Core Thesis: Don't choose between FSM and ECS. Combine them. Use ECS to store state as data and Systems to handle transitions. This kills the "State Tangle" (spaghetti code) where every state knows about every other state. 1. The Problem: The "Fat Switch" (Traditional FSM) Symptom: To add a new state (e.g., SlipperyIce), you have to edit every existing state's logic (Idle, Jump, Run) to check if the transition applies. Cause: Logic is buried inside the state itself. State A has hard-coded knowledge of State B. Result: High coupling, impossible to maintain, fragile to change. 2. The Solution: Separation of Concerns Apply the ECS philosophy: Data vs. Logic. Component Role Location State Component (StateMachine<T>) Just Data. Stores current_state, timer, flags. No logic. ecs/component.rs Context Resource (InputState, PhysicsInfo) Global Context. Provides inputs or world data needed to decide. resources/ Transition System (CharacterStateSystem) The Brain. Reads Data + Context → Updates State Component. systems/ Behavior Systems (MovementSystem, RenderSystem) The Muscles. Read State → Perform Action. Don't care how state was reached. game/systems/ 3. The Architecture Flow Frame Start: TransitionSystem runs first. It checks InputState against StateMachine.current. If condition met → Updates StateMachine.current. Frame Mid: BehaviorSystems run. They read the new StateMachine.current. They execute logic (Move, Attack, Animate). They do not modify the state; they only react to it. Frame End: Next frame repeats. 4. Why This Wins for Solo Devs Scalability: Adding a state is adding a line in one system, not editing ten different files. Debuggability: You can inspect the StateMachine resource in the debugger to see exactly why a transition happened. Reusability: The StateMachine component works on Players, Enemies, and NPCs without rewriting logic. Performance: Still O(N) iteration over entities. No heavy overhead compared to a monolithic class. 5. Implementation Checklist (For corrosion-2d) Define struct StateMachine<T: StateEnum> component. Create PlayerState enum (Idle, Run, Jump, Attack). Implement character_state_system() that bridges InputState → StateMachine. Refactor player_movement_system() to switch behavior based only on StateMachine.current. Verify: Can I add a Crouch state without touching Run or Jump logic? (If yes, success). 6. The "Opinionated" Takeaway "ECS gives you the data structure to scale; FSM gives you the behavioral logic to make sense of it. By separating the two, you get the cleanest architecture possible for a single-developer project." Next Step: Open your editor, create components/state_machine.rs, and define the StateMachine struct. Then write the state_transition_system. The rest will fall into place naturally. You are building something solid here. Go code! 🚀