FIX: reverse index output of bottleneck move_argmax/move_argmin functions

doc/whats-new.rst

@@ -38,6 +38,9 @@ Deprecations
 Bug fixes
 ~~~~~~~~~
 
+- Reverse index output of bottleneck's rolling move_argmax/move_argmin functions (:issue:`8541`, :pull:`8552`).
+  By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/rolling.py

@@ -596,6 +596,11 @@ def _bottleneck_reduce(self, func, keep_attrs, **kwargs):
             values = func(
                 padded.data, window=self.window[0], min_count=min_count, axis=axis
             )
+            # index 0 is at the rightmost edge of the window
+            # need to reverse index here
+            # see GH #8541
+            if func in [bottleneck.move_argmin, bottleneck.move_argmax]:
+                values = self.window[0] - 1 - values
 
         if self.center[0]:
             values = values[valid]

xarray/tests/test_rolling.py

@@ -95,7 +95,9 @@ def test_rolling_properties(self, da) -> None:
         ):
             da.rolling(foo=2)
 
-    @pytest.mark.parametrize("name", ("sum", "mean", "std", "min", "max", "median"))
+    @pytest.mark.parametrize(
+        "name", ("sum", "mean", "std", "min", "max", "median", "argmin", "argmax")
+    )
     @pytest.mark.parametrize("center", (True, False, None))
     @pytest.mark.parametrize("min_periods", (1, None))
     @pytest.mark.parametrize("backend", ["numpy"], indirect=True)
@@ -108,9 +110,15 @@ def test_rolling_wrapped_bottleneck(
 
         func_name = f"move_{name}"
         actual = getattr(rolling_obj, name)()
+        window = 7
         expected = getattr(bn, func_name)(
-            da.values, window=7, axis=1, min_count=min_periods
+            da.values, window=window, axis=1, min_count=min_periods
         )
+        # index 0 is at the rightmost edge of the window
+        # need to reverse index here
+        # see GH #8541
+        if func_name in ["move_argmin", "move_argmax"]:
+            expected = window - 1 - expected
 
         # Using assert_allclose because we get tiny (1e-17) differences in numbagg.
         np.testing.assert_allclose(actual.values, expected)