Merge pull request #270 from astronomy-commons/sean/fix-outer-alignme…

…nt-bug

Fix infinite loop in outer alignment edge case

src/hipscat/pixel_tree/pixel_alignment.py

@@ -237,9 +237,12 @@ def _add_pixels_until(
     """
     while add_from < add_to:
         # maximum power of 4 that is a factor of add_from
-        max_p4_from = add_from & -add_from
-        if max_p4_from & 0xAAAAAAAAAAAAAAA:
-            max_p4_from = max_p4_from >> 1
+        # If add_from is 0, can add any power of 4, so use largest healpix order of 4**29
+        max_p4_from = 1 << 58
+        if add_from != 0:
+            max_p4_from = add_from & -add_from
+            if max_p4_from & 0xAAAAAAAAAAAAAAA:
+                max_p4_from = max_p4_from >> 1
 
         # maximum power of 4 less than or equal to (add_to - add_from)
         max_p4_to_log2 = np.int64(np.log2(add_to - add_from))

tests/hipscat/pixel_tree/test_pixel_alignment.py

@@ -242,3 +242,14 @@ def test_catalog_align_outer(pixel_tree_2, pixel_tree_3, aligned_trees_2_3_outer
     alignment = left_cat_with_moc.align(right_cat_with_moc, alignment_type="outer")
     assert_trees_equal(alignment.pixel_tree, both_moc_correct_tree)
     assert_mapping_matches_tree(alignment)
+
+
+def test_outer_align_start_0():
+    left_tree = PixelTree.from_healpix([HealpixPixel(0, 0)])
+    right_tree = PixelTree.from_healpix([HealpixPixel(1, 1)])
+    alignment = align_trees(left_tree, right_tree, alignment_type="outer")
+    expected_tree = PixelTree.from_healpix(
+        [HealpixPixel(1, 0), HealpixPixel(1, 1), HealpixPixel(1, 2), HealpixPixel(1, 3)]
+    )
+    assert_trees_equal(alignment.pixel_tree, expected_tree)
+    assert_mapping_matches_tree(alignment)