from __future__ import annotations

from collections.abc import Mapping
from dataclasses import dataclass
from typing import Optional

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 OffPeakRate:
    """An Off-Peak Meter's dual-rate tariff — a cheaper ``night`` (low) rate and
    a dearer ``day`` (high) rate, plus the daily standing charge (ADR-0014).

    Off-peak electricity has no single unit rate, so it does not live in the
    single-rate ``FuelRates.rates`` map; each end use blends day/night by its own
    High-Rate Fraction (a calculator output)."""

    day_p_per_kwh: float
    night_p_per_kwh: float
    standing_charge_p_per_day: float

    def blended_p_per_kwh(self, high_rate_fraction: float) -> float:
        """Effective p/kWh for an end use billing ``high_rate_fraction`` of its
        kWh at the day (high) rate and the remainder at the night (low) rate."""
        return (
            high_rate_fraction * self.day_p_per_kwh
            + (1.0 - high_rate_fraction) * self.night_p_per_kwh
        )


@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]
    off_peak: Optional[OffPeakRate] = None

    def off_peak_blended_p_per_kwh(self, high_rate_fraction: float) -> float:
        """Blended day/night p/kWh for an Off-Peak Meter end use at the given
        High-Rate Fraction. Raises ``UnpricedFuel`` when the snapshot carries no
        off-peak entry."""
        if self.off_peak is None:
            raise UnpricedFuel(Fuel.ELECTRICITY_OFF_PEAK)
        return self.off_peak.blended_p_per_kwh(high_rate_fraction)

    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:
        if fuel is Fuel.ELECTRICITY_OFF_PEAK:
            if self.off_peak is None:
                raise UnpricedFuel(fuel)
            return self.off_peak.standing_charge_p_per_day
        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