Rework the AbstractCell interface
#679
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## master #679 +/- ##
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Is there a way to ensure that |
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Supersedes the prototype given in #519 ? |
That can be encoded in the type system: julia> abstract type AbstractRefShape{dim} end
julia> struct RefInterval <: AbstractRefShape{1} end
julia> struct RefTriangle <: AbstractRefShape{2} end
julia> abstract type AbstractCell{refdim, refshape <: AbstractRefShape{refdim}} end
julia> struct Nope <: AbstractCell{2, RefInterval} end
ERROR: TypeError: in AbstractCell, in refshape, expected refshape<:AbstractRefShape{2}, got Type{RefInterval}
Stacktrace:
[1] top-level scope
@ REPL[5]:1
julia> struct Yep <: AbstractCell{2, RefTriangle} end
Yea, this roughly corresponds to the Grid changes in that PR. |
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This patch reworks the `AbstractCell` interface (refer to #677 for motivation and discussion for alternatives to this patch). Specifically, this patch - introduces the reference dimension as a type parameter on `AbstractRefShape`, - introduces `RefLine`, `RefTriangle`, `RefQuadrilateral`, and `RefHexahedron` as new subtypes of `AbstractRefShape{refdim}` (note that interpolations are not updated to this system yet since it can be done in a separate patch), - deprecates the old `Cell{refdim, nnodes, nfaces}` struct, - introduces new individual structs for all supported cell types, e.g. `struct Triangle <: AbstractCell{2, RefTriangle}`, - defines the unique vertex/edge/face identifiers for DoF-distribution in terms of dispatch on the reference shape. The new parametrization makes it possible to dispatch on the reference shape instead of union of concrete implementations, i.e. `::AbstractCell{2, RefTriangle}` instead of `::Union{Triangle, QuadraticTriangle}`, and similarly for the reference dimension. One drawback is that it is no longer possible to use "anonymous celltypes" after this patch, i.e. using, e.g., `Cell{3, 20, 6}` and defining methods for that instead of explicitly naming it `SerendipityQuadraticHexahedron` or similar. Instead, you now have to define a struct and some methods. Arguably this is a good thing -- nobody understands the meaning of `Cell{3, 20, 6}`. For normal usage this patch is not breaking (see e.g. no required changes for examples), unless one dispatches on the `Cell` struct directly. Fixes #677.
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This patch reworks the
AbstractCellinterface (refer to #677 for motivation and discussion for alternatives to this patch). Specifically, this patchAbstractRefShape,RefInterval,RefTriangle,RefQuadrilateral, andRefHexahedronas new subtypes ofAbstractRefShape{refdim}(note that interpolations are not updated to this system yet since it can be done in a separate patch),Cell{refdim, nnodes, nfaces}struct,struct Triangle <: AbstractCell{RefTriangle},The new parametrization makes it possible to dispatch on the reference shape instead of union of concrete implementations, i.e.
::AbstractCell{RefTriangle}instead of::Union{Triangle, QuadraticTriangle}, and similarly for the reference dimension.One drawback is that it is no longer possible to use "anonymous celltypes" after this patch, i.e. using, e.g.,
Cell{3, 20, 6}and defining methods for that instead of explicitly naming itSerendipityQuadraticHexahedronor similar. Instead, you now have to define a struct and some methods. Arguably this is a good thing -- nobody understands the meaning ofCell{3, 20, 6}.For normal usage this patch is not breaking (see e.g. no required changes for examples), unless one dispatches on the
Cellstruct directly.Specific things that remain to be decided:
struct Triangle <: AbstractCell{2, RefTriangle}orstruct Triangle <: AbstractRefCell{2, RefTriangle} <: AbstractCell? I don't know if it is too restrictive to "force"{refdim, refshape}on all subtypes ofAbstractCell. On the other hand, if one implements a non-refshape based element one can define a dummy reference shape. For a real-world example, looking at e.g. https://github.com/kimauth/FerriteCohesiveZones.jl/blob/main/src/cohesive_cell.jl I think that would simply be implemented asjulia struct CohesiveQuadrilateral <: AbstractCell{2, RefQuadrilateral} nodes::NTuple{4, Int} endwhich I don't see a direct problem with, other than the fact that the cell will now have 4 edges (according to the fallback definitions). To solve this, one would have to implement a method forfaces(::CohesiveQuadrilateral)that only return the upper and lower edge. I don't think this is too much to ask for custom elements like this. Alternatively, with Wedge Support #581 merged it is possible to distribute different number of dofs for different edges, so this can also be solved on the (default) interpolation level by distributing 0 dofs on the left and right edge.AbstractCell{refdim, refshape}or justAbstractCell{refshape}? The dimension is implicit from the shape now, and you can dispatch onAbstractCell{<:AbstractRefShape{refdim}} where refdim. Personally I am in favor of including the dimension, even if redundant. It does not cost anything, and sometimes you want to dispatch on the dimension, sometimes on the refshape.Fixes #677.