12_state_and_visitation.cpp

#include <iostream>
#include <stdexcept>
#include <variant>

namespace state {

struct Depleted {};

struct Available {
  int count;
};

struct Discontinued {};
}  // namespace state

using State =
    std::variant<state::Depleted, state::Available, state::Discontinued>;

namespace event {

struct DeliveryArrived {
  int count;
};

struct Purchased {
  int count;
};

struct Discontinued {};

}  // namespace event

State on_event(state::Available available, event::DeliveryArrived delivered) {
  available.count += delivered.count;
  return available;
}

State on_event(state::Available available, event::Purchased purchased) {
  available.count -= purchased.count;
  if (available.count > 0) return available;
  return state::Depleted{};
}

template <typename S>
State on_event(S, event::Discontinued) {
  return state::Discontinued{};
}

State on_event(state::Depleted depleted, event::DeliveryArrived delivered) {
  return state::Available{delivered.count};
}

template <class... Ts>
struct overload : Ts... {
  using Ts::operator()...;
};
template <class... Ts>
overload(Ts...) -> overload<Ts...>;

class ItemStateMachine {
 public:
  template <typename Event>
  void process_event(Event &&event) {
    state_ = std::visit(overload{
        [&](const auto &state)
          requires std::is_same_v<decltype(on_event(
                                      state, std::forward<Event>(event))),
                                  State>
        { return on_event(state, std::forward<Event>(event)); },
        [](const auto &unsupported_state) -> State {
          throw std::logic_error{"Unsupported state transition"};
        }},
        state_);
  }

  std::string report_current_state() {
    return std::visit(
        overload{[](const state::Available &state) -> std::string {
                   return std::to_string(state.count) + " items available";
                 },
                 [](const state::Depleted) -> std::string {
                   return "Item is temporarily out of stock";
                 },
                 [](const state::Discontinued) -> std::string {
                   return "Item has been discontinued";
                 }},
        state_);
  }

 private:
  State state_;
};

int main() {
  auto fsm = ItemStateMachine{};
  std::cout << fsm.report_current_state() << '\n';
  fsm.process_event(event::DeliveryArrived{3});
  std::cout << fsm.report_current_state() << '\n';
  fsm.process_event(event::Purchased{2});
  std::cout << fsm.report_current_state() << '\n';
  fsm.process_event(event::DeliveryArrived{2});
  std::cout << fsm.report_current_state() << '\n';
  fsm.process_event(event::Purchased{3});
  std::cout << fsm.report_current_state() << '\n';
  fsm.process_event(event::Discontinued{});
  std::cout << fsm.report_current_state() << '\n';
  // fsm.process_event(event::DeliveryArrived{1});
}