from __future__ import annotations

import threading
import unittest
from concurrent.futures import ThreadPoolExecutor


class ThreadSafeCounter:
    """A small integer counter guarded by a lock."""

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

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

    def increment(self, amount: int = 1) -> int:
        # The read-modify-write operation must happen while locked.
        with self._lock:
            self._value += amount
            return self._value

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

    def reset(self, value: int = 0) -> int:
        with self._lock:
            self._value = value
            return self._value


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

        self.assertEqual(counter.increment(), 11)
        self.assertEqual(counter.increment(4), 15)
        self.assertEqual(counter.decrement(5), 10)
        self.assertEqual(counter.reset(), 0)
        self.assertEqual(counter.value, 0)

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

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

        with ThreadPoolExecutor(max_workers=workers) as executor:
            futures = [executor.submit(add_many) for _ in range(workers)]
            for future in futures:
                future.result()

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


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