Requirements

Let's dive into a fascinating realm: creating a robust Finite State Machine (FSM). Imagine you're building the core logic for a sophisticated vending machine, a complex traffic light system, or even the AI for a simple game character. At the heart of each of these lies a finite state machine, dictating the behavior and transitions between different states. The beauty of an FSM is its clear, deterministic nature. It allows us to model systems with a finite number of states and well-defined transitions between those states, triggered by specific events or inputs. Your task is to design and implement a flexible and extensible FSM that can be adapted to various scenarios. Consider how to represent states, transitions, and the actions performed during those transitions. How will you ensure the FSM remains consistent and predictable, even when faced with complex state changes or concurrent events? This isn't just about making it work; it's about crafting a design that's easy to understand, modify, and extend as the system evolves. Think about how to encapsulate behavior, decouple components, and handle errors gracefully. # Requirements ## Functional Requirements 1. **State Definition:** * The FSM must support the definition of states. Each state should have a unique identifier (e.g., a string or an enum value). * States can be created, retrieved, and potentially removed (depending on the design's immutability or mutability). 2. **Transition Definition:** * The FSM must allow defining transitions between states. A transition is triggered by an event and defines the next state to move to. * Transitions should be definable from one state to another. * Support "self-transitions" (a state transitioning back to itself on a specific event). * Transitions can have associated actions (see below). 3. **Event Handling:** * The FSM must accept events as input. * Upon receiving an event, the FSM must determine the appropriate transition based on the current state and the event type. * If a transition exists for the current state and event, the FSM must execute the transition and move to the new state. * If no transition exists for the current state and event, the FSM should handle the situation gracefully (e.g., by ignoring the event, logging an error, or throwing an exception). Define the strategy for handling unhandled events. 4. **Actions:** * Transitions can have associated actions that are executed when the transition occurs. * Actions should be decoupled from the FSM core logic. This allows for flexible and reusable actions. * Actions can modify the state of an external context object (e.g., update variables in a vending machine's inventory). 5. **State Retrieval:** * The FSM must provide a way to retrieve the current state. 6. **Start State:** * The FSM must have a designated start state. * Upon initialization, the FSM should automatically enter the start state. 7. **Context Passing:** * The FSM should be able to work with a context object, allowing the states and transitions to access and modify external data. This decouples the FSM's core logic from specific data structures. ## Non-Functional Requirements 1. **Extensibility:** * The FSM should be designed to be easily extensible. It should be possible to add new states, transitions, and actions without modifying the core FSM logic. * Consider the use of design patterns to achieve extensibility (e.g., Strategy pattern for actions). 2. **Modularity:** * The FSM should be modular, with clear separation of concerns. Different components of the FSM (e.g., state management, transition handling, action execution) should be decoupled and independently testable. 3. **Testability:** * The FSM should be designed to be easily testable. It should be possible to write unit tests for individual components of the FSM, as well as integration tests for the entire FSM. 4. **Thread Safety:** * The FSM should be thread-safe if it's expected to be used in a multithreaded environment. Consider potential race conditions when multiple threads attempt to trigger transitions simultaneously. Implement appropriate synchronization mechanisms (e.g., locks, atomic variables) to ensure data consistency. Concurrency should be handled at the FSM level, not left to the client code using the FSM. 5. **Error Handling:** * The FSM should handle errors gracefully. It should be able to recover from unexpected events or invalid transitions. Implement appropriate error handling mechanisms (e.g., exceptions, logging) to provide informative feedback. 6. **Immutability (Consideration):** * While not strictly required, consider whether parts of the FSM (e.g., the state definitions, transition table) could be made immutable to improve thread safety and simplify reasoning about the FSM's behavior. If so, which parts and why? ## Core Entities 1. **`State`:** Represents a state in the FSM. It needs a unique identifier. Consider if a State can hold data. 2. **`Event`:** Represents an event that triggers a transition. It needs a unique identifier. Consider if an Event can hold data. 3. **`Transition`:** Represents a transition between states. It defines the event that triggers the transition, the source state, the destination state, and the action to be executed. 4. **`Action`:** Represents an action to be performed during a transition. This should be an interface to allow for different types of actions. This will likely receive a Context. 5. **`FSM`:** The core class that manages the states, transitions, and event handling. It maintains the current state and processes events to trigger transitions. It uses the Transition table. 6. **`Context`:** An optional object that holds data relevant to the FSM's operation. It's passed to Actions, allowing them to interact with the external environment or modify data outside the FSM itself.

Implement a finite state machine.

Requirements

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