Make the user-defined distribcells an argument to the function.

include/openmc/geometry_aux.h

@@ -7,6 +7,7 @@
 #include <cstdint>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 
 #include "openmc/vector.h"
 
@@ -63,9 +64,12 @@ int32_t find_root_universe();
 
 //==============================================================================
 //! Populate all data structures needed for distribcells.
+//! \param user_distribcells A set of cell indices to create distribcell data
+//!   structures for regardless of whether or not they are part of a tally
+//!   filter.
 //==============================================================================
 
-void prepare_distribcell();
+void prepare_distribcell(const std::unordered_set<int32_t>* user_distribcells = nullptr);
 
 //==============================================================================
 //! Recursively search through the geometry and count cell instances.

include/openmc/settings.h

@@ -105,7 +105,6 @@ extern int trigger_batch_interval;   //!< Batch interval for triggers
 extern "C" int verbosity;                //!< How verbose to make output
 extern double weight_cutoff;         //!< Weight cutoff for Russian roulette
 extern double weight_survive;        //!< Survival weight after Russian roulette
-extern std::unordered_set<int32_t> distribcells; //!< Manually set distribcells
 } // namespace settings
 
 //==============================================================================

include/openmc/tallies/filter_cell_instance.h

@@ -44,6 +44,8 @@ public:
 
   const vector<CellInstance>& cell_instances() const { return cell_instances_; }
 
+  const std::unordered_set<int32_t>& cells() const { return cells_; }
+
   void set_cell_instances(gsl::span<CellInstance> instances);
 
 private: