def METHOD_NAME(M):
    """Returns the Bartlett window.
    The Bartlett window is defined as
    .. math::
            w(n) = \\frac{2}{M-1} \\left(
            \\frac{M-1}{2} - \\left|n - \\frac{M-1}{2}\\right|
            \\right)
    Args:
        M (int):
            Number of points in the output window. If zero or less, an empty
            array is returned.
    Returns:
        ~cupy.ndarray: Output ndarray.
    .. seealso:: :func:`numpy.bartlett`
    """
    if M == 1:
        return cupy.ones(1, dtype=cupy.float64)
    if M <= 0:
        return cupy.array([])
    alpha = (M - 1) / 2.0
    out = cupy.empty(M, dtype=cupy.float64)
    return _bartlett_kernel(alpha, out)