Stack-Based Finite State Machine (Push Down Automata)

PushDownAutomata.cs

using System.Collections.Generic;

namespace Automata {
    public abstract class State {
        protected readonly PushDownAutomata pda;

        protected State(PushDownAutomata pda) {
            this.pda = pda;
        }
        
        public abstract void Enter();
        public abstract void Update();
        public abstract void Exit();
    }
    
    public struct Context {}
    
    public class PushDownAutomata {
        readonly Stack<State> stateStack = new();
        
        public State CurrentState => stateStack.Count > 0 ? stateStack.Peek() : null;
        
        public Context context;

        public PushDownAutomata(Context context) {
            this.context = context;
        }

        public void PushState(State newState) {
            stateStack.Push(newState);
            newState.Enter();
        }

        public void PopState() {
            if (stateStack.Count == 0) return;
            
            CurrentState.Exit();
            stateStack.Pop();
        }

        public void Update() => CurrentState?.Update();
        
        public void SetContext(Context context) => this.context = context;
    }
}