Game Ai using Enum as Finite State Machine

Question

I want to implement an ai into a simple street fighter style game, and I want to do this with a finite state machine. For a simple example, this FSM has the states:

Attacking, Chasing, Fleeing

From what I have read online, a good way of implementing this would be to use an Enum, although I'm slightly confused how to do this.

At any one point the FMS is in a current state and should change occur in the game, this state could change through a transition function (next()). Using an Enum like below, how would I keep track of the current state, and how could I make this change when the next() function is called?

public enum FiniteStateAutomata {

  ATTACKING() {
    public FiniteStateAutomata next() {
      if (!gun.isInRange()) return CHASING;
      else if (health.isLow()) return FLEEING;
    }

  },
  CHASING() {
    public FiniteStateAutomata next() {
      if (gun.isInRange()) return ATTACKING;
      else if (health.isLow()) return FLEEING;
    }
  },
  FLEEING() {
    public FiniteStateAutomata next() {
      if (health.isHigh()) return CHASING;

    }
  };

  public abstract FiniteStateAutomata next();


}

Answer 1

how would I keep track of the current state, and how could I make this change when the next() function is called?

In your game you need to keep track of the current state :

FiniteStateAutomata state = FiniteStateAutomata.ATTACKING;

and change it when needed :

state = state.next();

Answer 2

The good solution for that is using functions in your enumerations. Like the next one:

 public enum FiniteStateAutomata {
        ATTACKING((gun, health) -> {
            if (!gun.isInRange()) return CHASING;
            else if (health.isLow()) return FLEEING;
        }),
        CHASING((gun, health) -> {
            if (gun.isInRange()) return ATTACKING;
            else if (health.isLow()) return FLEEING;
        }),
        FLEEING((gun, health) -> {
            if (health.isHigh()) return CHASING;
            else return FLEEING;
        });

        private BiFunction<Gun, Health, FiniteStateAutomata> next;

        FiniteStateAutomata(BiFunction<Gun, Health, FiniteStateAutomata> next) {
            this.next = next;
        }

        public FiniteStateAutomata next(Gun gun, Health health) {
            return next.apply(gun, health);
        }
    }

Answer 3

Sure you can make it work, but you would have to pass the game state to the next(...)-function as a parameter. E.g. the enum would return what the next state would be for a game in a given state. It would not hold references to any game state itself.

Personally... there's no point in involving the limitations of enum into this.

Answer 4

Implementing your own State Machine is not an easy undertaking because one theory of the State Machine that most people ignores but in my opinion it is impossible to model complex state machines, sub / nested State Machines which is critical to prevent State Explosion problem that I discussed here.

Or if you want an independent discussion about the subject.

I don't which platform you plan to develop a Java game but if you plan to implement a State Machine at server side, I advice you to check Akka FSM.

If you need an implementation example for Akka, please check blogs blog1 blog2