Neighbor Lists Never Populated #1827
Description
It appears that cell neighbor lists are not being populated in the current version of OpenMC -- i.e., the neighbor list push_back() function is never called. From what I can tell, this regression was introduced in #1784 when recursive calls were eliminated from the cell finding process.
The issue stems from a change made in find_cell_inner() of geometry.cpp. The old version basically had the following logic:
if( neighbor_list )
... check neighbor list cells
else
... do exhaustive search
The new version was changed to do
if( neighbor_list )
... check neighbor list cells
do exhaustive search
This means that the find_cell_inner() function will always find the cell, even if a neighbor list is passed and the cell is not in the neighbor list. This is a problem, due to the way find_cell_inner is called:
// Search for the particle in that cell's neighbor list. Return if we
// found the particle.
bool found = find_cell_inner(p, &c.neighbors_);
if (found) return found;
// The particle could not be found in the neighbor list. Try searching all
// cells in this universe, and update the neighbor list if we find a new
// neighboring cell.
found = find_cell_inner(p, nullptr);
if (found) c.neighbors_.push_back(p.coord_[coord_lvl].cell);
return found;This regression could potentially have a significant performance impact on a lot of problems. It may be worth doing some analysis to see if a new release with a patch is worthwhile, as this PR went in right before 12.1.