FiniteStateMachine.hpp

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// This file is part of the Acts project.
//
// Copyright (C) 2019 CERN for the benefit of the Acts project
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#pragma once

#include "Acts/Utilities/TypeTraits.hpp"

#include <optional>
#include <string_view>
#include <variant>

namespace Acts {

template <typename Derived, typename... States>
class FiniteStateMachine {
 public:
  struct Terminated {
    constexpr static std::string_view name = "Terminated";
  };

  using StateVariant = std::variant<Terminated, States...>;

 protected:
  using fsm_base = FiniteStateMachine<Derived, States...>;

  using event_return = std::optional<StateVariant>;

 public:
  FiniteStateMachine()
      : m_state(typename std::tuple_element<0, std::tuple<States...>>::type{}) {
  }

  FiniteStateMachine(StateVariant state) : m_state(std::move(state)) {}

  const StateVariant& getState() const noexcept { return m_state; }

 public:
  template <typename State, typename... Args>
  void setState(State state, Args&&... args) {
    Derived& child = static_cast<Derived&>(*this);

    // call on exit function
    std::visit([&](auto& s) { child.on_exit(s, std::forward<Args>(args)...); },
               m_state);

    m_state = std::move(state);

    // call on enter function, the type is known from the template argument.
    child.on_enter(std::get<State>(m_state), std::forward<Args>(args)...);
  }

  template <typename S>
  bool is(const S& state) const noexcept {
    (void)state;
    return is<S>();
  }

  template <typename S>
  bool is() const noexcept {
    if (std::get_if<S>(&m_state)) {
      return true;
    }
    return false;
  }

  bool terminated() const noexcept { return is<Terminated>(); }

 protected:
  template <typename Event, typename... Args>
  event_return process_event(Event&& event, Args&&... args) {
    Derived& child = static_cast<Derived&>(*this);

    child.on_process(event);

    auto new_state = std::visit(
        [&](auto& s) -> std::optional<StateVariant> {
          auto s2 = child.on_event(s, std::forward<Event>(event),
                                   std::forward<Args>(args)...);

          if (s2) {
            std::visit([&](auto& s2_) { child.on_process(s, event, s2_); },
                       *s2);
          } else {
            child.on_process(s, event);
          }
          return s2;
        },
        m_state);
    return new_state;
  }

 public:
  template <typename Event, typename... Args>
  void dispatch(Event&& event, Args&&... args) {
    auto new_state = process_event(std::forward<Event>(event), args...);
    if (new_state) {
      std::visit(
          [&](auto& s) { setState(std::move(s), std::forward<Args>(args)...); },
          *new_state);
    }
  }

 private:
  StateVariant m_state;
};

}  // namespace Acts