#include <iostream>
#include <iomanip>
#include <utility>
#include <future>
#include <chrono>

using namespace std;
using namespace std::chrono;

typedef pair<steady_clock::time_point, steady_clock::time_point> Timings;

/**
 * Calculate the nth fibonacci recursively.
 * (Generally a very bad idea.)
 * @param n which fibonacci number to calculate.
 */
long fibonacci(int n);

/**
 * Calculate the nth fibonacci recursively in a separate thread.
 * @return the start and end times as a pair.
 */
Timings fibonacci_thread(int n);

const int N1 = 46;
const int N2 = 45;

int main()
{
    // The first future result.
    cout << "Starting thread to calculate fibonacci(" << N1 << ")" << endl;
    auto times1 = async(launch::async, fibonacci_thread, N1);

    // The second future result.
    cout << "Starting thread to calculate fibonacci(" << N2 << ")" << endl;
    auto times2 = async(launch::async, fibonacci_thread, N2);

    // Wait for the results from each thread.
    Timings result1 = times1.get();
    Timings result2 = times2.get();

    // When the first thread started.
    auto first_start = (result1.first < result2.first) ? result1.first
                                                       : result2.first;

    // When the last thread ended.
    auto last_end = (result1.second > result2.second) ? result1.second
                                                      : result2.second;

    double elapsed = duration_cast<milliseconds>(last_end - first_start).count();
    cout << endl << "Total elapsed time = " << elapsed/1000 << " seconds" << endl;
}

Timings fibonacci_thread(int n)
{
   Timings result;
 
   auto   start   = steady_clock::now();
   long   value   = fibonacci(n);
   auto   end     = steady_clock::now();
   double elapsed = duration_cast<milliseconds>(end - start).count();

   cout << endl << "fibonacci(" << n << ") = " << value << endl;
   cout << "Calculation time = " << elapsed/1000 << " seconds" << endl;

   result.first  = start;
   result.second = end;

   return result;
}

long fibonacci(int n)
{                                                    
    if (n <= 1) return n;
    else        return fibonacci(n-2) + fibonacci(n-1);
}