Fix distribcell labels for lattices used as fill in multiple cells

src/geometry_aux.cpp

@@ -539,7 +539,7 @@ std::string distribcell_path_inner(int32_t target_cell, int32_t map,
 
       // The desired cell is the first cell that gives an offset smaller or
       // equal to the target offset.
-      if (temp_offset <= target_offset)
+      if (temp_offset <= target_offset - c.offset_[map])
         break;
     }
   }
@@ -561,11 +561,11 @@ std::string distribcell_path_inner(int32_t target_cell, int32_t map,
     for (ReverseLatticeIter it = lat.rbegin(); it != lat.rend(); ++it) {
       int32_t indx = lat.universes_.size() * map + it.indx_;
       int32_t temp_offset = offset + lat.offsets_[indx];
-      if (temp_offset <= target_offset) {
+      if (temp_offset <= target_offset - c.offset_[map]) {
         offset = temp_offset;
         path << "(" << lat.index_to_string(it.indx_) << ")->";
-        path << distribcell_path_inner(
-          target_cell, map, target_offset, *model::universes[*it], offset);
+        path << distribcell_path_inner(target_cell, map, target_offset,
+          *model::universes[*it], offset + c.offset_[map]);
         return path.str();
       }
     }

src/tallies/filter_distribcell.cpp

@@ -48,7 +48,8 @@ void DistribcellFilter::get_all_bins(
       auto& lat {*model::lattices[p.coord(i + 1).lattice]};
       const auto& i_xyz {p.coord(i + 1).lattice_i};
       if (lat.are_valid_indices(i_xyz)) {
-        offset += lat.offset(distribcell_index, i_xyz);
+        offset +=
+          lat.offset(distribcell_index, i_xyz) + c.offset_[distribcell_index];
       }
     }
     if (cell_ == p.coord(i).cell) {

tests/unit_tests/test_cell_instance.py

@@ -39,6 +39,18 @@ def double_lattice_model():
     cell_with_lattice2.translation = (2., 0., 0.)
     model.geometry = openmc.Geometry([cell_with_lattice1, cell_with_lattice2])
 
+    tally = openmc.Tally()
+    tally.filters = [openmc.DistribcellFilter(c)]
+    tally.scores = ['flux']
+    model.tallies = [tally]
+
+    # Add box source that covers the model space well
+    bbox = model.geometry.bounding_box
+    bbox[0][2] = -0.5
+    bbox[1][2] = 0.5
+    space = openmc.stats.Box(*bbox)
+    model.settings.source = openmc.IndependentSource(space=space)
+
     # Add necessary settings and export
     model.settings.batches = 10
     model.settings.inactive = 0
@@ -71,3 +83,9 @@ def double_lattice_model():
 def test_cell_instance_multilattice(r, expected_cell_instance):
     _, cell_instance = openmc.lib.find_cell(r)
     assert cell_instance == expected_cell_instance
+
+
+def test_cell_instance_multilattice_results():
+    openmc.lib.run()
+    tally_results = openmc.lib.tallies[1].mean
+    assert (tally_results != 0.0).all()