#include <atomic>
#include <thread>
#include <vector>
#include <iostream>

// Simple spinlock built from std::atomic_flag.
struct spinlock {
    std::atomic_flag flag = ATOMIC_FLAG_INIT;
    void lock() {
        while (flag.test_and_set(std::memory_order_acquire)) {
            // busy‑wait
        }
    }
    void unlock() {
        flag.clear(std::memory_order_release);
    }
};

constexpr int increments = 100'000;
spinlock mtx;
int protected_value = 0;

void worker() {
    for (int i = 0; i < increments; ++i) {
        mtx.lock();
        ++protected_value;
        mtx.unlock();
    }
}

int main() {
    std::thread t1(worker);
    std::thread t2(worker);
    t1.join();
    t2.join();
    std::cout << "protected value = " << protected_value << "\n";
    // EXPECT: protected value = 200000
    return 0;
}