def boss_distance(histo1, histo2):
    """
    BOSS distance between two histograms

    Not really a distance because it is not symmetric

    :param histo1:
    :param histo2:
    :return:
    """
    val = 0
    for w in histo1:
        if w in histo2:
            val += (histo1[w] - histo2[w]) ** 2
        else:
            val += histo1[w] * histo1[w]

    return val