get_neighbors_of_at_offset returning multiple references to a single cell in refined grids

[alhom]

The function get_neighbors_of_at_offset:

dccrg/dccrg.hpp

Line 3093 in b5ecae2

> get_neighbors_of_at_offset(

returns multiple references to a cell at the given offset if called in a refined grid: e.g. 8 copies of the same cell in the output in level 1 cells with a maxRefinement of 2.

[iljah]

Does this explain observed behavior: "Offset is in units of size of the given cell"?

[iljah]

If not, then program exhibiting this behavior would be best but knowing all input arguments, initial size and current structure of grid might suffice...

[markusbattarbee]

To clarify: the same neighbor cell (and it's indices) are returned multiple times. I think this occurs when both the cell and it's neighbor are at a lower refinement level than the grid includes. I'd wager that if it was called on level 0 cells on a maxRef 2 grid, we'd get 64 identical returns.

I think the problem arises from the fact that the function first calls indices_from_neighborhood(), which I think now returns all sets of indices contained within the queried cell or something. I'm not really sure why that is even called there, as then going through all neighbors, checking their indices, and seeing if they match the input values should already be enough.

[iljah]

Hmm doesn't look like this function is used anywhere within dccrg or by any tests so your mileage will probably vary :)

Looks like you're right, it's processing cells at index level instead of at level of cell given as argument and so returns many more results than it should. And it's implementation also looks suboptimal and doesn't use latest dccrg data structures, etc.

If you want exactly this functionality before I have time to fix this function (should be possible during this month) I suggest you do what I'll do and go through the neighbors_of iterator and skip cells that are not at desired offset. E.g. (after https://github.com/fmihpc/dccrg/blob/master/tests/advection/solve.hpp#L73)

for (const auto& cell: grid.local_cells(neighborhood_id)) {
  const int cell_length = grid.mapping.get_cell_length_in_indices(cell.id);
  for (const auto& neighbor: cell.neighbors_of) {
    if (neighbor.x > cell_length * desired_offset_x or neighbor.x < ...) { continue; }
    ...
    process(neighbor.id)
  }
}

[iljah]

Oh yeah neighbor.x,y,z should be documented in doxygen but I haven't really checked so looking at the source is probably best: https://github.com/fmihpc/dccrg/blob/master/dccrg.hpp#L7292