"""SAP 10.2 §13 Energy Cost Rating + §14 Environmental Impact Rating.

The Energy Cost Factor (ECF) blends total annual fuel cost with the
dwelling's floor area; the SAP rating maps ECF onto a 1-100+ scale that
rewards low cost per square metre. The EI rating is the same shape applied
to annual CO2 emissions.

Constants taken from SAP 10.2 Table 12 (page 191) per ADR-0010 (active
spec target is SAP 10.2, 14-03-2025):
- Energy Cost Deflator = 0.42
- Linear branch (ECF < 3.5): SAP = 100 − 13.95 × ECF
- Log branch   (ECF ≥ 3.5): SAP = 117 − 121 × log10(ECF)

(SAP 10.3 widens these to 0.36 / 16.21 / 108.8 / 120.5 — apply when the
spec target moves per a follow-up ADR amendment.)

Reference: SAP 10.2 specification (14-03-2025) §13 + §14 (pages 38-39),
Table 12 (page 191).
"""

from __future__ import annotations

from math import log10
from typing import Final


ENERGY_COST_DEFLATOR: Final[float] = 0.42
FLOOR_AREA_OFFSET_M2: Final[float] = 45.0
ECF_LOG_THRESHOLD: Final[float] = 3.5
_SAP_LINEAR_INTERCEPT: Final[float] = 100.0
_SAP_LINEAR_SLOPE: Final[float] = 13.95
_SAP_LOG_INTERCEPT: Final[float] = 117.0
_SAP_LOG_SLOPE: Final[float] = 121.0
_CF_LOG_THRESHOLD: Final[float] = 28.3
_EI_LINEAR_INTERCEPT: Final[float] = 100.0
_EI_LINEAR_SLOPE: Final[float] = 1.34
_EI_LOG_INTERCEPT: Final[float] = 200.0
_EI_LOG_SLOPE: Final[float] = 95.0


def energy_cost_factor(
    *,
    total_cost_gbp: float,
    total_floor_area_m2: float,
) -> float:
    """SAP 10.2 §13 equation (7): ECF = 0.36 × cost / (TFA + 45)."""
    return ENERGY_COST_DEFLATOR * total_cost_gbp / (total_floor_area_m2 + FLOOR_AREA_OFFSET_M2)


def sap_rating(*, ecf: float) -> float:
    """SAP 10.2 §13 equations (8)/(9). Un-rounded result so callers can
    inspect the continuous value; `sap_rating_integer` rounds and clamps."""
    if ecf >= ECF_LOG_THRESHOLD:
        return _SAP_LOG_INTERCEPT - _SAP_LOG_SLOPE * log10(ecf)
    return _SAP_LINEAR_INTERCEPT - _SAP_LINEAR_SLOPE * ecf


def sap_rating_integer(*, ecf: float) -> int:
    """SAP 10.2 §13: round the continuous SAP rating to the nearest integer
    and clamp to a minimum of 1 ("if the result of the calculation is less
    than 1 the rating should be quoted as 1"). The integer value is the
    one published on the EPC."""
    return max(1, round(sap_rating(ecf=ecf)))


def environmental_impact_rating(
    *,
    co2_emissions_kg_per_yr: float,
    total_floor_area_m2: float,
) -> float:
    """SAP 10.2 §14 equations (10)-(12). Un-rounded EI rating; mirrors the
    SAP rating curve but uses CO2 emissions per (TFA + 45) as the input."""
    cf = co2_emissions_kg_per_yr / (total_floor_area_m2 + FLOOR_AREA_OFFSET_M2)
    if cf >= _CF_LOG_THRESHOLD:
        return _EI_LOG_INTERCEPT - _EI_LOG_SLOPE * log10(cf)
    return _EI_LINEAR_INTERCEPT - _EI_LINEAR_SLOPE * cf