"""SAP 10.2 Table 9c step 10 — monthly space-heating requirement.

Final step of the heating worksheet: the heat the heating system has to
deliver to maintain the mean internal temperature, given the loss rate to
the outside and the gains the dwelling has already accumulated.

    L_m       = H × (T_i,m − T_e,m)              (W)
    Q_heat,m  = 0.024 × (L_m − η_m × G_m) × n_m  (kWh)

If Q_heat would be negative or below 1 kWh in any month, set it to 0 per
the Table 9c clamp.

Reference: SAP 10.2 specification (14-03-2025) Table 9c (page 185).
"""

from __future__ import annotations

from dataclasses import dataclass
from typing import Final

from domain.sap10_calculator.worksheet.heat_transmission import _round_half_up


_MIN_KWH_PER_MONTH: Final[float] = 1.0
_WH_TO_KWH_PER_DAY: Final[float] = 0.024  # 24 h / 1000
_DAYS_IN_MONTH: Final[tuple[int, ...]] = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
_WORKSHEET_DISPLAY_DP: Final[int] = 4
# SAP10.2 Table 9c step 10: "Include the heating requirement for each month
# from October to May (disregarding June to September)." Set Q_heat to zero
# in Jun..Sep regardless of computed value. Indices 5..8 inclusive (zero-based).
_SUMMER_MONTH_INDICES: Final[frozenset[int]] = frozenset({5, 6, 7, 8})


def monthly_heat_requirement_kwh(
    *,
    heat_transfer_coefficient_w_per_k: float,
    internal_temperature_c: float,
    external_temperature_c: float,
    utilisation_factor: float,
    total_gains_w: float,
    days_in_month: int,
) -> float:
    """SAP 10.2 Table 9c step 10. Returns delivered kWh required for the
    month; clamps to 0 when below 1 kWh or negative."""
    loss_rate_w = heat_transfer_coefficient_w_per_k * (
        internal_temperature_c - external_temperature_c
    )
    useful_loss_w = loss_rate_w - utilisation_factor * total_gains_w
    if useful_loss_w <= 0:
        return 0.0
    q_heat = _WH_TO_KWH_PER_DAY * useful_loss_w * days_in_month
    if q_heat < _MIN_KWH_PER_MONTH:
        return 0.0
    return q_heat


@dataclass(frozen=True)
class SpaceHeatingResult:
    """SAP 10.2 §8 worksheet line refs (95)..(99).

    Returned by `space_heating_monthly_kwh`. Downstream calculator consumes
    `total_space_heating_monthly_kwh` (98c) directly to drive fuel-cost +
    rating chains; per-line tuples are exposed for worksheet conformance +
    audit. Field names mirror the SAP 10.2 line refs.
    """

    useful_gains_monthly_w: tuple[float, ...]                   # (95)
    heat_loss_rate_monthly_w: tuple[float, ...]                 # (97)
    space_heating_requirement_monthly_kwh: tuple[float, ...]    # (98a)
    solar_space_heating_monthly_kwh: tuple[float, ...]          # (98b)
    total_space_heating_monthly_kwh: tuple[float, ...]          # (98c)
    space_heating_requirement_kwh_per_yr: float                 # Σ(98a) — FEE input
    total_space_heating_kwh_per_yr: float                       # Σ(98c)
    space_heating_per_m2_kwh: float                             # (99)


def space_heating_monthly_kwh(
    *,
    monthly_heat_transfer_coefficient_w_per_k: tuple[float, ...],
    monthly_internal_temperature_c: tuple[float, ...],
    monthly_external_temperature_c: tuple[float, ...],
    monthly_utilisation_factor: tuple[float, ...],
    monthly_total_gains_w: tuple[float, ...],
    total_floor_area_m2: float,
) -> SpaceHeatingResult:
    """SAP 10.2 §8 orchestrator — produce (95)..(99) line refs for all months.

    Composes the existing single-month leaf with the spec inclusion rule:
    Jun..Sep are zeroed (Table 9c step 10) regardless of computed value,
    on top of the per-month value clamp (< 1 kWh or negative).

    Solar space heating (98b) — Appendix H — is always 0 in this slice; no
    Elmhurst fixture lodges a solar space heating system. (98c) = (98a) for
    the current corpus.

    Inputs are length-12 Jan..Dec tuples. `total_floor_area_m2` only drives
    the (99) per-m² aggregate; everything else is per-month physics.
    """
    useful_gains: list[float] = []
    heat_loss_rate: list[float] = []
    q_heat_98a: list[float] = []
    q_solar_98b: list[float] = []
    q_total_98c: list[float] = []

    for m in range(12):
        h = monthly_heat_transfer_coefficient_w_per_k[m]
        t_i = monthly_internal_temperature_c[m]
        t_e = monthly_external_temperature_c[m]
        eta = monthly_utilisation_factor[m]
        gains = monthly_total_gains_w[m]

        useful_gains.append(eta * gains)
        heat_loss_rate.append(h * (t_i - t_e))

        q98a = monthly_heat_requirement_kwh(
            heat_transfer_coefficient_w_per_k=h,
            internal_temperature_c=t_i,
            external_temperature_c=t_e,
            utilisation_factor=eta,
            total_gains_w=gains,
            days_in_month=_DAYS_IN_MONTH[m],
        )
        # Spec inclusion rule: Jun..Sep do not contribute regardless of value.
        if m in _SUMMER_MONTH_INDICES:
            q98a = 0.0
        q_heat_98a.append(q98a)
        q_solar_98b.append(0.0)
        q_total_98c.append(q98a)

    # U985 worksheet lodges (98a)_m / (98c)_m at 4 d.p. half-up and reports
    # the annual as the Σ of those displayed monthlies. The full-precision Σ
    # diverges from the lodged annual by up to ~1.4e-4 (accumulated 4-d.p.
    # rounding over 8 heating months) — e.g. 000490 = 0.000132. Rounding
    # each monthly to 4 d.p. before summing reproduces the lodged annual
    # exactly for all 6 fixtures. SAP10.2 Table 9c step 10 (p.184) defines
    # (98a)_m without an explicit annual rounding rule; this matches the
    # worksheet display convention.
    annual_98a = sum(_round_half_up(q, _WORKSHEET_DISPLAY_DP) for q in q_heat_98a)
    annual_98c = sum(_round_half_up(q, _WORKSHEET_DISPLAY_DP) for q in q_total_98c)
    per_m2_99 = annual_98c / total_floor_area_m2 if total_floor_area_m2 > 0 else 0.0

    return SpaceHeatingResult(
        useful_gains_monthly_w=tuple(useful_gains),
        heat_loss_rate_monthly_w=tuple(heat_loss_rate),
        space_heating_requirement_monthly_kwh=tuple(q_heat_98a),
        solar_space_heating_monthly_kwh=tuple(q_solar_98b),
        total_space_heating_monthly_kwh=tuple(q_total_98c),
        space_heating_requirement_kwh_per_yr=annual_98a,
        total_space_heating_kwh_per_yr=annual_98c,
        space_heating_per_m2_kwh=per_m2_99,
    )