DOC: Added reverse rolling window examples

doc/source/user_guide/window.rst

@@ -50,16 +50,16 @@ As noted above, some operations support specifying a window based on a time offs
 
 .. ipython:: python
 
-   s = pd.Series(range(5), index=pd.date_range('2020-01-01', periods=5, freq='1D'))
-   s.rolling(window='2D').sum()
+   s = pd.Series(range(5), index=pd.date_range("2020-01-01", periods=5, freq="1D"))
+   s.rolling(window="2D").sum()
 
 Additionally, some methods support chaining a ``groupby`` operation with a windowing operation
 which will first group the data by the specified keys and then perform a windowing operation per group.
 
 .. ipython:: python
 
-   df = pd.DataFrame({'A': ['a', 'b', 'a', 'b', 'a'], 'B': range(5)})
-   df.groupby('A').expanding().sum()
+   df = pd.DataFrame({"A": ["a", "b", "a", "b", "a"], "B": range(5)})
+   df.groupby("A").expanding().sum()
 
 .. note::
 
@@ -95,8 +95,11 @@ be calculated with :meth:`~Rolling.apply` by specifying a separate column of wei
        arr[:, :2] = (x[:, :2] * x[:, 2]).sum(axis=0) / x[:, 2].sum()
        return arr
 
+
    df = pd.DataFrame([[1, 2, 0.6], [2, 3, 0.4], [3, 4, 0.2], [4, 5, 0.7]])
-   df.rolling(2, method="table", min_periods=0).apply(weighted_mean, raw=True, engine="numba")  # noqa:E501
+   df.rolling(2, method="table", min_periods=0).apply(
+       weighted_mean, raw=True, engine="numba"
+   )  # noqa:E501
 
 
 All windowing operations support a ``min_periods`` argument that dictates the minimum amount of
@@ -132,13 +135,13 @@ time based index must be monotonic.
 
 .. ipython:: python
 
-   times = ['2020-01-01', '2020-01-03', '2020-01-04', '2020-01-05', '2020-01-29']
+   times = ["2020-01-01", "2020-01-03", "2020-01-04", "2020-01-05", "2020-01-29"]
    s = pd.Series(range(5), index=pd.DatetimeIndex(times))
    s
    # Window with 2 observations
    s.rolling(window=2).sum()
    # Window with 2 days worth of observations
-   s.rolling(window='2D').sum()
+   s.rolling(window="2D").sum()
 
 For all supported aggregation functions, see :ref:`api.functions_rolling`.
 
@@ -198,6 +201,53 @@ from present information back to past information. This allows the rolling windo
 
    df
 
+.. _window.reverse_rolling_window:
+
+Reverse rolling window
+~~~~~~~~~~~~~~~~~~~~~~
+
+Get the window of a rolling function to look forward.
+
+We can achieve this by using slicing in python by applying rolling aggregation and then flipping the result
+as shown in example below:
+
+.. ipython:: python
+
+   df = pd.DataFrame(
+       data=[
+           [pd.Timestamp("2018-01-01 00:00:00"), 100],
+           [pd.Timestamp("2018-01-01 00:00:01"), 101],
+           [pd.Timestamp("2018-01-01 00:00:03"), 103],
+           [pd.Timestamp("2018-01-01 00:00:04"), 111],
+       ],
+       columns=["time", "value"],
+   ).set_index("time")
+   df
+
+   df1 = df[::-1].rolling("2s").sum()[::-1]
+   df1
+
+Or we can also do it using FixedForwardWindowIndexer which basically Creates window boundaries
+for fixed-length windows that include the current row.
+
+.. ipython:: python
+
+   df = pd.DataFrame(
+       data=[
+           [pd.Timestamp("2018-01-01 00:00:00"), 100],
+           [pd.Timestamp("2018-01-01 00:00:01"), 101],
+           [pd.Timestamp("2018-01-01 00:00:03"), 103],
+           [pd.Timestamp("2018-01-01 00:00:04"), 111],
+       ],
+       columns=["time", "value"],
+   ).set_index("time")
+   df
+
+   indexer = pd.api.indexers.FixedForwardWindowIndexer(window_size=2)
+
+   df2 = df.rolling(window=indexer, min_periods=1).sum()
+   df2
+
 
 .. _window.custom_rolling_window:
 
@@ -280,11 +330,23 @@ conditions. In these cases it can be useful to perform forward-looking rolling w
 This :func:`BaseIndexer <pandas.api.indexers.BaseIndexer>` subclass implements a closed fixed-width
 forward-looking rolling window, and we can use it as follows:
 
-.. ipython:: ipython
+.. ipython:: python
+
+   df = pd.DataFrame(
+       data=[
+           [pd.Timestamp("2018-01-01 00:00:00"), 100],
+           [pd.Timestamp("2018-01-01 00:00:01"), 101],
+           [pd.Timestamp("2018-01-01 00:00:03"), 103],
+           [pd.Timestamp("2018-01-01 00:00:04"), 111],
+       ],
+       columns=["time", "value"],
+   ).set_index("time")
+   df
 
-   from pandas.api.indexers import FixedForwardWindowIndexer
-   indexer = FixedForwardWindowIndexer(window_size=2)
-   df.rolling(indexer, min_periods=1).sum()
+   indexer = pd.api.indexers.FixedForwardWindowIndexer(window_size=2)
+
+   df_out = df.rolling(window=indexer, min_periods=1).sum()
+   df_out
 
 
 .. _window.rolling_apply:
@@ -302,6 +364,7 @@ the windows are cast as :class:`Series` objects (``raw=False``) or ndarray objec
    def mad(x):
        return np.fabs(x - x.mean()).mean()
 
+
    s = pd.Series(range(10))
    s.rolling(window=4).apply(mad, raw=True)
 
@@ -407,11 +470,7 @@ can even be omitted:
 
 .. ipython:: python
 
-   covs = (
-       df[["B", "C", "D"]]
-       .rolling(window=4)
-       .cov(df[["A", "B", "C"]], pairwise=True)
-   )
+   covs = df[["B", "C", "D"]].rolling(window=4).cov(df[["A", "B", "C"]], pairwise=True)
    covs