Add IndexMap::global_to_local to the Python interface (#2972)

* Add `global_to_local` to the Python interface.
Make it required to send in output array, as it makes sense from a performance perspective to let the user send in this array, as it might be used in loops.

* Add global indices

* Make indexmap local_to_global and global_to_local pure functions in python interface

* Simplify

python/dolfinx/wrappers/common.cpp

@@ -131,8 +131,18 @@ void common(nb::module_& m)
             self.local_to_global(std::span(local.data(), local.size()), global);
             return dolfinx_wrappers::as_nbarray(std::move(global));
           },
-          nb::arg("local"));
-
+          nb::arg("local"))
+      .def(
+          "global_to_local",
+          [](const dolfinx::common::IndexMap& self,
+             nb::ndarray<const std::int64_t, nb::ndim<1>, nb::c_contig> global)
+          {
+            std::vector<std::int32_t> local(global.size());
+            self.global_to_local(std::span(global.data(), global.size()),
+                                 local);
+            return dolfinx_wrappers::as_nbarray(std::move(local));
+          },
+          nb::arg("global"));
   // dolfinx::common::Timer
   nb::class_<dolfinx::common::Timer>(m, "Timer", "Timer class")
       .def(nb::init<>())

python/test/unit/common/test_index_map.py

@@ -91,6 +91,19 @@ def test_index_map_ghost_lifetime():
     map = dolfinx.common.IndexMap(comm, local_size, [dest, src], map_ghosts, src)
     assert map.size_global == local_size * comm.size
 
+    # Test global to local map
+    global_indices = np.arange(map.size_global, dtype=np.int64)
+    local_indices = map.global_to_local(global_indices)
+    for i, ghost in enumerate(map_ghosts):
+        assert local_indices[ghost] == local_size + i
+
+    # Create marker for all indices that are on process (local or ghost)
+    on_process = np.zeros(map.size_global, dtype=bool)
+    on_process[map.local_range[0]:map.local_range[1]] = True
+    on_process[map.ghosts] = True
+    assert (local_indices[on_process] >= 0).all()
+    assert np.allclose(local_indices[np.invert(on_process)], -1)
+
     # Test lifetime management
     ghosts = map.ghosts
     assert np.array_equal(ghosts, map_ghosts)
@@ -167,6 +180,5 @@ def test_create_submap_owner_change():
         assert np.array_equal(sub_imap.ghosts, [2 * (comm.rank + 1)])
         assert np.array_equal(sub_imap.owners, [comm.rank + 1])
         assert np.array_equal(sub_imap_to_imap, [0, 2, 3])
-
     global_indices = sub_imap.local_to_global(np.arange(sub_imap.size_local + sub_imap.num_ghosts, dtype=np.int32))
     assert np.array_equal(global_indices, np.arange(comm.rank * 2, comm.rank * 2 + 3))