add check for invalid start_index when inferring.

tests/test_grids/test_ugrid.py

@@ -387,6 +387,22 @@ def test_assign_ugrid_topology_start_index_zero_infer():
     assert ds["mesh_edge_nodes"].attrs["start_index"] == 0
     assert ds["mesh_boundary_nodes"].attrs["start_index"] == 0
 
+def test_assign_ugrid_topology_start_index_zero_infer_bad_data():
+    """
+    if the connectivity array has negative or no zero or one indexes -- not good.
+    """
+    # set a negative index
+    ds = xr.open_dataset(EXAMPLE_DATA / "small_ugrid_zero_based.nc")
+    faces = ds['mesh_face_nodes'].data[0, 0] = -3
+    with pytest.raises(ValueError):
+        ds = ugrid.assign_ugrid_topology(ds, face_node_connectivity="mesh_face_nodes")
+
+    # remove zero and one indexes
+    data = ds['mesh_face_nodes'].data
+    data[data < 2] = 100
+    with pytest.raises(ValueError):
+        ds = ugrid.assign_ugrid_topology(ds, face_node_connectivity="mesh_face_nodes")
+
 
 # NOTE: these tests are probably not complete -- but they are something.
 #       we really should have a complete UGRID example to test with.

xarray_subset_grid/grids/ugrid.py

@@ -464,8 +464,12 @@ def assign_ugrid_topology(
         mapping = [(dim, ds[dim].size) for dim in dims]
         mesh.face_dimension = next(x[0] for x in mapping if x[1] != 3 and x[1] != 4)
 
-    # check for start_index, and set it if there.
+    # check for start_index, and set it not there.
     if mesh.start_index is None:
+        start_index = int(ds[mesh.face_node_connectivity].min())
+        if start_index not in (0, 1):
+            raise ValueError(f"minimum index in face_node_connectivity array is {start_index}"
+                             " -- it should be zero (C-style indexing) or 1 (Fortran-style indexing)")
         mesh.start_index = int(ds[mesh.face_node_connectivity].min())
 
     if mesh.start_index not in (0, 1):