"""UK tax regime — bands, allowances, tapers."""
from decimal import Decimal

from hypothesis import given
from hypothesis import strategies as st

from fire_planner.tax.base import TaxInputs
from fire_planner.tax.uk import (
    PA_TAPER_CEILING,
    PERSONAL_ALLOWANCE,
    UkTaxRegime,
    taper_personal_allowance,
)


def test_pa_no_taper_below_100k() -> None:
    assert taper_personal_allowance(Decimal("80000")) == PERSONAL_ALLOWANCE


def test_pa_full_taper_at_ceiling() -> None:
    assert taper_personal_allowance(PA_TAPER_CEILING) == Decimal("0")


def test_pa_partial_taper_at_110k() -> None:
    # £10k above floor → £5k reduction off PA
    assert taper_personal_allowance(Decimal("110000")) == PERSONAL_ALLOWANCE - Decimal("5000")


def test_zero_income_zero_tax() -> None:
    b = UkTaxRegime().compute_tax(TaxInputs())
    assert b.total == Decimal("0")


def test_isa_only_zero_tax() -> None:
    b = UkTaxRegime().compute_tax(TaxInputs(isa_withdrawals=Decimal("100000")))
    assert b.total == Decimal("0")


def test_below_pa_zero_tax() -> None:
    b = UkTaxRegime().compute_tax(TaxInputs(earned_income=Decimal("12000")))
    # NI primary threshold matches PA so NI is zero too.
    assert b.total == Decimal("0")


def test_basic_rate_paye_smoke() -> None:
    # £30k earned: £17,430 taxable @ 20% = £3,486 income tax
    # NI: £17,430 @ 8% = £1,394.40
    b = UkTaxRegime().compute_tax(TaxInputs(earned_income=Decimal("30000")))
    assert b.income_tax == Decimal("3486.00")
    assert b.national_insurance == Decimal("1394.40")


def test_higher_rate_paye_100k() -> None:
    # £100k earned, PA still full (taper starts strictly above £100k):
    #   taxable = £87,430
    #   £37,700 @ 20% = £7,540
    #   £49,730 @ 40% = £19,892
    #   total income tax = £27,432
    b = UkTaxRegime().compute_tax(TaxInputs(earned_income=Decimal("100000")))
    assert b.income_tax == Decimal("27432.00")


def test_pa_taper_at_125k() -> None:
    # £125,000: PA = 12,570 - (25,000/2) = 12,570 - 12,500 = 70
    # taxable = 125,000 - 70 = 124,930
    # £37,700 @ 20% = £7,540
    # £87,230 @ 40% = £34,892
    # total = £42,432
    b = UkTaxRegime().compute_tax(TaxInputs(earned_income=Decimal("125000")))
    assert b.income_tax == Decimal("42432.00")


def test_additional_rate_above_125k() -> None:
    # £200k earned: PA fully tapered.
    # taxable income = £200,000
    #   £37,700 @ 20% = £7,540
    #   £87,440 @ 40% = £34,976
    #   £74,860 @ 45% = £33,687
    #   total = £76,203
    b = UkTaxRegime().compute_tax(TaxInputs(earned_income=Decimal("200000")))
    assert b.income_tax == Decimal("76203.00")


def test_cgt_basic_rate_only() -> None:
    # No earned income, £20k gains:
    # exempt £3k → £17k taxable @ 18% (basic band has plenty of room)
    # = £3,060
    b = UkTaxRegime().compute_tax(TaxInputs(capital_gains=Decimal("20000")))
    assert b.capital_gains_tax == Decimal("3060.00")


def test_cgt_spans_into_higher_band() -> None:
    # £30k earned (taxable income £17,430 — well under £37,700 band top)
    # £40k gains:
    #   exempt £3k → £37k taxable
    #   basic band remaining = 37,700 - 17,430 = 20,270 → @ 18% = £3,648.60
    #   higher = 37,000 - 20,270 = 16,730 → @ 24% = £4,015.20
    #   total CGT = £7,663.80
    b = UkTaxRegime().compute_tax(
        TaxInputs(earned_income=Decimal("30000"), capital_gains=Decimal("40000")))
    assert b.capital_gains_tax == Decimal("7663.80")


def test_dividend_basic_rate() -> None:
    # No other income, £10k dividends:
    # allowance £500 → £9,500 taxable
    # Stacked on top of taxable_ordinary=0, so basic band has £37,700 room.
    # All £9,500 @ 8.75% = £831.25
    b = UkTaxRegime().compute_tax(TaxInputs(dividends=Decimal("10000")))
    assert b.dividend_tax == Decimal("831.2500")


def test_pension_25pc_tax_free() -> None:
    # £40k pension drawdown, no other income:
    # PCLS = £10k tax-free
    # Taxable pension = £30k → £17,430 taxable @ 20% = £3,486
    b = UkTaxRegime().compute_tax(TaxInputs(pension_withdrawal=Decimal("40000")))
    assert b.income_tax == Decimal("3486.00")
    assert b.national_insurance == Decimal("0")  # NI not on pension


def test_total_equals_sum_of_components() -> None:
    inputs = TaxInputs(
        earned_income=Decimal("60000"),
        capital_gains=Decimal("15000"),
        dividends=Decimal("8000"),
    )
    b = UkTaxRegime().compute_tax(inputs)
    assert (b.total == b.income_tax + b.national_insurance + b.capital_gains_tax + b.dividend_tax +
            b.healthcare_levy + b.other)


@given(income=st.decimals(
    min_value=0, max_value=500_000, places=2, allow_nan=False, allow_infinity=False))
def test_tax_monotone_in_earned_income(income: Decimal) -> None:
    """Adding earned income never decreases total tax."""
    base = UkTaxRegime().compute_tax(TaxInputs(earned_income=income))
    plus = UkTaxRegime().compute_tax(TaxInputs(earned_income=income + Decimal("1000")))
    assert plus.total >= base.total


@given(gains=st.decimals(
    min_value=0, max_value=500_000, places=2, allow_nan=False, allow_infinity=False))
def test_cgt_monotone_in_gains(gains: Decimal) -> None:
    base = UkTaxRegime().compute_tax(TaxInputs(capital_gains=gains))
    plus = UkTaxRegime().compute_tax(TaxInputs(capital_gains=gains + Decimal("1000")))
    assert plus.capital_gains_tax >= base.capital_gains_tax