Add cell instance test with duplicated lattice

tests/unit_tests/test_cell_instance.py

@@ -0,0 +1,73 @@
+import numpy as np
+import pytest
+
+import openmc
+import openmc.lib
+
+from tests import cdtemp
+
+
+@pytest.fixture(scope='module', autouse=True)
+def double_lattice_model():
+    model = openmc.Model()
+
+    # Create a single material
+    m = openmc.Material()
+    m.add_nuclide('U235', 1.0)
+    m.set_density('g/cm3', 10.0)
+    model.materials.append(m)
+
+    # Create a universe with a single infinite cell
+    c = openmc.Cell(fill=m)
+    u = openmc.Universe(cells=[c])
+
+    # Create a 2x2 lattice filled with above universe
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (0.0, 0.0)
+    lattice.pitch = (1.0, 1.0)
+    lattice.universes = np.full((2, 2), u)
+
+    # Create two cells each filled with the same lattice, one from x=0..2 and
+    # y=0..2 and the other from x=2..4 and y=0..2
+    x0 = openmc.XPlane(0.0, boundary_type='vacuum')
+    x2 = openmc.XPlane(2.0)
+    x4 = openmc.XPlane(4.0, boundary_type='vacuum')
+    y0 = openmc.YPlane(0.0, boundary_type='vacuum')
+    y2 = openmc.YPlane(2.0, boundary_type='vacuum')
+    cell_with_lattice1 = openmc.Cell(fill=lattice, region=+x0 & -x2 & +y0 & -y2)
+    cell_with_lattice2 = openmc.Cell(fill=lattice, region=+x2 & -x4 & +y0 & -y2)
+    cell_with_lattice2.translation = (2., 0., 0.)
+    model.geometry = openmc.Geometry([cell_with_lattice1, cell_with_lattice2])
+
+    # Add necessary settings and export
+    model.settings.batches = 10
+    model.settings.inactive = 0
+    model.settings.particles = 100
+
+    with cdtemp():
+        model.export_to_xml()
+        openmc.lib.init()
+        yield
+        openmc.lib.finalize()
+
+
+# This shows the expected cell instance numbers for each lattice position:
+#      ┌─┬─┬─┬─┐
+#      │2│3│6│7│
+#      ├─┼─┼─┼─┤
+#      │0│1│4│5│
+#      └─┴─┴─┴─┘
+expected_results = [
+    ((0.5, 0.5, 0.0), 0),
+    ((1.5, 0.5, 0.0), 1),
+    ((0.5, 1.5, 0.0), 2),
+    ((1.5, 1.5, 0.0), 3),
+    ((2.5, 0.5, 0.0), 4),
+    ((3.5, 0.5, 0.0), 5),
+    ((2.5, 1.5, 0.0), 6),
+    ((3.5, 1.5, 0.0), 7),
+]
+@pytest.mark.parametrize("r,expected_cell_instance", expected_results)
+def test_cell_instance_multilattice(r, expected_cell_instance):
+    _, cell_instance = openmc.lib.find_cell(r)
+    assert cell_instance == expected_cell_instance