from __future__ import annotations

from collections import defaultdict
from typing import Final

from domain.fuel_rates.fuel import Fuel
from domain.fuel_rates.fuel_rates import FuelRates
from domain.property_baseline.bill import (
    Bill,
    BillSection,
    BillSectionCost,
    EnergyBreakdown,
)

_DAYS_PER_YEAR: Final[float] = 365.0
_PENCE_PER_POUND: Final[float] = 100.0


class BillDerivation:
    """Derives a Property's annual energy Bill by pricing a delivered-energy
    breakdown at current Fuel Rates (ADR-0014).

    Each end-use line is billed at its fuel's unit rate; **standing charges are
    added once per distinct fuel used** (a meter, not an end use — off-gas fuels
    carry a 0 standing charge so they contribute nothing); the SEG export credit
    is subtracted. Deterministic (ADR-0006). Raises ``UnpricedFuel`` (via
    ``FuelRates``) on a fuel the snapshot does not price.
    """

    def __init__(self, fuel_rates: FuelRates) -> None:
        self._rates = fuel_rates

    def derive(self, breakdown: EnergyBreakdown) -> Bill:
        section_kwh: defaultdict[BillSection, float] = defaultdict(float)
        section_cost_p: defaultdict[BillSection, float] = defaultdict(float)
        fuels_used: set[Fuel] = set()
        for line in breakdown.lines:
            section_kwh[line.section] += line.kwh
            section_cost_p[line.section] += (
                line.kwh * self._rates.unit_rate_p_per_kwh(line.fuel)
            )
            if line.kwh > 0:
                fuels_used.add(line.fuel)

        sections = {
            section: BillSectionCost(
                kwh=section_kwh[section], cost_gbp=section_cost_p[section] / _PENCE_PER_POUND
            )
            for section in section_kwh
        }
        standing_charges_gbp = (
            sum(
                (self._rates.standing_charge_p_per_day(fuel) * _DAYS_PER_YEAR for fuel in fuels_used),
                0.0,
            )
            / _PENCE_PER_POUND
        )
        seg_credit_gbp = (
            breakdown.exported_kwh * self._rates.seg_export_p_per_kwh / _PENCE_PER_POUND
        )
        total_gbp = (
            sum((section.cost_gbp for section in sections.values()), 0.0)
            + standing_charges_gbp
            - seg_credit_gbp
        )
        return Bill(
            sections=sections,
            standing_charges_gbp=standing_charges_gbp,
            seg_credit_gbp=seg_credit_gbp,
            total_gbp=total_gbp,
        )