Hierarchical State Machine

This project presents an implementation of a Hierarchical State Machine (HSM) using a media player system as an example. The HSM approach organizes the control logic into a hierarchy of states, making it easier to manage complex behaviors and transitions.

The media player system is modeled with two main levels:

• Superstate: Handles the device's power (On/Off).
• Substates: When powered on, the device can be in one of three modes: Playing, Paused, or Stopped.

The state transitions are illustrated in the following diagram:

---
title: Hierarchical State Machine 
---
stateDiagram-v2
    PowerOff --> PowerOn : Power Toggle
    PowerOn --> PowerOff : Power Toggle
    state PowerOn{
        [*] --> stop
        stop --> play : play/pause
        play --> pause : play/pause
        pause --> play : play/pause
        pause --> stop : stop
    }

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Key C++ Techniques Used

This implementation demonstrates several important C++ programming techniques:

• Polymorphism and Inheritance: All states inherit from a common interface, enabling dynamic (runtime) resolution of state transitions.
• Singleton Pattern: Each state is implemented as a singleton, ensuring only one instance of each state exists in the system.
• Encapsulation: State-specific logic and transitions are encapsulated within each state class, making the system modular and easy to extend.
• Runtime Resolution: State transitions are determined at runtime based on user input and the current state, reflecting the dynamic nature of state machines.

System Structure

The system is organized as follows:

• The main interface is established through the State and System classes.
• The superstate consists of two states: Power Off and Power On.
• When the device is powered on, the media player subsystem always starts in the Stopped state.
• The Power On state shares its interface with the substates (Playing, Paused, Stopped) through inheritance.

The class relationships are shown in this diagram:

classDiagram
    class System {
        +void systemEvent()
        +void systemStatus()
        +void runJobs()
        +void setCurrentState(shared_ptr~State~ newState)
        -shared_ptr~State~ currentState_
        -thread eventListernerJob
        -thread systemRunJob
    }
    class State {
        +void init()
        +void exit()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
        -string msg_
        -string commands_
    }
    class PowerOn {
        +static shared_ptr~State~ getIstance()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
    }
    class PowerOff {
        +static shared_ptr~State~ getIstance()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
    }
    class MediaPlayer {
        <<abstract>>
        +shared_ptr~State~ handleEvent(Events event)
        -string speaker
    }
    class Play {
        +static shared_ptr~State~ getIstance()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
    }
    class Pause {
        +static shared_ptr~State~ getIstance()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
    }
    class Stop {
        +static shared_ptr~State~ getIstance()
        +shared_ptr~State~ handleEvent(Events event)
        +void stateRun()
    }
    class KeyConfig {
        +static map~char, Events~ mapEvents
    }

    System --> State : manages
    State <|-- PowerOn : inherits
    State <|-- PowerOff : inherits
    PowerOn <|-- MediaPlayer : inherits
    MediaPlayer <|-- Play : inherits
    MediaPlayer <|-- Pause : inherits
    MediaPlayer <|-- Stop : inherits
    System --> KeyConfig : uses

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Building and Running

Ubuntu / Debian

Install required packages:

sudo apt-get update
sudo apt-get install build-essential cmake g++ 

Set up the build environment from the project root:

mkdir build
cmake -S . -B build

Build and run the application:

cmake --build build
./build/HSM_Media_Player

Press CTRL + C to exit the program.

Docker

Build the Docker image:

docker build -t HSM_Media_Player .

Run the application:

docker run -it HSM_Media_Player

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License

MIT License

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Author

Michele Di Lecce
michele.dilecce@hotmail.com