/////////////////////////////////////////////////////////////////////////////
// CustomAsync.cpp - how std::asynch works (Fawcett approximation thereof) //
//                                                                         //
// Jim Fawcett, CSE687 - Object Oriented Design, Spring 2018               //
/////////////////////////////////////////////////////////////////////////////

#include <thread>
#include <future>
#include <iostream>
#include <chrono>
#include <type_traits>

/*----< custom approximation to std::async >-------------------------*/

template<typename Ret, typename Callable, typename Arg>
std::future<Ret> asynchCustom(Callable co, Arg arg)
{
  std::promise<Ret> prm;
  std::future<Ret> fut(prm.get_future());
  /* 
    Note: must capture co by value, as original co may go out of scope before thread calls it 
  */
  auto lambda = [co](Arg arg, std::promise<Ret> p) {
    std::cout << "\n  starting thread's work";
    Ret ret = co(arg);
    std::cout << "\n  thread's work complete";
    p.set_value(ret);
  };
  /*
    Need to move ownership of promise to lambda
  */
  std::thread t(lambda, arg, std::move(prm));  // running callable on child thread
  t.detach();                                  // detaching so no assert at program end
  return fut;
}
/*----< a callable used to test asynchCustom >-----------------------*/

std::string announce(const std::string& msg)
{
  std::cout << "\n  announce function is running asynchronously";
  std::this_thread::sleep_for(std::chrono::milliseconds(2000));
  return msg;
}

using R = std::string;
using A = const std::string&;
using C = R(*)(A);

int main()
{
  std::cout << "\n  Demonstrating asynchCustom";
  std::cout << "\n ============================";

  C callable = announce;
  A arg = "That's all folks!";
  std::future<R> r = asynchCustom<R>(callable, arg);
  std::cout << "\n  returned from asynchCustom(...)";

  /////////////////////////////////////////////////////////////////////
  // this works too:
  //   std::future<R> r = asynchCustom<R, C, A>(announce, "That's all folks!");

  std::string result = r.get();
  std::cout << "\n  " << "asynchCustom returned \"" << result.c_str() << "\"";
  std::cout << "\n";

  std::cout << "\n  repeating with a lamda callable";
  A arg2 = "Here's more folks!";
  std::future<R> r2 = asynchCustom<R>(
    [=](A arg2) { std::cout << "\n  running lambda asynchronously"; return arg2; }, arg2
  );
  result = r2.get();
  std::cout << "\n  " << "asynchCustom returned \"" << result.c_str() << "\"";
  std::cout << "\n\n";
  return 0;
}