apacheGH-38768: Slicing an array backwards beyond the start now inclu…

…des first item

python/pyarrow/array.pxi

@@ -550,32 +550,36 @@ def _normalize_slice(object arrow_obj, slice key):
  Py_ssize_t start, stop, step
  Py_ssize_t n = len(arrow_obj)
 
- start = key.start or 0
- if start < 0:
- start += n
+ step = key.step or 1
+
+ if key.start is None:
+ if step < 0:
+ start = n - 1
+ else:
+ start = 0
+ elif key.start < 0:
+ start = key.start + n
  if start < 0:
  start = 0
- elif start >= n:
+ elif key.start >= n:
  start = n
+ else:
+ start = key.start
 
- stop = key.stop if key.stop is not None else n
- if stop < 0:
- stop += n
- if stop < 0:
+ if step < 0 and (key.stop is None or key.stop < -n):
+ stop = -1
+ elif key.stop is None:
+ stop = n
+ elif key.stop < 0:
+ stop = key.stop + n
+ if stop < 0: # step > 0 in this case.
  stop = 0
- elif stop >= n:
+ elif key.stop >= n:
  stop = n
+ else:
+ stop = key.stop
 
- step = key.step or 1
  if step != 1:
- if step < 0:
- # Negative steps require some special handling
- if key.start is None:
- start = n - 1
-
- if key.stop is None:
- stop = -1
-
  indices = np.arange(start, stop, step)
  return arrow_obj.take(indices)
  else:

python/pyarrow/tests/test_array.py

@@ -485,6 +485,7 @@ def test_array_slice_negative_step():
  slice(10, 2, -2),
  slice(None, None, 2),
  slice(0, 10, 2),
+ slice(15, -25, -1), # GH-38768
  ]
 
  for case in cases: