Warn/error if too many structures are of the same medium

Author: momchil-flex

tests/test_components/test_simulation.py

@@ -2146,19 +2146,19 @@ def test_tfsf_structures_grid():
 
 
 @pytest.mark.parametrize(
-    "size, num_struct, log_level", [(1, 1, None), (50, 1, "WARNING"), (1, 11000, "WARNING")]
+    "size, num_struct, log_level", [(1, 1, None), (50, 1, "WARNING"), (1, 11, "WARNING")]
 )
-def test_warn_large_epsilon(size, num_struct, log_level):
+def test_warn_large_epsilon(monkeypatch, size, num_struct, log_level):
     """Make sure we get a warning if the epsilon grid is too large."""
 
+    monkeypatch.setattr(simulation, "NUM_STRUCTURES_WARN_EPSILON", 10)
     structures = [
         td.Structure(
             geometry=td.Box(center=(0, 0, 0), size=(0.1, 0.1, 0.1)),
-            medium=td.Medium(permittivity=1.0),
+            medium=td.Medium(permittivity=eps),
         )
-        for _ in range(num_struct)
+        for eps in np.linspace(1, 2, num_struct)
     ]
-
     sim = td.Simulation(
         size=(size, size, size),
         grid_spec=td.GridSpec.uniform(dl=0.1),
@@ -3715,3 +3715,44 @@ def test_messages_contain_object_names():
     monitor = td.FieldMonitor(name=name, center=(-1.0, 0, 0), size=(0.5, 0, 1), freqs=[100e14])
     with pytest.raises(pydantic.ValidationError, match=name) as e:
         _ = sim.updated_copy(monitors=[monitor])
+
+
+def test_structures_per_medium(monkeypatch):
+    """Test if structures that share the same medium warn or error appropriately."""
+    import tidy3d.components.scene as scene
+
+    # Set low thresholds to keep the test fast; ensure len(structures) > MAX to avoid early return
+    monkeypatch.setattr(scene, "WARN_STRUCTURES_PER_MEDIUM", 2)
+    monkeypatch.setattr(scene, "MAX_STRUCTURES_PER_MEDIUM", 4)
+
+    shared_med = td.Medium(permittivity=2.0)
+    # 3 share the same medium -> triggers warning (> WARN), but <= MAX per-medium
+    same_medium_structs = [
+        td.Structure(geometry=td.Box(size=(1, 1, 1)), medium=shared_med) for _ in range(3)
+    ]
+    # Add two with different mediums so total structures > MAX but error should not be triggered
+    other_structs = [
+        td.Structure(geometry=td.Box(size=(1, 1, 1)), medium=td.Medium(permittivity=3.0)),
+        td.Structure(geometry=td.Box(size=(1, 1, 1)), medium=td.Medium(permittivity=4.0)),
+    ]
+    structs = same_medium_structs + other_structs  # total = 5 > MAX = 4
+
+    with AssertLogLevel("WARNING", contains_str="use the same medium"):
+        _ = td.Simulation(
+            size=(10, 10, 10),
+            run_time=1e-12,
+            grid_spec=td.GridSpec.uniform(dl=0.02),
+            structures=structs,
+        )
+
+    # Now test error
+    monkeypatch.setattr(scene, "MAX_STRUCTURES_PER_MEDIUM", 3, raising=False)
+    structs = [td.Structure(geometry=td.Box(size=(1, 1, 1)), medium=shared_med) for _ in range(4)]
+
+    with pytest.raises(pydantic.ValidationError, match="use the same medium"):
+        _ = td.Simulation(
+            size=(10, 10, 10),
+            run_time=1e-12,
+            grid_spec=td.GridSpec.uniform(dl=0.02),
+            structures=structs,
+        )

tidy3d/components/base.py

@@ -212,7 +212,7 @@ def _special_characters_not_in_name(cls, values):
         "operation of Tidy3D as it is not used internally. "
         "Note that, unlike regular Tidy3D fields, ``attrs`` are mutable. "
         "For example, the following is allowed for setting an ``attr`` ``obj.attrs['foo'] = bar``. "
-        "Also note that `Tidy3D`` will raise a ``TypeError`` if ``attrs`` contain objects "
+        "Also note that Tidy3D will raise a ``TypeError`` if ``attrs`` contain objects "
         "that can not be serialized. One can check if ``attrs`` are serializable "
         "by calling ``obj.json()``.",
     )

tidy3d/components/scene.py

@@ -79,6 +79,10 @@
 # maximum geometry count in a single structure
 MAX_GEOMETRY_COUNT = 100
 
+# warn and error out if the same medium is present in too many structures
+WARN_STRUCTURES_PER_MEDIUM = 200
+MAX_STRUCTURES_PER_MEDIUM = 1_000
+
 
 class Scene(Tidy3dBaseModel):
     """Contains generic information about the geometry and medium properties common to all types of
@@ -176,6 +180,43 @@ def _validate_num_geometries(cls, val):
 
         return val
 
+    @pd.validator("structures", always=True)
+    def _validate_structures_per_medium(cls, val):
+        """Error if too many structures share the same medium; suggest using GeometryGroup."""
+        if val is None:
+            return val
+
+        # if total structures are <= warn limit, the constraint cannot be violated.
+        if len(val) <= WARN_STRUCTURES_PER_MEDIUM:
+            return val
+
+        # Count structures per medium
+        counts = {}
+        get = counts.get
+        for structure in val:
+            key = structure.medium
+            new_count = get(key, 0) + 1
+            # Exit early to avoid slow counting if many structures present
+            if new_count > MAX_STRUCTURES_PER_MEDIUM:
+                raise SetupError(
+                    f"More than {MAX_STRUCTURES_PER_MEDIUM} structures use the same medium. "
+                    "For performance, use a 'GeometryGroup' or boolean operations to combine "
+                    "geometries that share a medium."
+                )
+            counts[key] = new_count
+
+        # Now check if we should warn
+        for count in counts.values():
+            if count > WARN_STRUCTURES_PER_MEDIUM:
+                log.warning(
+                    f"More than {WARN_STRUCTURES_PER_MEDIUM} structures use the same medium. "
+                    "For performance, use a 'GeometryGroup' or boolean operations to combine "
+                    "geometries that share a medium."
+                )
+                break
+
+        return val
+
     """ Accounting """
 
     @cached_property