Hana State Machine (HSM)

The hana state machine (hsm) is a finite state machine library based on the boost hana meta programming library. It follows the principles of the boost msm and boost sml libraries, but tries to reduce own complex meta programming code to a minimum.

The following table compares features among popular c++ state machine libraries. A click on a particular feature check mark will forward to the feature documentation.

Feature	Hsm	Sml	Msm	Statechart
External transition	✓	✓	✓	✓
Anonymous transition (Completion)	✓	✓	✓	✗
Internal transition	✓	✓	✓	✓
Direct transition	✓	✗	✓	✗
Guards / actions	✓	✓	✓	✓
Entry / exit actions	✓	✓	✓	✓
Orthogonal regions	✓	✓	✓	✓
Hierachies / sub state machines	✓	✓	✓	✓
Event defering	✓	✓	✓	✓
Transition logging	✓	✓	✓	?
Initial pseudo state	✓	✓	✓	✓
History pseudo state	✓	✓	✓	✓
eUml postfix frontend	✓	✓	✓	✗
eUml prefix frontend	✓	✓	✓	✗
Entry / exit pseudo state	✓	✗	✓	✗
State data members	✓	✓	✓	✓
Unexpected event / no transition handler	✓	✗	✓	✗
Dependency injection	✓	✓	✗	✗
Single amalgamation header	✓	✓	✗	✗

Example

• Try it online: https://godbolt.org/z/5eMzfv

#include "hsm/hsm.h"

#include <cassert>

// States
struct Locked {
};
struct Unlocked {
};

// Events
struct Push {
};
struct Coin {
};

// Guards
const auto noError = [](auto /*event*/, auto /*source*/, auto /*target*/){return true;};

// Actions
const auto beep = [](auto /*event*/, auto /*source*/, auto /*target*/){ 
  std::cout << "beep!" << std::endl;
};

const auto blink = [](auto /*event*/, auto /*source*/, auto /*target*/){ 
  std::cout << "blink, blink, blink!" << std::endl;
};

struct Turnstile {
    static constexpr auto make_transition_table()
    {
        // clang-format off
        return hsm::transition_table(
            // Source                 + Event               [Guard]   / Action = Target
            // +----------------------+---------------------+---------+--------+------------------------+
            * hsm::state<Locked> {}   + hsm::event<Push> {}           / beep   = hsm::state<Locked> {}  ,
              hsm::state<Locked> {}   + hsm::event<Coin> {} [noError] / blink  = hsm::state<Unlocked> {},
            // +----------------------+---------------------+---------+--------+------------------------+
              hsm::state<Unlocked> {} + hsm::event<Push> {} [noError]          = hsm::state<Locked> {}  ,
              hsm::state<Unlocked> {} + hsm::event<Coin> {}           / blink  = hsm::state<Unlocked> {}
            // +----------------------+---------------------+---------+--------+------------------------+
            );
        // clang-format on
    }
};

int main()
{
    hsm::sm<Turnstile> turnstileSm;

    // The turnstile is initially locked
    assert(turnstileSm.is(hsm::state<Locked> {}));

    // Inserting a coin unlocks it
    turnstileSm.process_event(Coin {});
    assert(turnstileSm.is(hsm::state<Unlocked> {}));

    // Entering the turnstile will lock it again
    turnstileSm.process_event(Push {});
    assert(turnstileSm.is(hsm::state<Locked> {}));

    return 0;
}

Runtime Benchmark Results

The benchmark result are taken from the state machine benchmark repository.

Benchmark	Hsm	Sml	Msm	Statechart
Simple state machine	49 ms	26 ms	38 ms	414ms
Complex state machine	526 ms	528 ms	541 ms	877 ms

Compiletime Benchmark Results

Benchmark	Hsm	Sml	Msm	Statechart
Simple state machine	10.010 s	0.786 s	5.380 s	1.480 s
Complex state machine	77.120 s	3.130 s	26.820 s	5.190 s

Dependencies

• Boost 1.72
• C++17
• >= g++-8
• >= clang-8

Play with it Online

• Follow the link to the compiler explorer: https://godbolt.org/z/P9b1dY

Usage as Single Header

• Download amalgamation header and put it into your project src folder
• Include amalgamation header:

  #include "path/to/amalgamation/header/hsm_gen.h"

Install with CMake

mkdir src/build && cd src/build
cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/tmp/
cmake --build . --target install

Install with Conan/Cmake

mkdir src/build && cd src/build
conan install ../..
cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/tmp/
cmake --build . --target install

Install from Conan

conan remote add conan-erikzenker https://api.bintray.com/conan/erikzenker/conan-erikzenker
conan install hsm/1.0@erikzenker/testing --build missing

Install from AUR

pacaur -S hsm-git

Compile and Run the Tests Using the Installed Library

mkdir test/build/ && cd test/build/
cmake .. -DCMAKE_BUILD_TYPE=Release
cmake --build . --target hsmTests
ctest -VV

Author

• erikzenker(at)hotmail.com