from __future__ import annotations

import pytest

from domain.fuel_rates.fuel import Fuel, UnpricedFuel
from domain.fuel_rates.fuel_rates import FuelRate, FuelRates, OffPeakRate


def _rates() -> FuelRates:
    return FuelRates(
        period="test",
        seg_export_p_per_kwh=15.0,
        rates={Fuel.MAINS_GAS: FuelRate(unit_rate_p_per_kwh=5.74, standing_charge_p_per_day=29.09)},
    )


def _off_peak_rates() -> FuelRates:
    return FuelRates(
        period="test",
        seg_export_p_per_kwh=15.0,
        rates={},
        off_peak=OffPeakRate(
            day_p_per_kwh=29.73, night_p_per_kwh=13.89, standing_charge_p_per_day=56.99
        ),
    )


def test_off_peak_blended_rate_at_zero_high_fraction_is_the_night_rate() -> None:
    # Arrange — an all-night load (storage heating, high-rate fraction 0.0).
    rates = _off_peak_rates()

    # Act
    blended = rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.0)

    # Assert — every kWh bills at the cheap night rate.
    assert blended == 13.89


def test_off_peak_blended_rate_at_full_high_fraction_is_the_day_rate() -> None:
    # Arrange — an all-day load (high-rate fraction 1.0).
    rates = _off_peak_rates()

    # Act / Assert — every kWh bills at the dearer day rate.
    assert rates.off_peak_blended_p_per_kwh(high_rate_fraction=1.0) == 29.73


def test_off_peak_blended_rate_blends_day_and_night_linearly() -> None:
    # Arrange — 25% of the load at the day rate, 75% at night.
    rates = _off_peak_rates()

    # Act / Assert — 0.25 * 29.73 + 0.75 * 13.89 = 17.85
    assert rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.25) == pytest.approx(17.85)


def test_off_peak_blend_without_a_snapshot_off_peak_entry_raises_unpriced_fuel() -> None:
    # Arrange — a snapshot that carries no off-peak entry at all.
    rates = _rates()

    # Act / Assert
    with pytest.raises(UnpricedFuel) as excinfo:
        rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.5)
    assert excinfo.value.fuel is Fuel.ELECTRICITY_OFF_PEAK


def test_off_peak_meter_standing_charge_reads_back() -> None:
    # Arrange — the bill adds the off-peak meter's standing charge once per meter.
    rates = _off_peak_rates()

    # Act / Assert
    assert rates.standing_charge_p_per_day(Fuel.ELECTRICITY_OFF_PEAK) == 56.99


def test_unit_rate_and_standing_charge_read_back_for_a_priced_fuel() -> None:
    # Arrange
    rates = _rates()

    # Act / Assert
    assert rates.unit_rate_p_per_kwh(Fuel.MAINS_GAS) == 5.74
    assert rates.standing_charge_p_per_day(Fuel.MAINS_GAS) == 29.09


def test_a_fuel_absent_from_the_snapshot_raises_unpriced_fuel() -> None:
    # Arrange — LPG is not in this snapshot.
    rates = _rates()

    # Act / Assert — the raise carries the offending fuel for the operator.
    with pytest.raises(UnpricedFuel) as excinfo:
        rates.unit_rate_p_per_kwh(Fuel.LPG)
    assert excinfo.value.fuel is Fuel.LPG