import os
import random
from stable_partition import stable_partition


def test_empty():
    assert stable_partition([], lambda x: True) == ([], [])


def test_all_true():
    assert stable_partition([1, 2, 3], lambda x: True) == ([1, 2, 3], [])


def test_all_false():
    assert stable_partition([1, 2, 3], lambda x: False) == ([], [1, 2, 3])


def test_mixed_preserves_order():
    items = [1, 2, 3, 4, 5, 6]
    yes, no = stable_partition(items, lambda x: x % 2 == 0)
    assert yes == [2, 4, 6]
    assert no == [1, 3, 5]


def test_none_items_raises():
    try:
        stable_partition(None, lambda x: True)
    except TypeError:
        return
    raise AssertionError("expected TypeError")


def test_random_stability():
    rng = random.Random(42)
    for _ in range(200):
        n = rng.randint(0, 30)
        items = [rng.randint(0, 9) for _ in range(n)]
        thr = rng.randint(0, 9)
        yes, no = stable_partition(items, lambda x, t=thr: x >= t)
        # membership
        assert sorted(yes + no) == sorted(items)
        # order preservation
        iy = [i for i, x in enumerate(items) if x >= thr]
        in_ = [i for i, x in enumerate(items) if x < thr]
        assert [items[i] for i in iy] == yes
        assert [items[i] for i in in_] == no