from __future__ import annotations

from enum import Enum


class Fuel(Enum):
    """A canonical billing fuel — the join key between the calculator's
    per-end-use fuel (mapped from SAP fuel codes) and the Fuel Rates snapshot
    (ADR-0014). Member names match the snapshot's keys.

    ``COAL`` (traditional house coal) and ``HEAT_NETWORK`` are carried as
    members so a cert lodging them maps to a Fuel, but they have no national
    rate — pricing them raises ``UnpricedFuel`` (house coal's domestic sale is
    illegal in England; heat networks are scheme-specific).
    """

    MAINS_GAS = "MAINS_GAS"
    ELECTRICITY = "ELECTRICITY"
    ELECTRICITY_OFF_PEAK = "ELECTRICITY_OFF_PEAK"
    OIL = "OIL"
    LPG = "LPG"
    COAL = "COAL"
    SMOKELESS = "SMOKELESS"
    WOOD_LOGS = "WOOD_LOGS"
    WOOD_PELLETS = "WOOD_PELLETS"
    HEAT_NETWORK = "HEAT_NETWORK"


class UnpricedFuel(ValueError):
    """Bill Derivation was asked for a rate on a fuel the current Fuel Rates
    snapshot does not price (ADR-0014).

    Raised rather than billing at a wrong default so the gap surfaces
    immediately — house coal and heat networks have no national rate, and
    off-peak electricity needs the day/night split that a later slice adds.
    """

    def __init__(self, fuel: Fuel) -> None:
        super().__init__(
            f"no rate for fuel {fuel.name} in the current Fuel Rates snapshot; "
            f"add it to the snapshot or map this end use to a priced fuel"
        )
        self.fuel = fuel