"""含税单位指导成本到总成本的唯一换算，不改写已封存的成本快照。"""

from decimal import ROUND_HALF_UP, Decimal


def guided_total(quantity, guided_price, cost_amount=None):
    if guided_price is None:
        return Decimal(str(cost_amount)) if cost_amount is not None else None
    total = (Decimal(str(quantity)) * Decimal(str(guided_price))).quantize(
        Decimal("0.01"), rounding=ROUND_HALF_UP
    )
    if cost_amount is not None and Decimal(str(cost_amount)) != total:
        raise ValueError("成本额与数量×含税单位指导成本不一致")
    return total