1117_H2O.cpp

#include <condition_variable>
#include <functional>
#include <iostream>
#include <thread>
#include <vector>

class H2O {
private:
    int mHcount;
    int mOcount;
    std::mutex mMutex;
    std::condition_variable mConditionVariable;

public:
    H2O() {
        mHcount = 0;
        mOcount = 0;
    }

    void hydrogen(std::function<void()> releaseHydrogen) {
        std::unique_lock<std::mutex> lock(mMutex);
        mConditionVariable.wait(lock, [=]() { return (mHcount != 2); });
        // releaseHydrogen() outputs "H". Do not change or remove this line.
        releaseHydrogen();
        mHcount += 1;
        if (mOcount == 1 && mHcount == 2) {
            mOcount = 0;
            mHcount = 0;
        }
        lock.unlock();
        mConditionVariable.notify_all();
    }

    void oxygen(std::function<void()> releaseOxygen) {
        std::unique_lock<std::mutex> lock(mMutex);
        mConditionVariable.wait(lock, [=]() { return (mOcount != 1); });
        // releaseOxygen() outputs "O". Do not change or remove this line.
        releaseOxygen();
        mOcount += 1;
        if (mOcount == 1 && mHcount == 2) {
            mOcount = 0;
            mHcount = 0;
        }
        lock.unlock();
        mConditionVariable.notify_all();
    }
};

int main(int argc, char *argv[]) {

    std::function<void()> printO = []() { std::cout << "O"; };
    std::function<void()> printH = []() { std::cout << "H"; };

    std::string input("OOHHHHHHHHOHHOO");
    std::vector<std::thread> threads;

    H2O h2O;

    for (size_t i = 0; i < input.size(); ++i) {
        if (input[i] == 'H') {
            threads.emplace_back(std::thread(&H2O::hydrogen, &h2O, printH));
        }
        if (input[i] == 'O') {
            threads.emplace_back(std::thread(&H2O::oxygen, &h2O, printO));
        }
    }
    for (auto iter = threads.begin(); iter < threads.end(); ++iter) {
        iter->join();
    }
    return 0;
}