TST: add test

Author: AlexKirko

pandas/tests/window/common.py

@@ -5,6 +5,7 @@
 
 from pandas import DataFrame, Series, bdate_range, notna
 import pandas._testing as tm
+from pandas.api.indexers import BaseIndexer
 
 N, K = 100, 10
 
@@ -384,3 +385,18 @@ def check_binary_ew_min_periods(name, min_periods, A, B):
         Series([1.0]), Series([1.0]), 50, name=name, min_periods=min_periods
     )
     tm.assert_series_equal(result, Series([np.NaN]))
+
+
+class ForwardIndexer(BaseIndexer):
+    # GH 32865
+    def get_window_bounds(self, num_values, min_periods, center, closed):
+        start = np.empty(num_values, dtype=np.int64)
+        end = np.empty(num_values, dtype=np.int64)
+        for i in range(num_values):
+            if i + min_periods <= num_values:
+                start[i] = i
+                end[i] = min(i + self.window_size, num_values)
+            else:
+                start[i] = i
+                end[i] = i + 1
+        return start, end

pandas/tests/window/test_rolling.py

@@ -10,7 +10,7 @@
 from pandas import DataFrame, Index, Series
 import pandas._testing as tm
 from pandas.core.window import Rolling
-from pandas.tests.window.common import Base
+from pandas.tests.window.common import Base, ForwardIndexer
 
 
 class TestRolling(Base):
@@ -465,3 +465,16 @@ def test_rolling_count_default_min_periods_with_null_values(constructor):
     result = constructor(values).rolling(3).count()
     expected = constructor(expected_counts)
     tm.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize("constructor", [Series, DataFrame])
+def test_rolling_forward_window_min(constructor):
+    # GH 32865
+    values = np.arange(10)
+    expected_min = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, np.nan]
+
+    indexer = ForwardIndexer(window_size=3)
+    rolling = constructor(values).rolling(window=indexer, min_periods=2)
+    result = rolling.min()
+    expected = constructor(rolling.apply(lambda x: min(x)))
+    tm.assert_equal(result, expected)