Add support for mesh loadbalancing for particles with no cells

arcane/src/arcane/mesh/MeshExchange.cc

@@ -559,7 +559,7 @@
 /*---------------------------------------------------------------------------*/
 
 void MeshExchange::
 _computeItemsToSend(bool send_dof)
 {
   for (const auto& iter : m_items_to_send)
     iter.second->resize(m_nb_rank);
@@ -609,7 +609,10 @@
       if (particle_family && particle_family->getEnableGhostItems() == true) {
         ArrayView<std::set<Int32>> to_send = _getItemsToSend(family);
         ENUMERATE_PARTICLE (iparticle, particle_family->allItems().own()) {
-          _addItemToSend(to_send, iparticle->localId(), iparticle->cell().localId(), use_itemfamily_network);
+          Cell cell = iparticle->cell();
+          // Do not send the particle if it has no cell.
+          if (!cell.null())
+            _addItemToSend(to_send, iparticle->localId(), cell.localId(), use_itemfamily_network);
         }
       }
       if (send_dof && family->itemKind() == IK_DoF) {
@@ -1416,13 +1419,16 @@
 {
   for (IItemFamily* family : m_mesh->itemFamilies()) {
     IParticleFamily* particle_family = family->toParticleFamily();
-    if (particle_family && particle_family->getEnableGhostItems() == true) {
+    if (particle_family && particle_family->getEnableGhostItems()) {
       mesh::NewItemOwnerBuilder owner_builder;
 
       ENUMERATE_PARTICLE (iparticle, particle_family->allItems()) {
         Particle particle = *iparticle;
         Cell cell = owner_builder.connectedCellOfItem(particle);
-        particle.mutableItemBase().setFlags(cell.itemBase().flags());
+        // NOTE GG: 10/2026. This code is very strange
+        // I think we should only modify some flags (II_NeedRemove?)
+        if (!cell.null())
+          particle.mutableItemBase().setFlags(cell.itemBase().flags());
       }
     }
   }

arcane/src/arcane/mesh/MeshExchanger.cc

@@ -240,7 +240,7 @@ computeExchangeInfos()
     // of ghost items, this list has already been determined during the call to
     // mesh_exchange.computeInfos().
     IParticleFamily* particle_family = family->toParticleFamily();
-    if (particle_family && particle_family->getEnableGhostItems() == false)
+    if (particle_family && !particle_family->getEnableGhostItems())
       exchanger->computeExchangeItems();
     else
       exchanger->setExchangeItems(mesh_exchange.getItemsToSend(family));

arcane/src/arcane/mesh/UnstructuredMeshUtilities.cc

@@ -169,7 +169,12 @@ changeOwnersFromCells()
     VariableItemInt32& particles_owner(family->itemsNewOwner());
     ENUMERATE_PARTICLE (i_particle, family->allItems()) {
       Particle particle = *i_particle;
-      particles_owner[particle] = cells_owner[particle.cell()];
+      Cell pcell = particle.cell();
+      if (!pcell.null())
+        particles_owner[particle] = cells_owner[pcell];
+      else
+        // If the particle has no cell, its new owner is the current owner.
+        particles_owner[particle] = particle.owner();
     }
   }
 

arcane/src/arcane/tests/ParallelTesterModule.cc

@@ -5,7 +5,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //-----------------------------------------------------------------------------
 /*---------------------------------------------------------------------------*/
-/* ParallelTesterModule.cc                                     (C) 2000-2024 */
+/* ParallelTesterModule.cc                                     (C) 2000-2026 */
 /*                                                                           */
 /* Parallelism test module.                                                  */
 /*---------------------------------------------------------------------------*/
@@ -174,8 +174,8 @@ class ParticleFamilyTester
     Integer particle_per_cell = 12;
     info() << " BuildParticleFamily increment=" << particle_per_cell
            << " nb_particle=" << m_family->nbItem();
-    Int64UniqueArray uids;
-    Int32UniqueArray cells_lid;
+    UniqueArray<Int64> uids;
+    UniqueArray<Int32> cells_lid;
     IParallelMng* pm = m_family->parallelMng();
     CellGroup own_cells = m_family->mesh()->ownCells();
     Integer nb_own_cell = own_cells.size();
@@ -191,7 +191,16 @@ class ParticleFamilyTester
         ++first_uid;
       }
     }
-
+    // To check load-balance with particles with no cells, make sure some
+    // particles have no cells.
+    // NOTE: Having particles without cells is not supported when using
+    // graph network.
+    if (!m_family->mesh()->itemFamilyNetwork()) {
+      Int32 nb_particle_to_add = uids.size();
+      for (Int32 i = 0; i < nb_particle_to_add; ++i)
+        if ((i % 10) == 0)
+          cells_lid[i] = ItemLocalId{};
+    }
     m_first_uid = m_first_uid + uid_increment * comm_size;
 
     info() << "Create " << uids.size() << " particles";
@@ -202,7 +211,7 @@ class ParticleFamilyTester
     m_family->partialEndUpdateVariable(m_values1.variable());
     ENUMERATE_PARTICLE (ipart, particles) {
       Particle particle = *ipart;
-      m_values1[ipart] = (Real)(particle.uniqueId().asInt64());
+      m_values1[ipart] = static_cast<Real>(particle.uniqueId().asInt64());
     }
     m_family->endUpdate();
   }