from __future__ import annotations

import unittest
from threading import Lock, Thread


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

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

    @property
    def value(self) -> int:
        # Reads are locked so callers never observe a partially updated value.
        with self._lock:
            return self._value

    def increment(self, amount: int = 1) -> int:
        # Return the new value to avoid requiring a separate read.
        with self._lock:
            self._value += amount
            return self._value

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

    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(3), 12)
        self.assertEqual(counter.reset(), 0)
        self.assertEqual(counter.value, 0)

    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: list[Thread] = [
            Thread(target=worker) for _ in range(thread_count)
        ]

        for thread in threads:
            thread.start()

        # Join all workers before checking the final shared state.
        for thread in threads:
            thread.join()

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


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