import threading
import unittest
from concurrent.futures import ThreadPoolExecutor


class ThreadSafeCounter:
    """A counter protected by a lock for safe concurrent access."""

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

    @property
    def value(self) -> int:
        with self._lock:
            return self._value

    def increment(self, amount: int = 1) -> int:
        with self._lock:
            self._value += amount
            return self._value

    def decrement(self, amount: int = 1) -> int:
        return self.increment(-amount)


class ThreadSafeCounterTests(unittest.TestCase):
    def test_increment_and_decrement(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()
        workers = 8
        increments_per_worker = 1_000

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

        # Exercise the counter from several threads at once.
        with ThreadPoolExecutor(max_workers=workers) as executor:
            futures = [executor.submit(increment_many) for _ in range(workers)]
            for future in futures:
                future.result()

        self.assertEqual(counter.value, workers * increments_per_worker)


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