import threading
import unittest
from concurrent.futures import ThreadPoolExecutor


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:
        """Increment the counter and return its new value."""
        with self._lock:
            self._value += amount
            return self._value

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


class ThreadSafeCounterTests(unittest.TestCase):
    def test_increment(self) -> None:
        counter = ThreadSafeCounter(10)
        self.assertEqual(counter.increment(), 11)
        self.assertEqual(counter.increment(4), 15)
        self.assertEqual(counter.value, 15)

    def test_concurrent_increments(self) -> None:
        counter = ThreadSafeCounter()
        increments = 10_000

        # Multiple workers update the same counter concurrently.
        with ThreadPoolExecutor(max_workers=8) as executor:
            list(executor.map(lambda _: counter.increment(), range(increments)))

        self.assertEqual(counter.value, increments)


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