From 971c5f77a5ebc276c6aa5c24879f115d1a49a8ed Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Mon, 15 Jan 2024 14:33:56 -0600
Subject: [PATCH] Ensure that implicit complement cells appear last in DAGMC
 universes (#2838)

---
 src/cell.cpp                                  | 30 ++++++++++++++++---
 .../regression_tests/dagmc/external/main.cpp  | 16 +++++++++-
 2 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/src/cell.cpp b/src/cell.cpp
index c96abf39be..cba52f1b55 100644
--- a/src/cell.cpp
+++ b/src/cell.cpp
@@ -1020,19 +1020,41 @@ void read_cells(pugi::xml_node node)
 
 void populate_universes()
 {
+  // Used to map universe index to the index of an implicit complement cell for
+  // DAGMC universes
+  std::unordered_map<int, int> implicit_comp_cells;
+
   // Populate the Universe vector and map.
-  for (int i = 0; i < model::cells.size(); i++) {
-    int32_t uid = model::cells[i]->universe_;
+  for (int index_cell = 0; index_cell < model::cells.size(); index_cell++) {
+    int32_t uid = model::cells[index_cell]->universe_;
     auto it = model::universe_map.find(uid);
     if (it == model::universe_map.end()) {
       model::universes.push_back(make_unique<Universe>());
       model::universes.back()->id_ = uid;
-      model::universes.back()->cells_.push_back(i);
+      model::universes.back()->cells_.push_back(index_cell);
       model::universe_map[uid] = model::universes.size() - 1;
     } else {
-      model::universes[it->second]->cells_.push_back(i);
+#ifdef DAGMC
+      // Skip implicit complement cells for now
+      Universe* univ = model::universes[it->second].get();
+      DAGUniverse* dag_univ = dynamic_cast<DAGUniverse*>(univ);
+      if (dag_univ && (dag_univ->implicit_complement_idx() == index_cell)) {
+        implicit_comp_cells[it->second] = index_cell;
+        continue;
+      }
+#endif
+
+      model::universes[it->second]->cells_.push_back(index_cell);
     }
   }
+
+  // Add DAGUniverse implicit complement cells last
+  for (const auto& it : implicit_comp_cells) {
+    int index_univ = it.first;
+    int index_cell = it.second;
+    model::universes[index_univ]->cells_.push_back(index_cell);
+  }
+
   model::universes.shrink_to_fit();
 }
 
diff --git a/tests/regression_tests/dagmc/external/main.cpp b/tests/regression_tests/dagmc/external/main.cpp
index 42f7d97c90..bebe088f60 100644
--- a/tests/regression_tests/dagmc/external/main.cpp
+++ b/tests/regression_tests/dagmc/external/main.cpp
@@ -33,7 +33,7 @@ int main(int argc, char* argv[])
   // Initialize acceleration data structures
   rval = dag_ptr->init_OBBTree();
   if (rval != moab::MB_SUCCESS) {
-    fatal_error("Failed to initialise OBB tree");
+    fatal_error("Failed to initialize OBB tree");
   }
 
   // Get rid of existing geometry
@@ -62,6 +62,20 @@ int main(int argc, char* argv[])
   // Add cells to universes
   openmc::populate_universes();
 
+  // Make sure implicit complement appears last
+  auto dag_univ = dynamic_cast<DAGUniverse*>(model::universes.back().get());
+  int n = dag_univ->cells_.size();
+  for (int i = 0; i < n - 1; ++i) {
+    if (dag_univ->cells_[i] == dag_univ->implicit_complement_idx()) {
+      fatal_error("Implicit complement cell should appear last in vector of "
+                  "cells for DAGMC universe.");
+    }
+  }
+  if (dag_univ->cells_.back() != dag_univ->implicit_complement_idx()) {
+    fatal_error(
+      "Last cell in DAGMC universe is not an implicit complement cell.");
+  }
+
   // Set root universe
   openmc::model::root_universe = openmc::find_root_universe();
   openmc::check_dagmc_root_univ();