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Behavior Trees

Finite State Machines (FSMs) are great for simple logic (Mario), but for complex AI (Halo, Uncharted), they become a “spaghetti” mess of transitions.

Behavior Trees (BTs) solve this by structuring AI as a hierarchy of tasks. They are modular, reusable, and easy to debug.

1. The Core Concept

A Behavior Tree is executed (Ticked) from the Root down. Every node returns one of three statuses:

1. Success: “I completed the task.”
2. Failure: “I cannot complete the task.”
3. Running: “I am still working on it. Check back next frame.”

2. The Nodes

Composites (Control Flow)

• Selector (“The Fallback”): Iterates through children. If a child Succeeds, the Selector stops and returns Success. If all fail, it returns Failure.
  • Example: “Try to Attack. If you can’t, Chase. If you can’t, Patrol.”
• Sequence (“The Checklist”): Iterates through children. If a child Fails, the Sequence stops and returns Failure. If all succeed, it returns Success.
  • Example: “Is Enemy Visible? -> Is In Range? -> Fire Weapon.”

Decorators (Wrappers)

• Inverter: Turns Success to Failure (NOT gate).
• Succeeder: Always returns Success (useful for optional tasks).

Leaves (Actions & Conditions)

• Condition: Checks world state (e.g., HasAmmo?). Returns Success/Failure immediately.
• Action: Performs logic (e.g., MoveToTarget). Often returns Running for multiple frames.

3. Implementation (C# Example)

public enum Status { Success, Failure, Running }

public abstract class Node {
    public abstract Status Tick();
}

public class Selector : Node {
    List<Node> children;
    public override Status Tick() {
        foreach (var child in children) {
            Status s = child.Tick();
            if (s != Status.Failure) return s; // Success or Running
        }
        return Status.Failure;
    }
}

4. Why use BTs?

• Decoupling: A “Reload” behavior can be dropped into any AI agent without knowing the rest of the logic.
• Reactiveness: Because the tree is traversed frequently, the AI can interrupt a lower-priority task (Patrol) immediately if a higher-priority condition (EnemyVisible) becomes true.