Add more unit tests

src/hats/catalog/healpix_dataset/healpix_dataset.py

@@ -200,8 +200,6 @@ def filter_by_moc(self, moc: MOC) -> Self:
         Returns:
             A new catalog with only the pixels that overlap with the moc. Note that we reset the total_rows
         to 0, as updating would require a scan over the new pixel sizes."""
-        if len(self.pixel_tree) == 0:
-            raise ValueError("Cannot filter empty catalog")
         filtered_tree = filter_by_moc(self.pixel_tree, moc)
         filtered_moc = self.moc.intersection(moc) if self.moc is not None else None
         filtered_catalog_info = self.catalog_info.copy_and_update(total_rows=0)

src/hats/pixel_tree/moc_filter.py

@@ -18,6 +18,8 @@ def filter_by_moc(
     Returns:
         A new PixelTree object with only the pixels from the input tree that overlap with the moc.
     """
+    if len(tree) == 0:
+        return tree
     moc_ranges = moc.to_depth29_ranges
     # Convert tree intervals to order 29 to match moc intervals
     tree_29_ranges = tree.tree << (2 * (29 - tree.tree_order))

tests/hats/catalog/test_catalog.py

@@ -10,6 +10,7 @@
 
 import hats.pixel_math.healpix_shim as hp
 from hats.catalog import Catalog, PartitionInfo, TableProperties
+from hats.catalog.healpix_dataset.healpix_dataset import HealpixDataset
 from hats.io import paths
 from hats.io.file_io import read_fits_image
 from hats.loaders import read_hats
@@ -147,6 +148,12 @@ def test_max_coverage_order(small_sky_order1_catalog):
     )
 
 
+def test_max_coverage_order_empty_catalog(catalog_info):
+    empty_catalog = HealpixDataset(catalog_info, PixelTree.from_healpix([]))
+    with pytest.raises(ValueError, match="empty catalog"):
+        empty_catalog.get_max_coverage_order()
+
+
 def test_cone_filter(small_sky_order1_catalog):
     ra = 315
     dec = -66.443

tests/hats/pixel_tree/test_moc_filter.py

@@ -3,6 +3,7 @@
 
 from hats.pixel_math import HealpixPixel
 from hats.pixel_tree.moc_filter import filter_by_moc
+from hats.pixel_tree.pixel_tree import PixelTree
 
 
 def test_moc_filter(pixel_tree_2):
@@ -43,6 +44,15 @@ def test_moc_filter_higher_order(pixel_tree_2):
     ]
 
 
+def test_moc_filter_with_empty_tree():
+    orders = np.array([1, 1, 2])
+    pixels = np.array([45, 46, 128])
+    moc = MOC.from_healpix_cells(pixels, orders, 2)
+    empty_tree = PixelTree.from_healpix([])
+    filtered_tree = filter_by_moc(empty_tree, moc)
+    assert filtered_tree.get_healpix_pixels() == []
+
+
 def test_moc_filter_empty_moc(pixel_tree_2):
     orders = np.array([])
     pixels = np.array([])