Use numpy & dask sliding_window_view for rolling

doc/user-guide/duckarrays.rst

@@ -42,10 +42,9 @@ the code will still cast to ``numpy`` arrays:
     :py:meth:`DataArray.interp` and :py:meth:`DataArray.interp_like` (uses ``scipy``):
     duck arrays in data variables and non-dimension coordinates will be casted in
     addition to not supporting duck arrays in dimension coordinates
+  * :py:meth:`Dataset.rolling` and :py:meth:`DataArray.rolling` (requires ``numpy>=1.20``)
   * :py:meth:`Dataset.rolling_exp` and :py:meth:`DataArray.rolling_exp` (uses
     ``numbagg``)
-  * :py:meth:`Dataset.rolling` and :py:meth:`DataArray.rolling` (uses internal functions
-    of ``numpy``)
   * :py:meth:`Dataset.interpolate_na` and :py:meth:`DataArray.interpolate_na` (uses
     :py:class:`numpy.vectorize`)
   * :py:func:`apply_ufunc` with ``vectorize=True`` (uses :py:class:`numpy.vectorize`)

xarray/core/dask_array_compat.py

@@ -93,3 +93,132 @@ def nanmedian(a, axis=None, keepdims=False):
         )
 
         return result
+
+
+if LooseVersion(dask_version) > LooseVersion("2.30.0"):
+    ensure_minimum_chunksize = da.overlap.ensure_minimum_chunksize
+else:
+
+    # copied from dask
+    def ensure_minimum_chunksize(size, chunks):
+        """Determine new chunks to ensure that every chunk >= size
+
+        Parameters
+        ----------
+        size: int
+            The maximum size of any chunk.
+        chunks: tuple
+            Chunks along one axis, e.g. ``(3, 3, 2)``
+
+        Examples
+        --------
+        >>> ensure_minimum_chunksize(10, (20, 20, 1))
+        (20, 11, 10)
+        >>> ensure_minimum_chunksize(3, (1, 1, 3))
+        (5,)
+
+        See Also
+        --------
+        overlap
+        """
+        if size <= min(chunks):
+            return chunks
+
+        # add too-small chunks to chunks before them
+        output = []
+        new = 0
+        for c in chunks:
+            if c < size:
+                if new > size + (size - c):
+                    output.append(new - (size - c))
+                    new = size
+                else:
+                    new += c
+            if new >= size:
+                output.append(new)
+                new = 0
+            if c >= size:
+                new += c
+        if new >= size:
+            output.append(new)
+        elif len(output) >= 1:
+            output[-1] += new
+        else:
+            raise ValueError(
+                f"The overlapping depth {size} is larger than your "
+                f"array {sum(chunks)}."
+            )
+
+        return tuple(output)
+
+
+if LooseVersion(dask_version) > LooseVersion("2021.03.0"):
+    sliding_window_view = da.lib.stride_tricks.sliding_window_view
+else:
+
+    def sliding_window_view(x, window_shape, axis=None):
+        from dask.array.overlap import map_overlap
+        from numpy.core.numeric import normalize_axis_tuple  # type: ignore
+
+        from .npcompat import sliding_window_view as _np_sliding_window_view
+
+        window_shape = (
+            tuple(window_shape) if np.iterable(window_shape) else (window_shape,)
+        )
+
+        window_shape_array = np.array(window_shape)
+        if np.any(window_shape_array <= 0):
+            raise ValueError("`window_shape` must contain positive values")
+
+        if axis is None:
+            axis = tuple(range(x.ndim))
+            if len(window_shape) != len(axis):
+                raise ValueError(
+                    f"Since axis is `None`, must provide "
+                    f"window_shape for all dimensions of `x`; "
+                    f"got {len(window_shape)} window_shape elements "
+                    f"and `x.ndim` is {x.ndim}."
+                )
+        else:
+            axis = normalize_axis_tuple(axis, x.ndim, allow_duplicate=True)
+            if len(window_shape) != len(axis):
+                raise ValueError(
+                    f"Must provide matching length window_shape and "
+                    f"axis; got {len(window_shape)} window_shape "
+                    f"elements and {len(axis)} axes elements."
+                )
+
+        depths = [0] * x.ndim
+        for ax, window in zip(axis, window_shape):
+            depths[ax] += window - 1
+
+        # Ensure that each chunk is big enough to leave at least a size-1 chunk
+        # after windowing (this is only really necessary for the last chunk).
+        safe_chunks = tuple(
+            ensure_minimum_chunksize(d + 1, c) for d, c in zip(depths, x.chunks)
+        )
+        x = x.rechunk(safe_chunks)
+
+        # result.shape = x_shape_trimmed + window_shape,
+        # where x_shape_trimmed is x.shape with every entry
+        # reduced by one less than the corresponding window size.
+        # trim chunks to match x_shape_trimmed
+        newchunks = tuple(
+            c[:-1] + (c[-1] - d,) for d, c in zip(depths, x.chunks)
+        ) + tuple((window,) for window in window_shape)
+
+        kwargs = dict(
+            depth=tuple((0, d) for d in depths),  # Overlap on +ve side only
+            boundary="none",
+            meta=x._meta,
+            new_axis=range(x.ndim, x.ndim + len(axis)),
+            chunks=newchunks,
+            trim=False,
+            window_shape=window_shape,
+            axis=axis,
+        )
+        # map_overlap's signature changed in https://github.com/dask/dask/pull/6165
+        if LooseVersion(dask_version) > "2.18.0":
+            return map_overlap(_np_sliding_window_view, x, align_arrays=False, **kwargs)
+        else:
+            return map_overlap(x, _np_sliding_window_view, **kwargs)

xarray/core/dask_array_ops.py

@@ -1,5 +1,3 @@
-import numpy as np
-
 from . import dtypes, nputils
 
 
@@ -26,92 +24,6 @@ def dask_rolling_wrapper(moving_func, a, window, min_count=None, axis=-1):
     return result
 
 
-def rolling_window(a, axis, window, center, fill_value):
-    """Dask's equivalence to np.utils.rolling_window"""
-    import dask.array as da
-
-    if not hasattr(axis, "__len__"):
-        axis = [axis]
-        window = [window]
-        center = [center]
-
-    orig_shape = a.shape
-    depth = {d: 0 for d in range(a.ndim)}
-    offset = [0] * a.ndim
-    drop_size = [0] * a.ndim
-    pad_size = [0] * a.ndim
-    for ax, win, cent in zip(axis, window, center):
-        if ax < 0:
-            ax = a.ndim + ax
-        depth[ax] = int(win / 2)
-        # For evenly sized window, we need to crop the first point of each block.
-        offset[ax] = 1 if win % 2 == 0 else 0
-
-        if depth[ax] > min(a.chunks[ax]):
-            raise ValueError(
-                "For window size %d, every chunk should be larger than %d, "
-                "but the smallest chunk size is %d. Rechunk your array\n"
-                "with a larger chunk size or a chunk size that\n"
-                "more evenly divides the shape of your array."
-                % (win, depth[ax], min(a.chunks[ax]))
-            )
-
-        # Although da.overlap pads values to boundaries of the array,
-        # the size of the generated array is smaller than what we want
-        # if center == False.
-        if cent:
-            start = int(win / 2)  # 10 -> 5,  9 -> 4
-            end = win - 1 - start
-        else:
-            start, end = win - 1, 0
-        pad_size[ax] = max(start, end) + offset[ax] - depth[ax]
-        drop_size[ax] = 0
-        # pad_size becomes more than 0 when the overlapped array is smaller than
-        # needed. In this case, we need to enlarge the original array by padding
-        # before overlapping.
-        if pad_size[ax] > 0:
-            if pad_size[ax] < depth[ax]:
-                # overlapping requires each chunk larger than depth. If pad_size is
-                # smaller than the depth, we enlarge this and truncate it later.
-                drop_size[ax] = depth[ax] - pad_size[ax]
-                pad_size[ax] = depth[ax]
-
-    # TODO maybe following two lines can be summarized.
-    a = da.pad(
-        a, [(p, 0) for p in pad_size], mode="constant", constant_values=fill_value
-    )
-    boundary = {d: fill_value for d in range(a.ndim)}
-
-    # create overlap arrays
-    ag = da.overlap.overlap(a, depth=depth, boundary=boundary)
-
-    def func(x, window, axis):
-        x = np.asarray(x)
-        index = [slice(None)] * x.ndim
-        for ax, win in zip(axis, window):
-            x = nputils._rolling_window(x, win, ax)
-            index[ax] = slice(offset[ax], None)
-        return x[tuple(index)]
-
-    chunks = list(a.chunks) + window
-    new_axis = [a.ndim + i for i in range(len(axis))]
-    out = da.map_blocks(
-        func,
-        ag,
-        dtype=a.dtype,
-        new_axis=new_axis,
-        chunks=chunks,
-        window=window,
-        axis=axis,
-    )
-
-    # crop boundary.
-    index = [slice(None)] * a.ndim
-    for ax in axis:
-        index[ax] = slice(drop_size[ax], drop_size[ax] + orig_shape[ax])
-    return out[tuple(index)]
-
-
 def least_squares(lhs, rhs, rcond=None, skipna=False):
     import dask.array as da
 

xarray/core/duck_array_ops.py

@@ -614,15 +614,15 @@ def last(values, axis, skipna=None):
     return take(values, -1, axis=axis)
 
 
-def rolling_window(array, axis, window, center, fill_value):
+def sliding_window_view(array, window_shape, axis):
     """
     Make an ndarray with a rolling window of axis-th dimension.
     The rolling dimension will be placed at the last dimension.
     """
     if is_duck_dask_array(array):
-        return dask_array_ops.rolling_window(array, axis, window, center, fill_value)
-    else:  # np.ndarray
-        return nputils.rolling_window(array, axis, window, center, fill_value)
+        return dask_array_compat.sliding_window_view(array, window_shape, axis)
+    else:
+        return npcompat.sliding_window_view(array, window_shape, axis)
 
 
 def least_squares(lhs, rhs, rcond=None, skipna=False):

xarray/core/npcompat.py

@@ -30,6 +30,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 import builtins
 import operator
+from distutils.version import LooseVersion
 from typing import Union
 
 import numpy as np
@@ -96,3 +97,99 @@ def __array_function__(self, *args, **kwargs):
 
 
 IS_NEP18_ACTIVE = _is_nep18_active()
+
+
+if LooseVersion(np.__version__) >= "1.20.0":
+    sliding_window_view = np.lib.stride_tricks.sliding_window_view
+else:
+    from numpy.core.numeric import normalize_axis_tuple  # type: ignore
+    from numpy.lib.stride_tricks import as_strided
+
+    # copied from numpy.lib.stride_tricks
+    def sliding_window_view(
+        x, window_shape, axis=None, *, subok=False, writeable=False
+    ):
+        """
+        Create a sliding window view into the array with the given window shape.
+
+        Also known as rolling or moving window, the window slides across all
+        dimensions of the array and extracts subsets of the array at all window
+        positions.
+
+        .. versionadded:: 1.20.0
+
+        Parameters
+        ----------
+        x : array_like
+            Array to create the sliding window view from.
+        window_shape : int or tuple of int
+            Size of window over each axis that takes part in the sliding window.
+            If `axis` is not present, must have same length as the number of input
+            array dimensions. Single integers `i` are treated as if they were the
+            tuple `(i,)`.
+        axis : int or tuple of int, optional
+            Axis or axes along which the sliding window is applied.
+            By default, the sliding window is applied to all axes and
+            `window_shape[i]` will refer to axis `i` of `x`.
+            If `axis` is given as a `tuple of int`, `window_shape[i]` will refer to
+            the axis `axis[i]` of `x`.
+            Single integers `i` are treated as if they were the tuple `(i,)`.
+        subok : bool, optional
+            If True, sub-classes will be passed-through, otherwise the returned
+            array will be forced to be a base-class array (default).
+        writeable : bool, optional
+            When true, allow writing to the returned view. The default is false,
+            as this should be used with caution: the returned view contains the
+            same memory location multiple times, so writing to one location will
+            cause others to change.
+
+        Returns
+        -------
+        view : ndarray
+            Sliding window view of the array. The sliding window dimensions are
+            inserted at the end, and the original dimensions are trimmed as
+            required by the size of the sliding window.
+            That is, ``view.shape = x_shape_trimmed + window_shape``, where
+            ``x_shape_trimmed`` is ``x.shape`` with every entry reduced by one less
+            than the corresponding window size.
+        """
+        window_shape = (
+            tuple(window_shape) if np.iterable(window_shape) else (window_shape,)
+        )
+        # first convert input to array, possibly keeping subclass
+        x = np.array(x, copy=False, subok=subok)
+
+        window_shape_array = np.array(window_shape)
+        if np.any(window_shape_array < 0):
+            raise ValueError("`window_shape` cannot contain negative values")
+
+        if axis is None:
+            axis = tuple(range(x.ndim))
+            if len(window_shape) != len(axis):
+                raise ValueError(
+                    f"Since axis is `None`, must provide "
+                    f"window_shape for all dimensions of `x`; "
+                    f"got {len(window_shape)} window_shape elements "
+                    f"and `x.ndim` is {x.ndim}."
+                )
+        else:
+            axis = normalize_axis_tuple(axis, x.ndim, allow_duplicate=True)
+            if len(window_shape) != len(axis):
+                raise ValueError(
+                    f"Must provide matching length window_shape and "
+                    f"axis; got {len(window_shape)} window_shape "
+                    f"elements and {len(axis)} axes elements."
+                )
+
+        out_strides = x.strides + tuple(x.strides[ax] for ax in axis)
+
+        # note: same axis can be windowed repeatedly
+        x_shape_trimmed = list(x.shape)
+        for ax, dim in zip(axis, window_shape):
+            if x_shape_trimmed[ax] < dim:
+                raise ValueError("window shape cannot be larger than input array shape")
+            x_shape_trimmed[ax] -= dim - 1
+        out_shape = tuple(x_shape_trimmed) + window_shape
+        return as_strided(
+            x, strides=out_strides, shape=out_shape, subok=subok, writeable=writeable
+        )