from __future__ import annotations

from collections.abc import Mapping
from dataclasses import dataclass

from domain.fuel_rates.fuel import Fuel, UnpricedFuel


@dataclass(frozen=True)
class FuelRate:
    """One fuel's current tariff: unit price + daily standing charge.

    Off-gas fuels (oil / LPG / solid / wood) carry a ``0.0`` standing charge —
    they are delivered, not metered, so there is no daily charge.
    """

    unit_rate_p_per_kwh: float
    standing_charge_p_per_day: float


@dataclass(frozen=True)
class FuelRates:
    """A current Fuel Rates snapshot — the rate per billing Fuel plus the SEG
    export credit (ADR-0014). ``period`` records which window it is for, since
    a committed snapshot goes stale on the Ofgem-cap (quarterly) cadence.

    Pricing a fuel the snapshot does not carry raises ``UnpricedFuel`` rather
    than defaulting — see [[reference-unmapped-sap-code]] for the same strict
    discipline on the calculator side.
    """

    period: str
    seg_export_p_per_kwh: float
    rates: Mapping[Fuel, FuelRate]

    def unit_rate_p_per_kwh(self, fuel: Fuel) -> float:
        return self._rate(fuel).unit_rate_p_per_kwh

    def standing_charge_p_per_day(self, fuel: Fuel) -> float:
        return self._rate(fuel).standing_charge_p_per_day

    def _rate(self, fuel: Fuel) -> FuelRate:
        rate = self.rates.get(fuel)
        if rate is None:
            raise UnpricedFuel(fuel)
        return rate