from __future__ import annotations

from threading import Lock, Thread


class ThreadSafeCounter:
    """A counter protected against concurrent access."""

    def __init__(self, initial: int = 0) -> None:
        self._value = initial
        self._lock = Lock()

    def increment(self, amount: int = 1) -> int:
        """Increment the counter and return its new value."""
        with self._lock:
            self._value += amount
            return self._value

    def value(self) -> int:
        """Return a consistent snapshot of the current value."""
        with self._lock:
            return self._value


def test_sequential_updates() -> None:
    counter = ThreadSafeCounter(10)
    assert counter.increment() == 11
    assert counter.increment(4) == 15
    assert counter.value() == 15


def test_concurrent_updates() -> None:
    counter = ThreadSafeCounter()
    workers = 8
    increments_per_worker = 10_000

    def increment_repeatedly() -> None:
        for _ in range(increments_per_worker):
            counter.increment()

    threads = [Thread(target=increment_repeatedly) for _ in range(workers)]
    for thread in threads:
        thread.start()
    for thread in threads:
        thread.join()

    assert counter.value() == workers * increments_per_worker


def main() -> None:
    test_sequential_updates()
    test_concurrent_updates()
    print("All tests passed.")


if __name__ == "__main__":
    main()