Fix wrong number of mesh levels when grid is multiple of refinement factor

CHANGELOG.md

@@ -18,6 +18,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Fix wrong number of mesh levels when grid is multiple of refinement factor
+  [\#26](https://github.com/mllam/weather-model-graphs/pull/26)
+  @joeloskarsson
+
 - Create different number of mesh nodes in x- and y-direction.
   [\#21](https://github.com/mllam/weather-model-graphs/pull/21)
   @joeloskarsson

src/weather_model_graphs/create/mesh/mesh.py

@@ -113,12 +113,14 @@ def create_multirange_2d_mesh_graphs(
     # Find the number of mesh levels possible in x- and y-direction,
     # and the number of leaf nodes that would correspond to
     # max_coord/(grid_refinement_factor*level_refinement_factor^mesh_levels) = 1
-    max_mesh_levels = (
-        (np.log(coord_extent) - np.log(grid_refinement_factor))
-        / np.log(level_refinement_factor)
-    ).astype(
-        int
-    )  # (2,)
+    max_mesh_levels_float = (
+        np.log(coord_extent) - np.log(grid_refinement_factor)
+    ) / np.log(level_refinement_factor)
+
+    # Need to add a small epsilon before rounding to int, due to numerical
+    # issues with the computation above
+    eps = 1e-8
+    max_mesh_levels = (max_mesh_levels_float + eps).astype(int)  # (2,)
     nleaf = grid_refinement_factor * (
         level_refinement_factor**max_mesh_levels
     )  # leaves at the bottom in each direction, if using max_mesh_levels

tests/test_graph_creation.py

@@ -120,3 +120,23 @@ def test_create_rectangular_graph(kind):
     fn_name = f"create_{kind}_graph"
     fn = getattr(wmg.create.archetype, fn_name)
     fn(xy_grid=xy, grid_refinement_factor=2)
+
+
+@pytest.mark.parametrize("grid_refinement_factor", (2, 3))
+@pytest.mark.parametrize("level_refinement_factor", (2, 3, 5))
+def test_create_exact_refinement(grid_refinement_factor, level_refinement_factor):
+    """
+    This test is to check that it is possible to create graph hierarchies when
+    the refinement factors are an exact multiple of the number of nodes. In these
+    situations it should be possible to create multi-level graphs, but it was not
+    earlier due to numerical issues.
+    """
+    N = grid_refinement_factor * (level_refinement_factor**2)
+    xy = _create_rectangular_fake_xy(Nx=N, Ny=N)
+
+    # Build hierarchical graph, should have 2 levels and not give error
+    wmg.create.archetype.create_oskarsson_hierarchical_graph(
+        xy,
+        grid_refinement_factor=grid_refinement_factor,
+        level_refinement_factor=level_refinement_factor,
+    )