From 70cb1110e935a9741ec8bf7aaa9e7df63a10b40b Mon Sep 17 00:00:00 2001
From: Gavin Ridley <gavin.keith.ridley@gmail.com>
Date: Wed, 28 Apr 2021 18:10:02 -0400
Subject: [PATCH 1/2] make sure failed neighbor list searches trigger
 exhaustive searches

---
 src/geometry.cpp | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/geometry.cpp b/src/geometry.cpp
index 06d35cacec..c100ebb5e7 100644
--- a/src/geometry.cpp
+++ b/src/geometry.cpp
@@ -91,11 +91,25 @@ find_cell_inner(Particle& p, const NeighborList* neighbor_list)
         break;
       }
     }
+
+    // If we're attempting a neighbor list search and fail, we
+    // now know we should return false. This will trigger an
+    // exhaustive search from neighbor_list_find_cell and make
+    // the result from that be appended to the neighbor list.
+    if (!found)
+      return false;
   }
 
   // Check successively lower coordinate levels until finding material fill
   for (;;++p.n_coord_) {
-    // If neighbor lists did not find a cell, must do exhaustive search
+    // If we did not attempt to use neighbor lists, i_cell is still C_NONE.  In
+    // that case, we should now do an exhaustive search to find the right value
+    // of i_cell.
+    //
+    // Alternatively, neighbor list searches could have succeeded, but we found
+    // that the fill of the neighbor cell was another universe. As such, in the
+    // code below this conditional, we set i_cell back to C_NONE to indicate
+    // that.
     if (i_cell == C_NONE) {
       int i_universe = p.coord_[p.n_coord_-1].universe;
       const auto& univ {*model::universes[i_universe]};

From 1155796253010f3cd9d9038131667209d2d4ccb8 Mon Sep 17 00:00:00 2001
From: Gavin Ridley <gavin.keith.ridley@gmail.com>
Date: Thu, 29 Apr 2021 13:26:08 -0400
Subject: [PATCH 2/2] if unable to find cell, get helpful error message rather
 than segfault

---
 src/geometry.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/geometry.cpp b/src/geometry.cpp
index c100ebb5e7..23622ebad0 100644
--- a/src/geometry.cpp
+++ b/src/geometry.cpp
@@ -96,8 +96,9 @@ find_cell_inner(Particle& p, const NeighborList* neighbor_list)
     // now know we should return false. This will trigger an
     // exhaustive search from neighbor_list_find_cell and make
     // the result from that be appended to the neighbor list.
-    if (!found)
-      return false;
+    if (!found) {
+      return found;
+    }
   }
 
   // Check successively lower coordinate levels until finding material fill
@@ -252,6 +253,7 @@ find_cell_inner(Particle& p, const NeighborList* neighbor_list)
       }
     }
     i_cell = C_NONE; // trip non-neighbor cell search at next iteration
+    found = false;
   }
 
   return found;