simplify roundrobinvolume redistribution

src/hyteg/primitivestorage/loadbalancing/DistributedBalancer.cpp

@@ -388,6 +388,7 @@ MigrationInfo roundRobinVolume( PrimitiveStorage& storage, uint_t minRank, uint_
    const uint_t numProcesses          = walberla::mpi::MPIManager::instance()->numProcesses();
    const uint_t numReceivingProcesses = maxRank - minRank + 1;
    const uint_t rank                  = uint_c( walberla::mpi::MPIManager::instance()->rank() );
+   const auto reduction = static_cast<double>(numReceivingProcesses) / static_cast<double>(numProcesses);
 
    MigrationMap_T migrationMap;
    uint_t         numReceivingPrimitives = 0;
@@ -404,27 +405,13 @@ MigrationInfo roundRobinVolume( PrimitiveStorage& storage, uint_t minRank, uint_
 
    // Heuristic: let's assign primitives with consecutive IDs to the same process.
    // Often, meshes are generated in a way that primitives with consecutive IDs are located next to each other.
+   // We are assuming here, that the current distribution was already obtained from loadbalancing::roundRobinVolume.
    auto setup_volume_ids = [&]( const auto& ids ) {
-      auto it = ids.begin();
-      for ( uint_t currentRank = 0; currentRank < numReceivingProcesses; currentRank++ )
-      {
-         uint_t numVolumesOnThisRank = 0;
-         while ( numVolumesOnThisRank < ids.size() / numReceivingProcesses )
-         {
-            migrationMap[it->getID()] = minRank + currentRank;
-            numVolumesOnThisRank++;
-            it++;
-         }
-
-         if ( currentRank < ids.size() % numReceivingProcesses )
-         {
-            migrationMap[it->getID()] = minRank + currentRank;
-            it++;
-         }
-         numPrimitivesPerRank[minRank + currentRank] =
-             ids.size() / numReceivingProcesses + ( currentRank < ids.size() % numReceivingProcesses );
+      auto target_rank = minRank + static_cast<uint_t>(static_cast<double>(rank) * reduction);
+      for(const auto& p: ids){
+         migrationMap[p.getID()] = target_rank;
       }
-      WALBERLA_CHECK( it == ids.end() );
+      numPrimitivesPerRank[target_rank] = ids.size();
    };
    if ( storage.hasGlobalCells() )
    {