"""SAP10 §20.1 cost reconstruction: predicted total fuel cost + ECF.

ECF = 0.42 * total_cost / (TFA + 45)   (SAP rating relationship)

Total cost (gbp/yr) = (space_kwh * space_fuel_price + dhw_kwh * dhw_fuel_price
                       + lighting_kwh * elec_price) / 100   [pence -> pounds]

Standing charges are deliberately omitted at this slice -- they add a
fuel-mix-conditional offset the tree-based model can learn (ADR-0008,
"+ Lighting" scope).
"""

from __future__ import annotations

from math import log10
from typing import Final, Optional

from domain.sap10_ml.sap_efficiencies import fuel_unit_price_p_per_kwh


# SAP10 deflator applied to total cost before the rating equation (Table 32).
_DEFLATOR: float = 0.42

# Electricity standard tariff fuel code (Table 32) — used for lighting + PV credit.
_ELECTRICITY_STANDARD_CODE: int = 30

# Annual PV yield (kWh / kWp / yr) by SAP10.2 region.  Derived from Table 6e
# climate data integrated over an average roof (South-facing, 30 deg pitch,
# average overshading).  Southern England ~ 900, Scotland ~ 700, Highland ~ 600.
_PV_YIELD_BY_REGION: Final[dict[int, float]] = {
    1: 920, 2: 950, 3: 970, 4: 960, 5: 920, 6: 880, 7: 850, 8: 830,
    9: 820, 10: 820, 11: 830, 12: 880, 13: 850, 14: 770, 15: 740,
    16: 700, 17: 650, 18: 700, 19: 690, 20: 680, 21: 870, 22: 870,
}
_PV_YIELD_UK_AVG: Final[float] = 850.0


def predicted_pv_generation_kwh(
    pv_total_peak_power_kw: Optional[float],
    region_code: Optional[str],
) -> float:
    """Annual PV generation (kWh/yr) for the dwelling's PV array(s).

    Linear in peak power; uses a SAP-region yield factor with South-facing,
    30 deg pitch, average overshading assumptions (SAP10.2 Table 6e).
    """
    if pv_total_peak_power_kw is None or pv_total_peak_power_kw <= 0:
        return 0.0
    yield_factor = _PV_YIELD_UK_AVG
    if region_code is not None:
        try:
            yield_factor = _PV_YIELD_BY_REGION.get(int(region_code), _PV_YIELD_UK_AVG)
        except (TypeError, ValueError):
            pass
    return pv_total_peak_power_kw * yield_factor


def predicted_total_fuel_cost_gbp(
    predicted_space_heating_kwh: float,
    predicted_hot_water_kwh: float,
    predicted_lighting_kwh: float,
    main_fuel_code: Optional[int],
    water_heating_fuel_code: Optional[int],
    predicted_pv_kwh: float = 0.0,
) -> float:
    """Annual regulated fuel cost (gbp/yr).

    Skips standing charges; sums delivered kWh * unit price across the
    three included end uses (lighting always at standard electricity).

    Slice 17a: subtracts predicted_pv_kwh * standard electricity price as
    a flat PV credit.  SAP10.2 splits PV between self-consumption and
    export with separate rates; both are 13.19 p/kWh in Table 32 so a
    single rate is fine at this fidelity.
    """
    space_p_per_kwh = fuel_unit_price_p_per_kwh(main_fuel_code)
    dhw_p_per_kwh = fuel_unit_price_p_per_kwh(water_heating_fuel_code)
    light_p_per_kwh = fuel_unit_price_p_per_kwh(_ELECTRICITY_STANDARD_CODE)
    pv_p_per_kwh = fuel_unit_price_p_per_kwh(_ELECTRICITY_STANDARD_CODE)
    total_pence = (
        predicted_space_heating_kwh * space_p_per_kwh
        + predicted_hot_water_kwh * dhw_p_per_kwh
        + predicted_lighting_kwh * light_p_per_kwh
        - predicted_pv_kwh * pv_p_per_kwh
    )
    return total_pence / 100.0


def predicted_ecf(
    predicted_total_cost_gbp: float,
    total_floor_area_m2: Optional[float],
) -> float:
    """SAP rating Energy Cost Factor: 0.42 * total_cost / (TFA + 45)."""
    if total_floor_area_m2 is None or total_floor_area_m2 <= 0:
        return 0.0
    return _DEFLATOR * predicted_total_cost_gbp / (total_floor_area_m2 + 45.0)


def predicted_log10_ecf(predicted_ecf_value: float) -> float:
    """log10(ECF). Returns 0.0 for non-positive input so the feature is
    finite for the (rare) all-PV property.

    The SAP rating formula uses log10(ECF) for ECF >= 3.5 (low-SAP region);
    in the high-SAP linear region the model can still use log10_ecf as a
    monotone proxy for SAP."""
    if predicted_ecf_value <= 0:
        return 0.0
    return log10(predicted_ecf_value)