import threading
import unittest


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

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

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

    def decrement(self, amount: int = 1) -> int:
        """Decrease the counter and return its new value."""
        return self.increment(-amount)

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


class ThreadSafeCounterTests(unittest.TestCase):
    def test_basic_operations(self) -> None:
        counter = ThreadSafeCounter(10)

        self.assertEqual(counter.increment(5), 15)
        self.assertEqual(counter.decrement(3), 12)
        self.assertEqual(counter.value, 12)

    def test_concurrent_increments(self) -> None:
        counter = ThreadSafeCounter()
        thread_count = 8
        increments_per_thread = 10_000

        def worker() -> None:
            for _ in range(increments_per_thread):
                counter.increment()

        threads = [threading.Thread(target=worker) for _ in range(thread_count)]
        for thread in threads:
            thread.start()
        for thread in threads:
            thread.join()

        self.assertEqual(counter.value, thread_count * increments_per_thread)


if __name__ == "__main__":
    unittest.main()