Fix cumulative argmax/argmin returning window-local indices (GH#11336)

xarray/computation/rolling.py

@@ -177,7 +177,7 @@ def _reduce_method(  # type: ignore[misc]
         def method(self, keep_attrs=None, **kwargs):
             keep_attrs = self._get_keep_attrs(keep_attrs)
 
-            return self._array_reduce(
+            result = self._array_reduce(
                 array_agg_func=array_agg_func,
                 bottleneck_move_func=bottleneck_move_func,
                 numbagg_move_func=numbagg_move_func,
@@ -188,6 +188,11 @@ def method(self, keep_attrs=None, **kwargs):
                 **kwargs,
             )
 
+            if name in ("argmax", "argmin"):
+                result = self._adjust_argminmax_result(result)
+
+            return result
+
         method.__name__ = name
         method.__doc__ = _ROLLING_REDUCE_DOCSTRING_TEMPLATE.format(name=name)
         return method
@@ -580,6 +585,33 @@ def reduce(
         counts = self._counts(keep_attrs=False)
         return result.where(counts >= self.min_periods)
 
+    def _is_cumulative(self) -> bool:
+        for i, d in enumerate(self.dim):
+            if self.center[i]:
+                return False
+            if self.window[i] != self.obj.sizes[d]:
+                return False
+        return True
+
+    def _adjust_argminmax_result(self, result: DataArray) -> DataArray:
+        if not self._is_cumulative():
+            return result
+
+        for i, d in enumerate(self.dim):
+            window_size = self.window[i]
+            if window_size <= 1:
+                continue
+
+            n = result.sizes[d]
+            position = np.arange(n)
+            # GH#11336: cumulative windows have NaN padding on the left,
+            # so window-local indices need adjustment to become absolute:
+            # absolute = local - (window_size - 1 - position)
+            offset = window_size - 1 - position
+            result = result - offset
+
+        return result
+
     def _counts(self, keep_attrs: bool | None) -> DataArray:
         """Number of non-nan entries in each rolling window."""
 

xarray/tests/test_rolling.py

@@ -49,6 +49,31 @@ def test_cumulative_vs_cum(d) -> None:
     assert_identical(result, expected)
 
 
+@pytest.mark.parametrize("func", ["argmax", "argmin"])
+def test_cumulative_argminmax(func) -> None:
+    # Regression test for GH#11336: cumulative argmax/argmin should return
+    # absolute indices, not window-local indices
+    da = DataArray([3, 1, 4, 2, 5], dims=["time"])
+    result = getattr(da.cumulative("time"), func)()
+
+    if func == "argmax":
+        expected = DataArray([0, 0, 2, 2, 4], dims=["time"])
+    else:
+        expected = DataArray([0, 1, 1, 1, 1], dims=["time"])
+
+    assert_identical(result, expected)
+
+
+def test_cumulative_argmax_2d() -> None:
+    da = DataArray(
+        [[1, 3, 2], [4, 1, 5]],
+        dims=("x", "time"),
+    )
+    result = da.cumulative("time").argmax()
+    expected = DataArray([[0, 1, 1], [0, 0, 2]], dims=("x", "time"))
+    assert_identical(result, expected)
+
+
 class TestDataArrayRolling:
     @pytest.mark.parametrize("da", (1, 2), indirect=True)
     @pytest.mark.parametrize("center", [True, False])