Merge branch 'master' into 116/ascending-slice

Author: flexatone

performance/reference/util.py

@@ -25,6 +25,8 @@
 DTYPES_BOOL = (DTYPE_BOOL,)
 DTYPES_INEXACT = (DTYPE_FLOAT_DEFAULT, DTYPE_COMPLEX_DEFAULT)
 
+EMPTY_SLICE = slice(0, 0) # gathers nothing
+
 
 def mloc(array: np.ndarray) -> int:
     '''Return the memory location of an array.
@@ -261,5 +263,44 @@ def count_iteration(iterable: tp.Iterable):
     return count
 
 
+def slice_to_ascending_slice(
+        key: slice,
+        size: int
+        ) -> slice:
+    '''
+    Given a slice, return a slice that, with ascending integers, covers the same values.
+
+    Args:
+        size: the length of the container on this axis
+    '''
+    key_step = key.step
+    key_start = key.start
+    key_stop = key.stop
+
+    if key_step is None or key_step > 0:
+        return key
+
+    # will get rid of all negative values greater than the size; but will replace None with an appropriate number for usage in range
+    norm_key_start, norm_key_stop, norm_key_step = key.indices(size)
+
+    # everything else should be descending, but we might have non-descending start, stop
+    if key_start is not None and key_stop is not None:
+        if norm_key_start <= norm_key_stop: # an ascending range
+            return EMPTY_SLICE
+
+    norm_range = range(norm_key_start, norm_key_stop, norm_key_step)
+
+    # derive stop
+    if key_start is None:
+        stop = None
+    else:
+        stop = norm_range[0] + 1
+
+    if key_step == -1:
+        # gets last realized value, not last range value
+        return slice(None if key_stop is None else norm_range[-1], stop, 1)
+
+    return slice(norm_range[-1], stop, key_step * -1)
+
 
 

test/test_util.py

@@ -27,6 +27,7 @@
 from arraykit import get_new_indexers_and_screen
 
 from performance.reference.util import mloc as mloc_ref
+from performance.reference.util import slice_to_ascending_slice as slice_to_ascending_slice_ref
 
 
 class TestUnit(unittest.TestCase):