def iterator_strategy(cube, axis=None):
    """
    Guess the most efficient iteration strategy
    for iterating over a cube, given its size and layout

    Parameters
    ----------
    cube : SpectralCube instance
        The cube to iterate over
    axis : [0, 1, 2]
        For reduction methods, the axis that is
        being collapsed

    Returns
    -------
    strategy : ['cube' | 'ray' | 'slice']
        The recommended iteration strategy.
        *cube* recommends working with the entire array in memory
        *slice* recommends working with one slice at a time
        *ray*  recommends working with one ray at a time
    """
    # pretty simple for now
    if cube.size < 1e8:  # smallish
        return 'cube'
    return 'slice'