Add factory for container descriptors specialized for different pre-basis types

Summary

This MR implements the first step towards container descriptors for dune-functions bases. The descriptors for leaf bases and for power/composite bases with blocked-lexicographic IMS is provided. All other cases return a ContainerDescriptors::Unknown tag, indicating: not implemented.

Design

The container descriptors of a pre-basis can be obtained by calling preBasis.containerDescriptor() and by using the factory method

template <class PreBasis>
auto constainerDescriptor(const PreBasis& preBasis)
{
  if constexpr (/* preBasis implements .containerDescriptor() */)
    return preBasis.containerDescriptor();
  else
    return ContainerDescriptors::Unknown{};
}

with a fallback to ContainerDescriptors::Unknown in case the pre-basis does not provide a definition.

Specializations for leaf pre-bases in dune-functions are provided. Additionally, in this MR the specialization for composie/power/dynamic-power pre-basis with blocked-lexicographic index-merging strategy is implemented. In follow-up MRs the other index-merging strategy specializations will be added.

Example

using namespace Dune::Functions;
using namespace Dune::Functions::BasisFactory;
namespace CD = Dune::Functions::ContainerDescriptors;

auto basis1 = makeBasis(gridView, power<2>(lagrange<1>(), blockedLexicographic()));
auto descriptor1 = containerDescriptor(basis1.preBasis());
  // -> CD::UniformArray<CD::FlatVector,2>

auto basis2 = makeBasis(gridView, 
  composite(power<3>(lagrange<2>(),blockedLexicographic()), lagrange<1>(), blockedLexicographic()));
auto descriptor2 = containerDescriptor(basis2.preBasis());
  // -> CD::Tuple<CD::UniformArray<CD::FlatVector,2>,CD::FlatVector>

auto basis3 = makeBasis(gridView, power<2>(lagrange<1>(), flatInterleaved()));
auto descriptor3 = containerDescriptor(basis3.preBasis());
  // -> CD::Unknown

Changes

• Each pre-basis gets a new member-function containerDescriptor() const returning a type from the namespace ContainerDescriptors.
• The GenericIndexingTransformation is parametrized additionally with a ContainerDescriptorImplementation, i.e. a callable returning the container-descriptor associated to the pre-basis after index transformation. This callable is passed as additional last constructor argument. The factory function indexTransformation also gets an additional parameter, but for backwards-compatibility, there is an overload that just returns ContainerDescriptor::Unkown if nothing is provided.

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• 0c8df434 - Remove container descriptor merge utility for flat indexmerging

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• e07b240a - Remove blocked-interleaved size-merging

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marked this merge request as ready

@carsten.graeser, @oliver.sander, @christi, @santiago.ospina: This MR implements the second step of the container-descriptor design we have discussed at the last dune-functions meeting. Instead of having a member-function preBasis.containerDescriptor() I propose to introduce a factory class. Thus, the container-descriptor is not directly a part of the basis interface, but is still provided for all bases available in dune-functions.

In this MR, the descriptors are implemented for the leaf-pre-bases, a few special bases (e.g. Taylor-Hood) and for composite/power bases with blocked-lexicographic index-merging. In these cases, the descriptor can be defined explicitly in terms of the container-descriptor tree, without any merging/appending algorithm. That will be added in a next MR after we have decided on a general pattern.

Can you please give a clear voting whether you accept my proposal of the factory class instead of the member-function?!

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• 712b02cf - Move containerDescriptor factory into member functions

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• 947b379c - Undo some whitespace changes

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• 66dc4981 - Add a changelog entry

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• 66dc4981...03dc66b6 - 28 commits from branch master
• 0f15a4d3 - Add factory for container discretors specialized for different pre basis types
• 88820dba - Add a changelog entry

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This is ready from my side

Looks good.

Due to the missing implementations for most merging strategies this is not very useful so far. Do you expect any obstacles implementing these, that may require to change the mechanism?

The implementation for other merging strategies was left out on purpose in this MR to first concentrate on the general interface. All cases can be covered, see !350 (closed).

In doing the MRs step-by-step we can add for all steps test cases. This was partially missing in the big MR. Also, some of the merging-strategies require in the current implementation of the container-descriptors quiet complex algorithms. Maybe these can be cleaned up in the follow-up MRs.

Thanks for clarification.

mentioned in commit 664f68c6

merged