fix roll_generic in window.pyx

add new test method for min_periods in test_window::TestRolling

Author: FXocena

pandas/_libs/window.pyx

@@ -1428,14 +1428,15 @@ def roll_generic(ndarray[float64_t, cast=True] input,
     if n == 0:
         return input
 
-    counts = roll_sum(np.concatenate([np.isfinite(input).astype(float),
-                                      np.array([0.] * offset)]),
-                      win, minp, index, closed)[offset:]
-    
     start, end, N, win, minp, is_variable = get_window_indexer(input, win,
                                                                minp, index,
                                                                closed,
                                                                floor=0)
+    
+    counts = roll_sum(np.concatenate([np.isfinite(input).astype(float),
+                                      np.array([0.] * offset)]),
+                      win, minp, index, closed)[offset:]
+
     output = np.empty(N, dtype=float)
 
 

pandas/tests/test_window.py

@@ -423,6 +423,26 @@ def test_constructor_with_timedelta_window(self):
             expected = df.rolling('3D').sum()
             tm.assert_frame_equal(result, expected)
 
+    def test_constructor_with_timedelta_window_and_minperiods(self):
+        # GH 15440
+        n = 10
+        df = pd.DataFrame({'value': np.arange(n)},
+                          index=pd.date_range('2015-12-24',
+                                              periods=n,
+                                              freq="D"))
+        expected_data = np.append([np.NaN, 1.], np.arange(3., 27., 3))
+        for window in [timedelta(days=3), pd.Timedelta(days=3)]:
+            result_roll_sum = df.rolling(window=window, min_periods=2).sum()
+            result_roll_generic = df.rolling(window=window, min_periods=2).apply(sum)
+            expected = pd.DataFrame({'value': expected_data},
+                                    index=pd.date_range('2015-12-24',
+                                                        periods=n,
+                                                        freq="D"))
+            tm.assert_frame_equal(result_roll_sum, expected)
+            tm.assert_frame_equal(result_roll_generic, expected)
+            expected = df.rolling('3D', min_periods=2).sum()
+            tm.assert_frame_equal(result_roll_generic, expected)
+            
     def test_numpy_compat(self):
         # see gh-12811
         r = rwindow.Rolling(Series([2, 4, 6]), window=2)