main.cpp

#include <ctime>
#include <future>
#include <iostream>
#include <thread>

using namespace std;

#define EPSILON 1e-5
#define ASYNCMODE
#define DELAY

double f1(double x) { return 1 / x; }
double f2(double x) { return x; }
double f3(double x) { return 1 / x; }
double f4(double x) { return 1 / x; }
double f5(double x) { return 1 / x; }

double quad(double left, double right, double fleft, double fright,
            double lrarea, double f(double)) {
#ifdef DELAY
  std::chrono::milliseconds timespan(1); // or whatever
  std::this_thread::sleep_for(timespan);
#endif
  double mid = (left + right) / 2;
  double fmid = f(mid);
  double larea = (fleft + fmid) * (mid - left) / 2;
  double rarea = (fmid + fright) * (right - mid) / 2;
  if (abs(larea + rarea - lrarea) > EPSILON) {
#ifdef ASYNCMODE
    future<double> larea_promise = async(&quad, left, mid, fleft, fmid, larea, f);
    future<double> rarea_promise =
        async(&quad, mid, right, fmid, fright, rarea, f);
    larea = larea_promise.get();
    rarea = rarea_promise.get();
#else
    larea = quad(left, mid, fleft, fmid, larea);
    rarea = quad(mid, right, fmid, fright, rarea);
#endif
  }
  return rarea + larea;
}

double quad(double left, double right, double f(double)) {
  return quad(left, right, f(left), f(right), (f(left) + f(right)) * (right - left) / 2, f);
}

int main(int argc, char *argv[]) {
  double a, b;
  a = 1;
  b = 1e9;
#ifdef ASYNCMODE
  cerr << "Async..." << endl;
#else
  cerr << "Single thread..." << endl;
#endif
  cout << quad(a, b, f1) << endl;
  return 0;
}