A container owns variables. Each variable is named, and the container knows about various types of variables, such as whether the variable is for cell-centered data, face-centered data, sparse data, or dense data. (For more details on anonymous variables, see here.) Variables in a container can be different shapes, e.g., scalar, tensor, etc.A variable can be added to a container as:
parthenon::Container.Add(name, metadata, shape)where the name is a string, the metadata is a std::vector of
metadata flags, and the shape is a std::vector of integers
specifying the dimensions.
Note that if a location, such as Metadata::Cell or Metadata::Face,
then shape is the shape of the variable at a given point. If you have
a scalar, you do not need to specify the shape in Add. If you want a
vector field with 3 components, you would use
shape = std::vector<int>({3});and if you wanted a 3x3 rank-2 tensor field, you would use
shape = std::vector<int>({3,3});If you do not specify a location on the grid which the variable
lives, then shape is the shape of the full array. I.e., a
11x12x13x14 array would be added with a shape of
shape = std::vector<int>({11,12,13,14});It is often desirable to extract from a container a specific set of variables that have desired names, sparse ids, or conform to specific metadata flags. This set of variables must be collected in such a way that it can be accessed easily and performantly on a GPU, and such that one can index into the collection in a known way. This capability is provided by variable packs.
To extract a variable pack for variables with a set of names, call
container.PackVariables(names, map)where names is a std::vector of strings and map is an
instance of a parthenon::PackIndexMap.
This will return a VariablePack object, which
is essentially a Kokkos::view of parthenon::ParArray3Ds.
The map will be filled by reference as a map from
variable names to indices in the VariablePack.
Similar methods are available for metadata and sparse IDs:
container.PackVariables(metadata, ids, map)
container.PackVariables(metadata, map)If you would like all variables in a container, you can ommit the metadata or name arguments:
container.PackVariables(map)If you do not care about indexing into variables by name,
you can ommit the map argument in any of the above calls.
For examples of use, see here.