Fix DatetimeIndex.loc for all types of ordering cases (#15761)

Fixes: #15742 

This PR resolves issues with returning incorrect ranges for `DatetimeIndex.loc` when the index objects are monotonically decreasing. Additionally, I went ahead and fixed it for all cases, (i.e., random ordering) too.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)

URL: #15761

python/cudf/cudf/core/indexed_frame.py

@@ -194,7 +194,6 @@ def _get_label_range_or_mask(index, start, stop, step):
     if (
         not (start is None and stop is None)
         and type(index) is cudf.core.index.DatetimeIndex
-        and index.is_monotonic_increasing is False
     ):
         start = pd.to_datetime(start)
         stop = pd.to_datetime(stop)
@@ -205,19 +204,28 @@ def _get_label_range_or_mask(index, start, stop, step):
                 # when we have a non-monotonic datetime index, return
                 # values in the slice defined by index_of(start) and
                 # index_of(end)
-                start_loc = index.get_loc(start.to_datetime64())
-                stop_loc = index.get_loc(stop.to_datetime64()) + 1
+                start_loc = index.get_loc(start)
+                stop_loc = index.get_loc(stop) + 1
                 return slice(start_loc, stop_loc)
             else:
                 raise KeyError(
                     "Value based partial slicing on non-monotonic "
                     "DatetimeIndexes with non-existing keys is not allowed.",
                 )
         elif start is not None:
-            boolean_mask = index >= start
+            if index.is_monotonic_increasing:
+                return index >= start
+            elif index.is_monotonic_decreasing:
+                return index <= start
+            else:
+                return index.find_label_range(slice(start, stop, step))
         else:
-            boolean_mask = index <= stop
-        return boolean_mask
+            if index.is_monotonic_increasing:
+                return index <= stop
+            elif index.is_monotonic_decreasing:
+                return index >= stop
+            else:
+                return index.find_label_range(slice(start, stop, step))
     else:
         return index.find_label_range(slice(start, stop, step))
 

python/cudf/cudf/tests/test_indexing.py

@@ -2275,3 +2275,77 @@ def test_loc_setitem_empty_dataframe():
     gdf.loc[["index_1"], "new_col"] = "A"
 
     assert_eq(pdf, gdf)
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        [15, 14, 12, 10, 1],
+        [1, 10, 12, 14, 15],
+    ],
+)
+@pytest.mark.parametrize(
+    "scalar",
+    [
+        1,
+        10,
+        15,
+        14,
+        0,
+        2,
+    ],
+)
+def test_loc_datetime_monotonic_with_ts(data, scalar):
+    gdf = cudf.DataFrame(
+        {"a": [1, 1, 1, 2, 2], "b": [1, 2, 3, 4, 5]},
+        index=cudf.Index(data, dtype="datetime64[ns]"),
+    )
+    pdf = gdf.to_pandas()
+
+    i = pd.Timestamp(scalar)
+
+    actual = gdf.loc[i:]
+    expected = pdf.loc[i:]
+
+    assert_eq(actual, expected)
+
+    actual = gdf.loc[:i]
+    expected = pdf.loc[:i]
+
+    assert_eq(actual, expected)
+
+
+@pytest.mark.parametrize("data", [[15, 14, 3, 10, 1]])
+@pytest.mark.parametrize("scalar", [1, 10, 15, 14, 0, 2])
+def test_loc_datetime_random_with_ts(data, scalar):
+    gdf = cudf.DataFrame(
+        {"a": [1, 1, 1, 2, 2], "b": [1, 2, 3, 4, 5]},
+        index=cudf.Index(data, dtype="datetime64[ns]"),
+    )
+    pdf = gdf.to_pandas()
+
+    i = pd.Timestamp(scalar)
+
+    if i not in pdf.index:
+        assert_exceptions_equal(
+            lambda: pdf.loc[i:],
+            lambda: gdf.loc[i:],
+            lfunc_args_and_kwargs=([],),
+            rfunc_args_and_kwargs=([],),
+        )
+        assert_exceptions_equal(
+            lambda: pdf.loc[:i],
+            lambda: gdf.loc[:i],
+            lfunc_args_and_kwargs=([],),
+            rfunc_args_and_kwargs=([],),
+        )
+    else:
+        actual = gdf.loc[i:]
+        expected = pdf.loc[i:]
+
+        assert_eq(actual, expected)
+
+        actual = gdf.loc[:i]
+        expected = pdf.loc[:i]
+
+        assert_eq(actual, expected)