using System;

using System.Linq;

using System.Collections.Generic;

​

//https://stackoverflow.com/questions/31166013/what-is-the-best-way-to-implement-a-rust-enum-in-c

​

public interface IState 

{

    public virtual Action<string> GetTokenHandler() => (s) => Console.WriteLine(s);

}

​

public sealed class State : IState

{

    private string state;

    private State(string state) => this.state = state;

​

    // Enum-like states with no internal data

    public static State Initial { get; } = new State(nameof(Initial));

    public static State Final { get; } = new State(nameof(Final));

    public static State Error { get; } = new State(nameof(Error));

    public override string ToString() => state;

}

​

// Record states with internal data

public record class StateA(string Token, int Count) : IState

{

    public Action<string> GetTokenHandler() => (s) => Console.WriteLine("The current count is {0} and the current token is {1}", Count, Token);

}

public record class StateB(string Token) : IState;

​

public class TestClass

{

    const string terminalString = "stop";

    IEnumerator<string> inputs = Enumerable.Range(1, 6).Select(i => "token " + i.ToString()).Concat(new [] {terminalString}).GetEnumerator();

    string GetNextToken()

    {

        inputs.MoveNext();

        return inputs.Current;

    }

    public void Test()

    {

        string terminalString = "stop";

        int maxIterations = 100;

        (int count, IState state) = (0, State.Initial);

​

        while (state != State.Final && state != State.Error)

        {

            // Get the next token

            string token = GetNextToken();

            // Do some work with the current state + next token

            state.GetTokenHandler()(token);

            Console.WriteLine("State = {0}", state);

​

            // Transition to the new state

            state = state switch // Switch Expression: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/switch-expression

            {

                _ when count > maxIterations =>

                    state = State.Error,

                State s when s == State.Initial => 

                    new StateA(token, 0),

                StateA s when s is { Count : > 3 } =>  //https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/patterns#property-pattern

                    new StateB (token),

                StateA s =>

                    s with { Token = token, Count = s.Count + 1 },  // https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/record#nondestructive-mutation

                StateB s when s.Token == terminalString =>

                    State.Final,

                StateB s =>

                    s with { Token = token },

                _ => throw new Exception($"Unknown state {state}"),

            };

            // Do some additional work with the new state

        }

        Console.WriteLine("State = {0}", state);

    }

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Environment version: {0} ({1}), {2}\n", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

​

        new TestClass().Test();

    }

}