from typing import Sequence

from .hr_absence_bradford_factor_types import BradfordFactor


def bradford_factor(spell_days: Sequence[int]) -> BradfordFactor:
    """B = S x S x D over the spells given."""
    days = 0
    for d in spell_days:
        if isinstance(d, bool) or not isinstance(d, int) or d < 1:
            raise ValueError("each spell must be at least one whole day, received %s" % (d,))
        days += d
    spells = len(spell_days)
    return BradfordFactor(spells=spells, days=days, score=spells * spells * days)