"""ELO rating calculation."""


def calculate_elo(
    winner_rating: float,
    loser_rating: float,
    k_factor: float = 32.0,
) -> tuple[float, float]:
    """Calculate new ELO ratings after a match.

    Uses the standard ELO formula:
        E = 1 / (1 + 10^((R_opponent - R_self) / 400))
        R_new = R_old + K * (S - E)

    where S = 1 for a win, S = 0 for a loss.

    Returns:
        (new_winner_rating, new_loser_rating)
    """
    expected_winner = 1.0 / (1.0 + 10.0 ** ((loser_rating - winner_rating) / 400.0))
    expected_loser = 1.0 - expected_winner

    new_winner = winner_rating + k_factor * (1.0 - expected_winner)
    new_loser = loser_rating + k_factor * (0.0 - expected_loser)

    return round(new_winner, 2), round(new_loser, 2)