coroutine(コルーチン)を使ってお手軽なState Machine(ステートマシン)を作ってみた

コルーチンを使って、お手軽なステートマシンを作ってみました。

ステートマシンというと、Boost.MSMをはじめ色々ライブラリーがあり、いまさらな感じなのですが、

• 重かったり（ビルド時間が）

• 面倒くさかったり（コードジェネレーターを使うのが）

するので、そんな大げさなことをしたくないとき用です。

#include <iostream>

#include <boost/assert.hpp>
#include <boost/asio/coroutine.hpp>

constexpr int ANY_EVENT = ~0,
              EVENT1 = 1 << 1,
              EVENT2 = 1 << 2,
              EVENT3 = 1 << 3;

enum state_t {
    STATE1 = 1,
    STATE2,
};

#include <boost/asio/yield.hpp>
template <typename Action>
class state_machine
{
public:
    state_machine()
    :   ctx_(), state_(), mask_(ANY_EVENT), action_()
    {
        do_process_event(0);
    }

    void process_event(int event)
    {
        if (event & mask_)
            do_process_event(event);
        else
            action_.on_error(state_, event);
    }

    bool is_complete() const
    {
        return ctx_.is_complete();
    }

private:
    void do_process_event(int event)
    {
        reenter(ctx_) {
            while (1) {
                yield accept_event(STATE1, EVENT1 | EVENT2);
                if (event == EVENT2) {
                    action_.do_action2();
                    break;
                }
                else {
                    action_.do_action1();
                }
            }

            yield accept_event(STATE2, EVENT3);
            action_.do_action3();
        }
    }

    void accept_event(state_t state, int mask) {
        if (state != state_) {
            state_t tmp = state_;
            state_ = state;
            action_.on_state_changed(tmp, state);
        }
        mask_ = mask;
    }

    boost::asio::coroutine ctx_;
    state_t state_;
    int mask_;
    Action action_;
};
#include <boost/asio/unyield.hpp>

//-------------------------------------------------------------------
// BEGIN TEST CODE

using namespace std;

int check = 0;
struct dummy_action {
    void on_state_changed(state_t before, state_t after) { cout << "State changed: " << before << " -> " << after << endl; }
    void do_action1() { check = 1; cout << "Action 1 is called" << endl; }
    void do_action2() { check = 2; cout << "Action 2 is called" << endl; }
    void do_action3() { check = 3; cout << "Action 3 is called" << endl; }
    void on_error(state_t state, int event) { cout << state << ": Event " << event << " is ignored." << endl; }
};

int main() {
    state_machine<dummy_action> sm;

    check = 0;
    sm.process_event(EVENT1);
    BOOST_ASSERT(check == 1);

    check = 0;
    sm.process_event(EVENT3);
    BOOST_ASSERT(check == 0);

    check = 0;
    sm.process_event(EVENT1);
    BOOST_ASSERT(check == 1);

    check = 0;
    sm.process_event(EVENT2);
    BOOST_ASSERT(check == 2);

    check = 0;
    sm.process_event(EVENT3);
    BOOST_ASSERT(check == 3);

    BOOST_ASSERT(sm.is_complete() == true);

    return 0;
}

ビルド方法

g++ -std=c++11 -I$BOOST_ROOT state_machine_demo.cpp

実行結果

State changed: 0 -> 1
Action 1 is called
1: Event 8 is ignored.
Action 1 is called
Action 2 is called
State changed: 1 -> 2
Action 3 is called