From 390cee15cd0a3f3606fea0058edec4e778eddda7 Mon Sep 17 00:00:00 2001
From: Cora E <cprather@radimant.org>
Date: Fri, 28 Aug 2026 11:26:14 -0400
Subject: [PATCH] Fix ownership for a self-bordering block (possible in small
 periodic single forest)

---
 src/mesh/forest/block_ownership.cpp | 18 ++++++++++++++++++
 src/mesh/forest/block_ownership.hpp |  1 +
 src/mesh/mesh-gmg.cpp               | 19 +++++++++++++++++--
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/mesh/forest/block_ownership.cpp b/src/mesh/forest/block_ownership.cpp
index 0bfaf266cff22..915bc7a43a0f2 100644
--- a/src/mesh/forest/block_ownership.cpp
+++ b/src/mesh/forest/block_ownership.cpp
@@ -41,6 +41,7 @@ namespace parthenon {
 block_ownership_t
 DetermineOwnership(const LogicalLocation &main_block,
                    const std::vector<forest::NeighborLocation> &allowed_neighbors,
+                   const std::array<bool, 3> self_border_block,
                    const std::unordered_set<LogicalLocation> &newly_refined) {
   block_ownership_t main_owns;
 
@@ -63,6 +64,19 @@ DetermineOwnership(const LogicalLocation &main_block,
     return a.morton() < b.morton();
   };
 
+  // Does this block border itself over this face, and are we on the left side?
+  // Then Morton number would say we own the face/edge/corner, but we don't
+  // (the right side does).
+  auto self_border_left = [self_border_block](int ox1, int ox2, int ox3) {
+    // Can take care of each direction independently, corners follow
+    if ((self_border_block[0] && ox1 < 0) || (self_border_block[1] && ox2 < 0) ||
+        (self_border_block[2] && ox3 < 0)) {
+      return true;
+    } else {
+      return false;
+    }
+  };
+
   for (int ox1 : {-1, 0, 1}) {
     for (int ox2 : {-1, 0, 1}) {
       for (int ox3 : {-1, 0, 1}) {
@@ -72,6 +86,10 @@ DetermineOwnership(const LogicalLocation &main_block,
               main_block.IsNeighborOfTE(n.origin_loc, {ox1, ox2, ox3})) {
             main_owns(ox1, ox2, ox3) = false;
             break;
+          } else if (main_block.level() == 0 && main_block == n.global_loc &&
+                     self_border_left(ox1, ox2, ox3)) {
+            main_owns(ox1, ox2, ox3) = false;
+            break;
           }
         }
       }
diff --git a/src/mesh/forest/block_ownership.hpp b/src/mesh/forest/block_ownership.hpp
index 5e7f3f8f74574..f8d5a180a1d6f 100644
--- a/src/mesh/forest/block_ownership.hpp
+++ b/src/mesh/forest/block_ownership.hpp
@@ -44,6 +44,7 @@ namespace parthenon {
 block_ownership_t
 DetermineOwnership(const LogicalLocation &main_block,
                    const std::vector<forest::NeighborLocation> &allowed_neighbors,
+                   const std::array<bool, 3> self_border_block = {false, false, false},
                    const std::unordered_set<LogicalLocation> &newly_refined = {});
 
 // Given a topological element, ownership array of the sending block, and offset indices
diff --git a/src/mesh/mesh-gmg.cpp b/src/mesh/mesh-gmg.cpp
index a6fb0b0553cc3..d047ac8f80f6c 100644
--- a/src/mesh/mesh-gmg.cpp
+++ b/src/mesh/mesh-gmg.cpp
@@ -62,6 +62,21 @@ void SetMeshBlockNeighbors(Mesh *pmesh, GridIdentifier grid_id, BlockList_t &blo
                     {ndim > 2 ? -1 : 0, ndim > 2 ? 1 : 0});
   BufferID buffer_id(ndim, multilevel);
 
+  // If we're periodic with just 1 block in any direction,
+  // we must treat ownership specially
+  auto block_size = pmesh->GetDefaultBlockSize();
+  std::array<bool, 3> self_border_block{
+      !pmesh->mesh_size.symmetry(X1DIR) &&
+          pmesh->mesh_size.nx(X1DIR) / block_size.nx(X1DIR) == 1 &&
+          pmesh->mesh_bcs[BoundaryFace::inner_x1] == BoundaryFlag::periodic,
+      !pmesh->mesh_size.symmetry(X2DIR) &&
+          pmesh->mesh_size.nx(X2DIR) / block_size.nx(X2DIR) == 1 &&
+          pmesh->mesh_bcs[BoundaryFace::inner_x2] == BoundaryFlag::periodic,
+      !pmesh->mesh_size.symmetry(X3DIR) &&
+          pmesh->mesh_size.nx(X3DIR) / block_size.nx(X3DIR) == 1 &&
+          pmesh->mesh_bcs[BoundaryFace::inner_x3] == BoundaryFlag::periodic,
+  };
+
   for (auto &pmb : block_list) {
     std::vector<NeighborBlock> all_neighbors;
     const auto &loc = pmb->loc;
@@ -90,8 +105,8 @@ void SetMeshBlockNeighbors(Mesh *pmesh, GridIdentifier grid_id, BlockList_t &blo
       auto &nb = all_neighbors.back();
       auto neighbor_neighbors = forest.FindNeighbors(nloc.global_loc, grid_id);
 
-      nb.ownership =
-          DetermineOwnership(nloc.global_loc, neighbor_neighbors, newly_refined);
+      nb.ownership = DetermineOwnership(nloc.global_loc, neighbor_neighbors,
+                                        self_border_block, newly_refined);
       nb.ownership.initialized = true;
 
       // Set logical coordinate transformation from this block to the neighbor