Chapter 9: Behavior Trees

In a preceding chapter, we saw a basic but effective way to implement and manage character states and behaviors: finite state machines (FSMs). FSMs are simple to implement and intuitive, but they have a fatal flaw: it is tough to make them scale once there are many states and transitions. For example, imagine a character that behaves differently depending on its health and mana (high, medium, or low). We have a state in which both health and mana are high, one in which health is medium and mana is high, one in which they are both medium, and so on. In total, we have nine states just for those. If we add other conditions (such as player proximity, time of day, equipment, player's score, or whatever you may imagine), the number of states grows exponentially.

Luckily, we have a solution: behavior trees (BTs). In essence, BTs are just another way to visualize complex FSMs, but they are fast, provide reusability, and are easy to maintain. After their introduction...