using System;
using System.Collections.Generic;

public class TCP {
  private static bool IsInitialized = false;
  private static Dictionary<string, Dictionary<string, string>> Transitions = new Dictionary<string, Dictionary<string, string>>();

  private static void InitializeIfNeeded() {
    if (IsInitialized) {
      return;
    }
    IsInitialized = true;

    var transitions = new List<(string, string, string)>() {
      ("CLOSED", "APP_PASSIVE_OPEN" , "LISTEN"),
      ("CLOSED", "APP_ACTIVE_OPEN"  , "SYN_SENT"),
      ("LISTEN", "RCV_SYN"          , "SYN_RCVD"),
      ("LISTEN", "APP_SEND"         , "SYN_SENT"),
      ("LISTEN", "APP_CLOSE"        , "CLOSED"),
      ("SYN_RCVD", "APP_CLOSE"      , "FIN_WAIT_1"),
      ("SYN_RCVD", "RCV_ACK"        , "ESTABLISHED"),
      ("SYN_SENT", "RCV_SYN"        , "SYN_RCVD"),
      ("SYN_SENT", "RCV_SYN_ACK"    , "ESTABLISHED"),
      ("SYN_SENT", "APP_CLOSE"      , "CLOSED"),
      ("ESTABLISHED", "APP_CLOSE"   , "FIN_WAIT_1"),
      ("ESTABLISHED", "RCV_FIN"     , "CLOSE_WAIT"),
      ("FIN_WAIT_1", "RCV_FIN"      , "CLOSING"),
      ("FIN_WAIT_1", "RCV_FIN_ACK"  , "TIME_WAIT"),
      ("FIN_WAIT_1", "RCV_ACK"      , "FIN_WAIT_2"),
      ("CLOSING", "RCV_ACK"         , "TIME_WAIT"),
      ("FIN_WAIT_2", "RCV_FIN"      , "TIME_WAIT"),
      ("TIME_WAIT", "APP_TIMEOUT"   , "CLOSED"),
      ("CLOSE_WAIT", "APP_CLOSE"    , "LAST_ACK"),
      ("LAST_ACK", "RCV_ACK"        , "CLOSED"),
    };

    foreach (var (fromState, byEvent, toState) in transitions) {
      var fromStateDictionary = Transitions.GetValueOrDefault(fromState, new Dictionary<string, string>());
      fromStateDictionary.Add(byEvent, toState);
      Transitions[fromState] = fromStateDictionary;
    }
  }

  public static string TraverseStates(string[] events) {
    InitializeIfNeeded();

    var state = "CLOSED"; // initial state, always

    foreach (var ev in events) {
      state = Transitions
        .GetValueOrDefault(state, new Dictionary<string, string>())
        .GetValueOrDefault(ev, "ERROR");
    }

    return state;
  }
}