cpp-concurrency/ZeroEvenOdd.cpp at main · ashutoshdhande/cpp-concurrency · GitHub

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/*

Example 1:

Input: n = 2
Output: "0102"
Explanation: There are three threads being fired asynchronously.
One of them calls zero(), the other calls even(), and the last one calls odd().
"0102" is the correct output.
Example 2:

Input: n = 5
Output: "0102030405"

*/

#include <iostream>
#include <thread>
#include <condition_variable>
#include <mutex>

using namespace std;

mutex mtx;
condition_variable cv;

int n = 5;
int num = 0;

bool zeroTurn = true, oddTurn = false, evenTurn = false;

void Zero()
{
    while (true)
    {
        unique_lock<mutex> ulock(mtx);
        cv.wait(ulock, [&]
                { return zeroTurn || num >= n; });
        if (num >= n)
        {
            cv.notify_all();
            return;
        }
        cout << 0;
        zeroTurn = false;
        oddTurn = evenTurn = false;
        if (num % 2 == 0)
        {
            oddTurn = true;
        }
        else
        {
            evenTurn = true;
        }

        ulock.unlock();
        cv.notify_all();
    }
}

void Odd()
{
    while (num < n)
    {
        unique_lock<mutex> ulock(mtx);
        cv.wait(ulock, [&]
                { return oddTurn || num >= n; });
        if (num >= n)
        {
            cv.notify_all();
            return;
        }
        num++;
        cout << num;
        oddTurn = false;
        zeroTurn = true;
        ulock.unlock();
        cv.notify_all();
    }
}

void Even()
{
    while (num < n)
    {
        unique_lock<mutex> ulock(mtx);
        cv.wait(ulock, [&]
                { return evenTurn || num >= n; });
        if (num >= n)
        {
            cv.notify_all();
            return;
        }
        num++;
        cout << num;
        evenTurn = false;
        zeroTurn = true;
        ulock.unlock();
        cv.notify_all();
    }
}

int main()
{
    thread t1(Zero);
    thread t2(Even);
    thread t3(Odd);
    // cout << "All threads fired start routine" << endl;
    t1.join();
    t2.join();
    t3.join();
    cout << endl;
    // cout << "Ahead of thread joins" << endl;

    return 0;
}