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	✓	✓	✗	✗
Custom target state construction	✓	✗	✗	✗
Chain actions	✓	✓	✓	?

Example (Run)

#include "hsm/hsm.h"

#include <iostream>
#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
constexpr auto beep
    = [](auto /*event*/, auto /*source*/, auto /*target*/) { std::cout << "beep!" << std::endl; };
constexpr 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
    }
};

auto main() -> int
{
    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;
}

Play with it Online

• Follow the link to the compiler explorer: https://godbolt.org/z/r9sTrMfqE

Runtime Benchmark Results

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

Benchmark	Hsm	Sml	Msm	Statechart
Simple state machine	99 ms	17 ms	18 ms	443 ms
Complex state machine	818 ms	978 ms	881 ms	1374 ms

Compiletime Benchmark Results

Benchmark	Hsm	Sml	Msm	Statechart
Simple state machine	6.41 s	0.62 s	5.17 s	1.52 s
Complex state machine	41.99 s	3.01 s	25.54 s	4.27 s

Compilememory Benchmark Results

Benchmark	Hsm	Sml	Msm	Statechart
Simple state machine	174.649 MB	28.474 MB	404.621 MB	70.976 MB
Complex state machine	815.720 MB	188.333 MB	1323.477 MB	122.720 MB

Dependencies

• Boost 1.72
• C++17
• >= g++-8
• >= clang-8
• Cmake 3.14

Dev Dependencies

• Gtest

Integration

Usage as Single Header

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

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

CMake

To use this library from a CMake project, you can locate it directly with find_package() and use the namespaced imported target from the generated package configuration:

# CMakeLists.txt
find_package(hsm 1.3.5 REQUIRED)
...
add_library(foo ...)
...
target_link_libraries(foo PRIVATE hsm::hsm)

Since CMake v3.11, FetchContent can be used to automatically download the repository as a dependency at configure time. You can follow this example and implement the following snippet:

include(FetchContent)

FetchContent_Declare(hsm
  GIT_REPOSITORY https://github.com/erikzenker/hsm.git
  GIT_TAG v1.4.7)

FetchContent_GetProperties(hsm)
if(NOT hsm_POPULATED)
  FetchContent_Populate(hsm)
  add_subdirectory(${hsm_SOURCE_DIR} ${hsm_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()

target_link_libraries(foo PRIVATE hsm::hsm)

If you are using CPM.cmake, you can follow this example. Implement the following snippet:

include(cmake/CPM.cmake)

CPMAddPackage(
    NAME hsm
    GITHUB_REPOSITORY erikzenker/hsm
    VERSION 1.4.7)

target_link_libraries(foo PRIVATE hsm::hsm)

Package Managers

If you are using Conan to manage your dependencies, merely add hsm/x.y.z to your conanfile's requires, where x.y.z is the release version you want to use. Please file issues here if you experience problems with the packages.

Install

CMake

cmake -S . -B build
cmake --install build/ --prefix /tmp/

Conan/Cmake

mkdir -p build/dependencies/conan
conan install . -if build/dependencies/conan -s compiler.libcxx=libstdc++11 --build missing
cmake -S . -B build -DCMAKE_BUILD_TYPE=Release -D "CMAKE_PREFIX_PATH=${PWD}/build/dependencies/conan"
cmake --install build/ --prefix /tmp/

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 Arch Linux AUR

pacaur -S hsm-git

Compile and Run the Tests Using the Installed Library

cmake -S test -B build
cmake --build build/test
cd build/test
ctest --output-on-failure

Compile and Run the Tests Using Conan

mkdir -p build/dependencies/conan
conan install . -if build/dependencies/conan -s compiler.libcxx=libstdc++11 --build missing
cmake -S test/ -B build/test -DCMAKE_BUILD_TYPE=Release -D "CMAKE_PREFIX_PATH=${PWD}/build/dependencies/conan"
cmake --build build/test/
cd build/test
ctest --output-on-failure

Author

• erikzenker(at)hotmail.com