Patch sky map when catalog has empty partitions (#474)

* Handle empty partitions in inner skymap routine

* Improve test readability

src/lsdb/catalog/dataset/healpix_dataset.py

@@ -336,13 +336,14 @@ def skymap_data(
             have the default_value as its result, as well as any pixels for which the aggregate
             function returns None.
         """
+        results = {}
         partitions = self.to_delayed()
         if order is None:
             results = {
                 pixel: delayed(func)(partitions[index], pixel, **kwargs)
                 for pixel, index in self._ddf_pixel_map.items()
             }
-        else:
+        elif len(self.hc_structure.pixel_tree) > 0:
             if order < self.hc_structure.pixel_tree.get_max_depth():
                 raise ValueError(
                     f"order must be greater than or equal to max order in catalog "

src/lsdb/core/plotting/skymap.py

@@ -19,6 +19,12 @@ def perform_inner_skymap(
     **kwargs,
 ) -> np.ndarray:
     """Splits a partition into pixels at a target order and performs a given function on the new pixels"""
+    delta_order = target_order - pixel.order
+    img = np.full(1 << 2 * delta_order, fill_value=default_value)
+
+    if len(partition) == 0:
+        return img
+
     spatial_index = partition.index.to_numpy()
     order_pixels = spatial_index_to_healpix(spatial_index, target_order=target_order)
 
@@ -28,8 +34,6 @@ def apply_func(df):
         return func(df, HealpixPixel(target_order, p), **kwargs)
 
     gb = partition.groupby(order_pixels, sort=False).apply(apply_func)
-    delta_order = target_order - pixel.order
-    img = np.full(1 << 2 * delta_order, fill_value=default_value)
     min_pixel_value = pixel.pixel << 2 * delta_order
     img[gb.index.to_numpy() - min_pixel_value] = gb.to_numpy(na_value=default_value)
     return img

tests/lsdb/catalog/test_catalog.py

@@ -458,6 +458,45 @@ def func(df, healpix):
         assert np.array_equal(expected_arr, arr)
 
 
+def test_skymap_histogram_order_empty(small_sky_order1_catalog):
+    order = 3
+
+    def func(df, healpix):
+        return len(df) / hp.nside2pixarea(hp.order2nside(healpix.order), degrees=True)
+
+    catalog = small_sky_order1_catalog.cone_search(0, 0, 1)
+    _, non_empty_partitions = catalog._get_non_empty_partitions()
+    assert len(non_empty_partitions) == 0
+
+    img = catalog.skymap_histogram(func, order)
+    expected_img = np.zeros(hp.order2npix(order))
+    assert (img == expected_img).all()
+
+
+def test_skymap_histogram_order_some_partitions_empty(small_sky_order1_catalog):
+    order = 3
+
+    def func(df, healpix):
+        return len(df) / hp.nside2pixarea(hp.order2nside(healpix.order), degrees=True)
+
+    catalog = small_sky_order1_catalog.query("ra > 350 and dec < -50")
+    _, non_empty_partitions = catalog._get_non_empty_partitions()
+    assert 0 < len(non_empty_partitions) < catalog._ddf.npartitions
+
+    img = catalog.skymap_histogram(func, order)
+
+    pixel_map = catalog.skymap_data(func, order)
+    pixel_map = {pixel: value.compute() for pixel, value in pixel_map.items()}
+    expected_img = np.zeros(hp.order2npix(order))
+    for pixel, value in pixel_map.items():
+        dorder = order - pixel.order
+        start = pixel.pixel * (4**dorder)
+        end = (pixel.pixel + 1) * (4**dorder)
+        img_order_pixels = np.arange(start, end)
+        expected_img[img_order_pixels] = value
+    assert (img == expected_img).all()
+
+
 # pylint: disable=no-member
 def test_skymap_plot(small_sky_order1_catalog, mocker):
     mocker.patch("lsdb.catalog.dataset.healpix_dataset.plot_healpix_map")