A ported sparse version of np.diff.
    Uses recursion to compute higher order differences

    Parameters
    ----------
    array : sparse array
    n : int, default: 1
        differencing order
    axis : int, default: -1
        axis along which differences are computed

    Returns
    -------
    diff_array : sparse array
                 same shape as input array,
                 but 'axis' dimension is smaller by 'n'.

def sparse_diff(array, n=1, axis=-1):
    """
    A ported sparse version of np.diff.
    Uses recursion to compute higher order differences

    Parameters
    ----------
    array : sparse array
    n : int, default: 1
        differencing order
    axis : int, default: -1
        axis along which differences are computed

    Returns
    -------
    diff_array : sparse array
                 same shape as input array,
                 but 'axis' dimension is smaller by 'n'.
    """
    if (n < 0) or (int(n) != n):
        raise ValueError('Expected order is non-negative integer, '\
                         'but found: {}'.format(n))
    if not sp.sparse.issparse(array):
        warnings.warn('Array is not sparse. Consider using numpy.diff')

    if n == 0:
        return array

    nd = array.ndim
    slice1 = [slice(None)]*nd
    slice2 = [slice(None)]*nd
    slice1[axis] = slice(1, None)
    slice2[axis] = slice(None, -1)
    slice1 = tuple(slice1)
    slice2 = tuple(slice2)

    A = sparse_diff(array, n-1, axis=axis)
    return A[slice1] - A[slice2]