"""Regression tests for the demo pricing helper."""

import pytest

from src.discount import apply_discount


def test_no_discount_returns_subtotal() -> None:
    assert apply_discount(100.0, 0) == 100.0


def test_ten_percent_off() -> None:
    assert apply_discount(100.0, 10) == 90.0


def test_full_discount_is_free() -> None:
    assert apply_discount(59.99, 100) == 0.0


def test_fractional_rounding() -> None:
    assert apply_discount(19.99, 15) == 16.99


def test_rejects_out_of_range_percent() -> None:
    with pytest.raises(ValueError):
        apply_discount(10.0, 101)