticed that when
   porting the numerical differentiation classes to the new, weighted residual and jacobian
   containers.

Christian and I now came up with the idea that what you actually want to do when accessing
a container in a LocalOperator is to retrieve or write a value associated with DOF i of some
local function space lfs. By adjusting the interface of the containers to reflect that notion,
we get rid of all three problems noted above. The new interface would look like this:

access to vector: x(lfsu,i)
access to matrix: jac(lfsv,i,lfsu,j)
accumulate residual: x.accumulate(lfsu,i,value)
accumulate jacobian: jac.accumulate(lfsv,i,lfsu,j,value)

The new containers would also still allow access to the underlying raw containers using
a common interface method like base().

Moreover, we would initially keep the existing access methods (operator[] for the vector and two-argument
operator() for the matrix), but deprecate them.

I implemented that change and encountered no real problems (apart from the flood of deprecation
warnings), but haven't committed anything yet. Are there any objections or ideas for improvement?

Steffen


Steffen Müthing
Universität Stuttgart
Institut für Parallele und Verteilte Systeme
Universitätsstr. 38
70569 Stuttgart
Tel: +49 711 685 88429
Fax: +49 711 685 88340
Email: steffen.muething at ipvs.uni-stuttgart.de