from datetime import date
from decimal import Decimal

from app.engine.loans import add_months, annuity_payment, loan_schedule


def test_add_months_clamps_month_end():
    assert add_months(date(2026, 1, 31), 1) == date(2026, 2, 28)


def test_annuity_payment():
    # 10.000 EUR, 6 % p.a., 48 Monate -> 234,85 EUR Monatsrate
    assert annuity_payment(Decimal("10000"), Decimal("6"), 48) == Decimal("234.85")


def test_annuity_schedule_amortizes_fully():
    plan = loan_schedule(Decimal("10000"), Decimal("6"), 48, date(2026, 8, 1), "annuity")
    assert len(plan) == 48
    assert plan[-1].remaining == Decimal("0.00")
    assert plan[0].due == date(2026, 9, 1)
    assert plan[0].interest == Decimal("50.00")  # 10000 * 0,5 %
    assert plan[0].payment < 0


def test_bullet_loan():
    plan = loan_schedule(Decimal("10000"), Decimal("6"), 12, date(2026, 8, 1), "bullet")
    assert all(i.principal == 0 for i in plan[:-1])
    assert plan[-1].principal == Decimal("10000")
    assert plan[-1].remaining == Decimal("0.00")