BehaviourTrees.cs

protected delegate State State(bool? prev);
protected readonly State PASS = x => PASS;
protected readonly State FAIL = x => FAIL;
protected readonly State TICK = x => TICK;

Stack<State> stateStack = new Stack<State>();
protected abstract State GetBaseState(); //put your behaviour here
protected void Update() {
  bool? prev = null;
  if(stateStack.Count == 0) stateStack.Push(GetBaseState());
  while(stateStack.count > 0) {
    var top = stateStack.Pop();
    var res = top(prev);
    if(res == PASS) {
      prev = true;
      continue;
    }
    if(res == FAIL) {
      prev = false;
      continue;
    }
    stateStack.Push(top);
    if(res == TICK) {
      break;
    }
    else {
      stateStack.Push(res);
      prev = null;
      continue;
    }
  }
}
protected State Sequence(params State[] states) {
    int i = 0;
    State self = null;
    return self = prev => {
        if(i >= states.Length) return PASS;
        if(prev == false) return FAIL;
        return states[i++];
    };
}
protected State Selector(params State[] states) {
    int i = 0;
    State self = null;
    return self = prev => {
        if(i >= states.Length) return FAIL;
        if(prev == true) return PASS;
        return states[i++];
    };
}