"""The ventilation Recommendation Generator.

Detects a dwelling that lacks adequate mechanical ventilation and emits a
Recommendation whose single Measure Option installs decentralised mechanical
extract ventilation (MEV), priced per installed unit. Like the wall/roof/floor
generators it does detection + pricing and carries no scores (ADR-0016).

Unlike them it is **not** run by the candidate-pool runner: ventilation is a
forced Measure Dependency of fabric insulation (it only ever costs SAP, so the
Optimiser would never choose it), so this Recommendation is consumed by
``optimisation.measure_dependency`` and injected into the package, never freely
selected. The legacy intervention was "mechanical, extract only"; the guard
mirrors legacy ``Property.has_ventilation``.
"""

from typing import Optional

from datatypes.epc.domain.epc_property_data import EpcPropertyData
from domain.modelling.measure_type import MeasureType
from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
from domain.modelling.simulation import EpcSimulation, VentilationOverlay
from repositories.product.product_repository import ProductRepository

_VENTILATION_MEASURE_TYPE = MeasureType.MECHANICAL_VENTILATION

# The SAP10.2 §2 mechanical-ventilation kind installed: decentralised MEV
# ("mechanical extract, decentralised (MEV dc)" → MechanicalVentilationKind
# name), the legacy "mechanical, extract only" intervention.
_MEV_KIND = "EXTRACT_OR_PIV_OUTSIDE"

# Best practice installs one MEV unit per wet zone; the legacy recommendation
# fits two units per dwelling.
_VENTILATION_UNIT_COUNT = 2


def recommend_ventilation(
    epc: EpcPropertyData, products: ProductRepository
) -> Optional[Recommendation]:
    """Return a mechanical-ventilation Recommendation for a dwelling that is not
    already mechanically ventilated, else None. The single Option installs MEV
    and is priced at two fully-loaded units."""
    if _already_mechanically_ventilated(epc):
        return None

    product = products.get(_VENTILATION_MEASURE_TYPE)
    cost = Cost(
        total=product.unit_cost_per_m2 * _VENTILATION_UNIT_COUNT,
        contingency_rate=product.contingency_rate,
    )
    option = MeasureOption(
        measure_type=_VENTILATION_MEASURE_TYPE,
        description=f"Install {_VENTILATION_UNIT_COUNT} mechanical extract ventilation units",
        overlay=EpcSimulation(
            ventilation=VentilationOverlay(mechanical_ventilation_kind=_MEV_KIND)
        ),
        cost=cost,
        material_id=product.id,
    )
    return Recommendation(surface="Ventilation", options=(option,))


def _already_mechanically_ventilated(epc: EpcPropertyData) -> bool:
    """True when the dwelling already lodges a mechanical ventilation kind
    (MEV/MVHR) — the legacy `has_ventilation` guard."""
    return (
        epc.sap_ventilation is not None
        and epc.sap_ventilation.mechanical_ventilation_kind is not None
    )