Model/tests/domain/sap10_calculator/worksheet/test_rating.py

"""Tests for SAP 10.2 §13 Energy Cost Rating and §14 Environmental Impact
Rating formulas.

  (7)  ECF = deflator × total_cost / (TFA + 45)
  (8)  if ECF ≥ 3.5,  SAP10 = 117 − 121 × log10(ECF)
  (9)  if ECF < 3.5,  SAP10 = 100 − 13.95 × ECF
  (10) CF  = CO2_emissions / (TFA + 45)
  (11) if CF ≥ 28.3,  EI = 200 − 95 × log10(CF)
  (12) if CF < 28.3,  EI = 100 − 1.34 × CF

Energy Cost Deflator = 0.42 per Table 12 (page 191). SAP and EI ratings
are rounded to nearest integer; if the result is less than 1 it is quoted
as 1.

Reference: SAP 10.2 specification (14-03-2025) §13 + §14 (pages 38-39),
Table 12 Energy Cost Deflator (page 191). Per ADR-0010, active spec
target is SAP 10.2.
"""

import pytest

from domain.sap10_calculator.worksheet.rating import (
    energy_cost_factor,
    environmental_impact_rating,
    sap_rating,
    sap_rating_integer,
)


def test_ecf_applies_deflator_0_42_to_total_cost_per_adjusted_area() -> None:
    # Arrange — Equation (7): ECF = 0.42 × total_cost / (TFA + 45). For a
    # 100 m² dwelling costing £500/year:
    #   ECF = 0.42 × 500 / 145 ≈ 1.4483

    # Act
    result = energy_cost_factor(total_cost_gbp=500.0, total_floor_area_m2=100.0)

    # Assert
    assert result == pytest.approx(1.4483, abs=0.005)


def test_sap_rating_uses_linear_branch_when_ecf_below_3_5() -> None:
    # Arrange — Equation (9): SAP = 100 − 13.95 × ECF when ECF < 3.5.
    # For ECF = 1.4483 → SAP = 100 − 20.204 ≈ 79.80, rounds to 80.

    # Act
    result = sap_rating(ecf=1.4483)

    # Assert — un-rounded SAP value for hand-verification.
    assert result == pytest.approx(79.80, abs=0.05)


def test_sap_rating_uses_log_branch_when_ecf_above_3_5() -> None:
    # Arrange — Equation (8): SAP = 117 − 121 × log10(ECF) for ECF ≥ 3.5.
    # For a high-cost dwelling, ECF = 4.965:
    #   log10(4.965) ≈ 0.6960
    #   SAP = 117 − 121 × 0.6960 ≈ 32.78

    # Act
    result = sap_rating(ecf=4.965)

    # Assert
    assert result == pytest.approx(32.78, abs=0.05)


def test_sap_rating_log_threshold_boundary_is_continuous() -> None:
    # Arrange — At ECF = 3.5 (the branch boundary) both formulas should
    # yield the same value within tolerance, since the rating curve has
    # been calibrated to be continuous at the transition.
    #   linear: 100 − 13.95 × 3.5 = 51.175
    #   log:    117 − 121 × log10(3.5) = 117 − 65.83 ≈ 51.17

    # Act
    at_threshold = sap_rating(ecf=3.5)
    just_below = sap_rating(ecf=3.499)

    # Assert
    assert at_threshold == pytest.approx(just_below, abs=0.05)


def test_net_energy_exporter_returns_sap_above_100() -> None:
    # Arrange — §13 explicitly: "The SAP rating scale has been set so that
    # SAP 100 is achieved at zero-ECF. It can rise above 100 if the dwelling
    # is a net exporter of energy." For ECF = -0.3:
    #   SAP = 100 − 13.95 × (−0.3) = 100 + 4.185 = 104.185

    # Act
    result = sap_rating(ecf=-0.3)

    # Assert
    # 100 − 13.95 × (−0.3) = 104.185 is exact arithmetic — no float drift.
    assert result == pytest.approx(104.185, abs=1e-12)
    assert result > 100.0


def test_sap_rating_integer_rounds_to_nearest_and_clamps_to_minimum_one() -> None:
    # Arrange — §13: "The SAP rating is rounded to the nearest integer. If
    # the result of the calculation is less than 1 the rating should be
    # quoted as 1." So a catastrophically high-cost dwelling (e.g. ECF ≈ 10)
    # → SAP = 117 − 121 × 1 = −4, clamps to 1.
    # A typical case ECF = 1.4483 → 79.80 → 80.

    # Act
    typical = sap_rating_integer(ecf=1.4483)
    catastrophic = sap_rating_integer(ecf=10.0)

    # Assert
    assert typical == 80
    assert catastrophic == 1


def test_environmental_impact_rating_linear_branch_below_threshold() -> None:
    # Arrange — Equations (10)/(12): CF = CO2/(TFA+45); when CF < 28.3,
    # EI = 100 − 1.34 × CF. For a 100 m² home emitting 1500 kg CO2/yr:
    #   CF = 1500 / 145 ≈ 10.345
    #   EI = 100 − 1.34 × 10.345 ≈ 86.14

    # Act
    result = environmental_impact_rating(
        co2_emissions_kg_per_yr=1500.0,
        total_floor_area_m2=100.0,
    )

    # Assert
    assert result == pytest.approx(86.14, abs=0.05)


def test_environmental_impact_rating_log_branch_above_threshold() -> None:
    # Arrange — When CF ≥ 28.3, EI = 200 − 95 × log10(CF). For a 100 m²
    # home emitting 5000 kg CO2/yr:
    #   CF = 5000 / 145 ≈ 34.48
    #   log10(34.48) ≈ 1.5377
    #   EI = 200 − 95 × 1.5377 ≈ 53.92

    # Act
    result = environmental_impact_rating(
        co2_emissions_kg_per_yr=5000.0,
        total_floor_area_m2=100.0,
    )

    # Assert
    assert result == pytest.approx(53.92, abs=0.05)