From 42fef925d50f6cf8a6c5efb04b1a6e6f12ca5626 Mon Sep 17 00:00:00 2001
From: sbstndb/sbstndbs <sebastiendubois4@gmail.com>
Date: Wed, 10 Sep 2025 10:02:29 +0200
Subject: [PATCH 1/2] perf: factorise MPI calls

	-	Previously, update_tag_subdomains in a simple for loop
	-	Now, factorized MPI calls in the simple for loop.
---
 include/samurai/algorithm/update.hpp | 81 ++++++++++++++++++++++++++++
 include/samurai/mr/adapt.hpp         |  5 +-
 2 files changed, 82 insertions(+), 4 deletions(-)

diff --git a/include/samurai/algorithm/update.hpp b/include/samurai/algorithm/update.hpp
index 196e8fed5..9ae5454cd 100644
--- a/include/samurai/algorithm/update.hpp
+++ b/include/samurai/algorithm/update.hpp
@@ -738,6 +738,87 @@ namespace samurai
 #endif
     }
 
+    template <class Field>
+    void update_tag_subdomains([[maybe_unused]] std::size_t min_level,
+                             [[maybe_unused]] std::size_t max_level,
+                             [[maybe_unused]] Field& tag,
+                             [[maybe_unused]] bool erase = false)
+    {
+#ifdef SAMURAI_WITH_MPI
+        using mesh_t    = typename Field::mesh_t;
+        using value_t   = typename Field::value_type;
+        using mesh_id_t = typename mesh_t::mesh_id_t;
+        std::vector<mpi::request> req;
+
+        auto& mesh = tag.mesh();
+        mpi::communicator world;
+        std::vector<std::vector<value_t>> to_send(mesh.mpi_neighbourhood().size());
+
+        std::size_t i_neigh = 0;
+        for (auto& neighbour : mesh.mpi_neighbourhood())
+        {
+            for (std::size_t level = min_level; level <= max_level; ++level)
+            {
+                if (!mesh[mesh_id_t::reference][level].empty() && !neighbour.mesh[mesh_id_t::reference][level].empty())
+                {
+                    auto out_interface = intersection(mesh[mesh_id_t::reference][level],
+                                                      neighbour.mesh[mesh_id_t::reference][level],
+                                                      mesh.subdomain())
+                                             .on(level);
+                    out_interface(
+                        [&](const auto& i, const auto& index)
+                        {
+                            std::copy(tag(level, i, index).begin(), tag(level, i, index).end(), std::back_inserter(to_send[i_neigh]));
+                        });
+                }
+            }
+            req.push_back(world.isend(neighbour.rank, neighbour.rank, to_send[i_neigh]));
+            i_neigh++;
+        }
+
+        i_neigh = 0;
+        for (auto& neighbour : mesh.mpi_neighbourhood())
+        {
+            std::vector<value_t> to_recv;
+            world.recv(neighbour.rank, world.rank(), to_recv);
+            std::ptrdiff_t count = 0;
+
+            for (std::size_t level = min_level; level <= max_level; ++level)
+            {
+                if (!mesh[mesh_id_t::reference][level].empty() && !neighbour.mesh[mesh_id_t::reference][level].empty())
+                {
+                    auto in_interface = intersection(mesh[mesh_id_t::reference][level],
+                                                     neighbour.mesh[mesh_id_t::reference][level],
+                                                     neighbour.mesh.subdomain())
+                                            .on(level);
+                    in_interface(
+                        [&](const auto& i, const auto& index)
+                        {
+                            if (count + static_cast<std::ptrdiff_t>(i.size()) <= to_recv.size())
+                            {
+                                xt::xtensor<value_t, 1> neigh_tag = xt::empty_like(tag(level, i, index));
+                                std::copy(to_recv.begin() + count,
+                                          to_recv.begin() + count + static_cast<std::ptrdiff_t>(i.size()),
+                                          neigh_tag.begin());
+                                if (erase)
+                                {
+                                    tag(level, i, index) = neigh_tag;
+                                }
+                                else
+                                {
+                                    tag(level, i, index) |= neigh_tag;
+                                }
+                                count += static_cast<std::ptrdiff_t>(i.size());
+                            }
+                        });
+                }
+            }
+            i_neigh++;
+        }
+        mpi::wait_all(req.begin(), req.end());
+#endif
+    }
+
     template <class Field>
     void update_tag_subdomains([[maybe_unused]] std::size_t level, [[maybe_unused]] Field& tag, [[maybe_unused]] bool erase = false)
     {
diff --git a/include/samurai/mr/adapt.hpp b/include/samurai/mr/adapt.hpp
index 958c14a7f..c2d1b9b85 100644
--- a/include/samurai/mr/adapt.hpp
+++ b/include/samurai/mr/adapt.hpp
@@ -262,10 +262,7 @@ namespace samurai
                           m_tag[cell] = static_cast<int>(CellFlag::keep);
                       });
 
-        for (std::size_t level = min_level; level <= max_level; ++level)
-        {
-            update_tag_subdomains(level, m_tag, true);
-        }
+        update_tag_subdomains(min_level, max_level, m_tag, true);
 
         times::timers.stop("mesh adaptation");
         update_ghost_mr(m_fields);

From 47ce2a669b7dd71771a18adeca3fe7e58f9665a7 Mon Sep 17 00:00:00 2001
From: sbstndb/sbstndbs <sebastiendubois4@gmail.com>
Date: Wed, 10 Sep 2025 10:12:11 +0200
Subject: [PATCH 2/2] fix: pre-commit

---
 include/samurai/algorithm/update.hpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/samurai/algorithm/update.hpp b/include/samurai/algorithm/update.hpp
index 9ae5454cd..8d32607e9 100644
--- a/include/samurai/algorithm/update.hpp
+++ b/include/samurai/algorithm/update.hpp
@@ -740,9 +740,9 @@ namespace samurai
 
     template <class Field>
     void update_tag_subdomains([[maybe_unused]] std::size_t min_level,
-                             [[maybe_unused]] std::size_t max_level,
-                             [[maybe_unused]] Field& tag,
-                             [[maybe_unused]] bool erase = false)
+                               [[maybe_unused]] std::size_t max_level,
+                               [[maybe_unused]] Field& tag,
+                               [[maybe_unused]] bool erase = false)
     {
 #ifdef SAMURAI_WITH_MPI
         using mesh_t    = typename Field::mesh_t;