import pytest
from decimal import Decimal
from src.contribution_util import prod, system_availability, get_all_components, birnbaum_importance

def test_prod():
    assert prod([1, 2, 3]) == 6.0
    assert prod([0.5, 0.5]) == 0.25
    assert prod([]) == 1.0

def test_system_availability_series():
    structure = {"series": ["A", "B"]}
    availabilities = {"A": 0.9, "B": 0.8}
    # 0.9 * 0.8 = 0.72
    assert system_availability(structure, availabilities) == pytest.approx(0.72)

def test_system_availability_parallel():
    structure = {"parallel": ["A", "B"]}
    availabilities = {"A": 0.9, "B": 0.8}
    # 1 - (1-0.9)*(1-0.8) = 1 - 0.1*0.2 = 1 - 0.02 = 0.98
    assert system_availability(structure, availabilities) == pytest.approx(0.98)

def test_system_availability_nested():
    # (A in series with (B in parallel with C))
    structure = {
        "series": [
            "A",
            {"parallel": ["B", "C"]}
        ]
    }
    availabilities = {"A": 0.9, "B": 0.8, "C": 0.7}
    # B||C = 1 - (1-0.8)*(1-0.7) = 1 - 0.2*0.3 = 0.94
    # A && (B||C) = 0.9 * 0.94 = 0.846
    assert system_availability(structure, availabilities) == pytest.approx(0.846)

def test_get_all_components():
    structure = {
        "series": [
            "A",
            {"parallel": ["B", "C"]}
        ]
    }
    assert get_all_components(structure) == {"A", "B", "C"}

def test_birnbaum_importance():
    structure = {"series": ["A", "B"]}
    availabilities = {"A": 0.9, "B": 0.8}
    # I_B(A) = A_sys(A=1) - A_sys(A=0)
    # A_sys(A=1, B=0.8) = 1 * 0.8 = 0.8
    # A_sys(A=0, B=0.8) = 0 * 0.8 = 0
    # I_B(A) = 0.8
    assert birnbaum_importance(structure, availabilities, "A") == pytest.approx(0.8)
    # I_B(B) = A_sys(B=1, A=0.9) - A_sys(B=0, A=0.9) = 0.9 - 0 = 0.9
    assert birnbaum_importance(structure, availabilities, "B") == pytest.approx(0.9)