diff --git a/src/core/p3m/fft.cpp b/src/core/p3m/fft.cpp
index a14c9a2d00..bd6ad27e38 100644
--- a/src/core/p3m/fft.cpp
+++ b/src/core/p3m/fft.cpp
@@ -411,6 +411,7 @@ void back_grid_comm(fft_forw_plan plan_f, fft_back_plan plan_b,
                      plan_f.element);
   }
 }
+} // namespace
 
 /** Calculate 'best' mapping between a 2D and 3D grid.
  *  Required for the communication from 3D regular domain
@@ -461,7 +462,6 @@ int map_3don2d_grid(int const g3d[3], int g2d[3]) {
   }
   return row_dir;
 }
-} // namespace
 
 /** Calculate most square 2D grid. */
 static void calc_2d_grid(int n, int grid[3]) {
diff --git a/src/core/p3m/fft.hpp b/src/core/p3m/fft.hpp
index c87f11a14c..03dd4a0de6 100644
--- a/src/core/p3m/fft.hpp
+++ b/src/core/p3m/fft.hpp
@@ -232,4 +232,6 @@ void fft_pack_block(double const *in, double *out, int const start[3],
 void fft_unpack_block(double const *in, double *out, int const start[3],
                       int const size[3], int const dim[3], int element);
 
+int map_3don2d_grid(int const g3d[3], int g2d[3]);
+
 #endif // defined(P3M) || defined(DP3M)
diff --git a/src/core/p3m/math.hpp b/src/core/p3m/math.hpp
index a9f2bdf7fd..2da9b38dd1 100644
--- a/src/core/p3m/math.hpp
+++ b/src/core/p3m/math.hpp
@@ -61,7 +61,7 @@ template <typename T> DEVICE_QUALIFIER auto sinc(T x) {
   if (::math::abs(x) > epsilon)
     return sin(pix) / pix;
 
-  auto constexpr factorial = [](int n) {
+  auto constexpr factorial = [](int n) consteval {
     int acc{1}, c{1};
     while (c < n) {
       acc *= ++c;
diff --git a/src/core/unit_tests/fft_test.cpp b/src/core/unit_tests/fft_test.cpp
index bb27335279..6e301a48da 100644
--- a/src/core/unit_tests/fft_test.cpp
+++ b/src/core/unit_tests/fft_test.cpp
@@ -67,6 +67,97 @@ BOOST_AUTO_TEST_CASE(fft_find_comm_groups_mismatch) {
   }
 }
 
+BOOST_AUTO_TEST_CASE(fft_map_grid) {
+  {
+    auto g3d = Utils::Vector3i{{3, 2, 1}};
+    auto g2d = Utils::Vector3i{{3, 2, 1}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 2);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{2, 1, 6}};
+    auto g2d = Utils::Vector3i{{6, 2, 1}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 2);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{2, 6, 2}};
+    auto g2d = Utils::Vector3i{{6, 2, 6}};
+    auto ref = Utils::Vector3i{{6, 1, 2}};
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{3, 6, 6}};
+    auto g2d = Utils::Vector3i{{6, 3, 6}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, -1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{4, 1, 6}};
+    auto g2d = Utils::Vector3i{{6, 4, 1}};
+    auto ref = Utils::Vector3i{{4, 6, 1}};
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 2);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{5, 7, 7}};
+    auto g2d = Utils::Vector3i{{7, 7, 5}};
+    auto ref = Utils::Vector3i{{1, 7, 7}};
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 0);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{5, 7, 5}};
+    auto g2d = Utils::Vector3i{{7, 7, 5}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, -1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{4, 5, 6}};
+    auto g2d = Utils::Vector3i{{6, 4, 5}};
+    auto ref = Utils::Vector3i{{4, 1, 6}};
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{5, 4, 6}};
+    auto g2d = Utils::Vector3i{{6, 4, 5}};
+    auto ref = Utils::Vector3i{{1, 4, 6}};
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, 0);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{5, 6, 8}};
+    auto g2d = Utils::Vector3i{{8, 7, 5}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, -1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+  {
+    auto g3d = Utils::Vector3i{{5, 6, 9}};
+    auto g2d = Utils::Vector3i{{8, 7, 5}};
+    auto ref = g2d;
+    auto dir = map_3don2d_grid(g3d.data(), g2d.data());
+    BOOST_CHECK_EQUAL(dir, -1);
+    BOOST_CHECK_EQUAL(g2d, ref);
+  }
+}
+
 BOOST_AUTO_TEST_CASE(fft_exceptions) {
   auto constexpr size_max = std::numeric_limits<std::size_t>::max();
   auto constexpr bad_size = size_max / sizeof(int) + 1ul;
diff --git a/testsuite/scripts/tutorials/test_ferrofluid_1.py b/testsuite/scripts/tutorials/test_ferrofluid_1.py
index f6ce5e78ca..7da0aa14ef 100644
--- a/testsuite/scripts/tutorials/test_ferrofluid_1.py
+++ b/testsuite/scripts/tutorials/test_ferrofluid_1.py
@@ -22,7 +22,7 @@
 
 tutorial, skipIfMissingFeatures = importlib_wrapper.configure_and_import(
     "@TUTORIALS_DIR@/ferrofluid/ferrofluid_part1.py",
-    EQUIL_STEPS=200, EQUIL_ROUNDS=10,
+    EQUIL_STEPS=200, EQUIL_ROUNDS=20,
     CI_DP3M_PARAMS={'cao': 3, 'r_cut': 8.34, 'mesh': [8, 8, 8], 'alpha': 0.2115, 'tune': False})
 
 
@@ -33,7 +33,7 @@ class Tutorial(ut.TestCase):
     def test(self):
         self.assertEqual(
             int(np.sum(tutorial.n_clusters)), len(tutorial.cluster_sizes))
-        for i in range(8):
+        for i in range(7):
             self.assertLess(
                 tutorial.size_dist[0][i + 1],
                 tutorial.size_dist[0][i])