Game AI - Behavior Trees

Question

How do you make a robust AI/scripting system for a game?

1) For all NPC's/environment/entities do you give them a SEPARATE single behavior tree (etc. patrolBehavior, allyBehavior, vendorBehavior, doorBehavior)? If there are 500 units on the screen, should I do a full pass on the tree (going from root -> node/action) or should I do 1-node progress for all the units?

2) I am doing the AI logic in the update() function... but I heard some games have their separate AI thread, any ideas?

3) I'm wondering how to divide my game into sections/chapters... do I use a simple variable (EVENT="Mission 3") to denote how for the player's been, and make it all linear? And then utilize variable in the trees above?

Answer 1

I'll try to answer your question.

1. I do all branching logic and behavior trees in a static class for example:

     public static class Behavior
    {
        //Branch
        public static Action Selector(Func<bool> cond, Action ifTrue, Action ifFalse) {
            return () => { if (cond()) { ifTrue(); } else { ifFalse(); } };
            }
        public static Action Sequencer(Action a, Action b) {
              return () => { a(); b(); }
            }

        //Example trees
        public static Func<bool> ifPlayerIsInSight = () => { return true; /*...true iff WorldState shows guard can see player...*/};

        public static Action shootAtPlayer = () => { /*...aim guard's weapon at player and fire...*/ };

        public static Func<bool> ifUnderFire = () => { return true; /*...true iff WorldState shows guard hears player gunfire...*/};

        public static Action takeCover = () => { /*...guard runs for nearest shelter... */};

        public static Action walkBackAndForthGuardingDoorway = () => { /*...default guard patrol behaviour...*/ };

        public static Action patrollingGuardBehaviour =
          Selector(Behavior.ifPlayerIsInSight, Behavior.shootAtPlayer,
            Selector(Behavior.ifUnderFire, Behavior.takeCover,
              Behavior.walkBackAndForthGuardingDoorway));
    }

2. Do it in LateUpdate() or last so it does not lag the main loop.

3. It is upto you. You could implement a "State" in each behavior tree or split it out and manage which get used at what time.