Fix negative slice indexing when combined with int indexing (#935)

Author: tomwhite

cubed/core/indexing.py

@@ -47,9 +47,13 @@ def index(x, key):
 
     # Use trick from xarray for negative step values
     where_negative_step = []
+    num_integer_dims = 0
     for i, ia in enumerate(idx.args):
-        if isinstance(ia, ndindex.Slice) and ia.step < 0:
-            where_negative_step.append(i)
+        if isinstance(ia, ndindex.Integer):
+            num_integer_dims += 1
+        elif isinstance(ia, ndindex.Slice) and ia.step < 0:
+            # Axis in the output drops integer-indexed dimensions, so adjust
+            where_negative_step.append(i - num_integer_dims)
             pos_slice = _convert_slice_with_negative_step(selection[i], x.shape[i])
             selection[i] = pos_slice
     where_negative_step = tuple(where_negative_step)

cubed/tests/test_array_api.py

@@ -299,6 +299,7 @@ def test_index_1d(spec, ind):
         (slice(None), slice(2, 4), xp.newaxis),
         (slice(None), 1),
         (1, slice(2, 4)),
+        (0, slice(1, None, -1)),
     ],
 )
 def test_index_2d(spec, ind):
@@ -375,6 +376,14 @@ def test_index_1d_step(spec, shape, chunks, ind, new_chunks_expected):
             (slice(14, 3, -2), slice(14, 3, -3)),
             ((4, 2), (4,),),
         ),
+        # integer index on axis 0 drops a dimension; negative step on axis 1
+        # must flip on axis 0 of the output, not axis 1 of the input
+        (
+            (1, 2),
+            (1, 2),
+            (0, slice(1, None, -1)),
+            ((2,),),
+        ),
     ],
 )
 # fmt: on