Because CellSetExplicit is a templated class, the implementation of
most of its features is part of the header files. One of the things that
was included was the code to build the reverse connectivity links. That
is, it figured out which cells were incident on each point using the
standard connections of which points comprise which cells.
Of course, building these links is non-trivial, and it used multiple DPPs to engage the device. It meant that header had to include the device adapter algorithms and therefore required a device compiler. We want to minimize this where possible.
To get around this issue, a non-templated function was added to find the
reverse connections of a CellSetExplicit. It does this by passing in
UnknownArrayHandles for the input arrays. (The output visit-points-
with-cells arrays are standard across all template instances.) The
implementation first iterates over all CellSetExplicit versions in
VTKM_DEFAULT_CELL_SETS and attempts to retrieve arrays of those types.
In the unlikely event that none of these arrays work, it copies the data
to ArrayHandle<vtkm::Id> and uses those.