Initial extraction from monorepo

tests/test_tax_base.py

@@ -0,0 +1,70 @@
+"""Bracket-arithmetic and breakdown invariants."""
+from decimal import Decimal
+
+from hypothesis import given
+from hypothesis import strategies as st
+
+from fire_planner.tax.base import TaxBreakdown, TaxInputs, apply_brackets
+
+
+def test_apply_brackets_zero_input() -> None:
+    assert apply_brackets(Decimal("0"), [(Decimal("100"), Decimal("0.2"))]) == Decimal("0")
+
+
+def test_apply_brackets_negative_input() -> None:
+    # Negative income shouldn't generate a refund — clamp to zero.
+    assert apply_brackets(Decimal("-1000"), [(Decimal("100"), Decimal("0.2"))]) == Decimal("0")
+
+
+def test_apply_brackets_within_first_band() -> None:
+    brackets = [(Decimal("100"), Decimal("0.2")), (Decimal("Infinity"), Decimal("0.4"))]
+    assert apply_brackets(Decimal("50"), brackets) == Decimal("10")
+
+
+def test_apply_brackets_spans_two_bands() -> None:
+    # 100 @ 20% = 20; next 50 @ 40% = 20 → total 40
+    brackets = [(Decimal("100"), Decimal("0.2")), (Decimal("Infinity"), Decimal("0.4"))]
+    assert apply_brackets(Decimal("150"), brackets) == Decimal("40")
+
+
+def test_apply_brackets_uk_paye_2026_smoke() -> None:
+    # Taxable income £80,000 (gross £92,570 less £12,570 PA):
+    #  £37,700 @ 20% = £7,540
+    #  £42,300 @ 40% = £16,920
+    #  total = £24,460
+    brackets = [
+        (Decimal("37700"), Decimal("0.20")),
+        (Decimal("112570"), Decimal("0.40")),
+        (Decimal("Infinity"), Decimal("0.45")),
+    ]
+    assert apply_brackets(Decimal("80000"), brackets) == Decimal("24460")
+
+
+@given(amount=st.decimals(min_value=0, max_value=10_000_000, allow_nan=False, allow_infinity=False))
+def test_apply_brackets_monotone_in_amount(amount: Decimal) -> None:
+    """More taxable income → never less tax."""
+    brackets = [
+        (Decimal("37700"), Decimal("0.20")),
+        (Decimal("112570"), Decimal("0.40")),
+        (Decimal("Infinity"), Decimal("0.45")),
+    ]
+    extra = Decimal("100")
+    assert apply_brackets(amount + extra, brackets) >= apply_brackets(amount, brackets)
+
+
+def test_breakdown_total_is_sum_of_components() -> None:
+    b = TaxBreakdown(
+        income_tax=Decimal("10000"),
+        national_insurance=Decimal("3000"),
+        capital_gains_tax=Decimal("500"),
+        dividend_tax=Decimal("200"),
+        healthcare_levy=Decimal("100"),
+        other=Decimal("50"),
+    )
+    assert b.total == Decimal("13850")
+
+
+def test_inputs_default_to_zero() -> None:
+    i = TaxInputs()
+    assert i.earned_income == Decimal("0")
+    assert i.years_since_uk_departure == 0