blueshell

Blueshell is a Promise-based Behavior Tree implementation using modern ES6 syntax.

Node Types

Actions (aka Tasks or Execution Nodes)

• Action
• Condition

Composites (aka Control Flow Nodes)

• Selector (aka Priority or Fallback)

  • Sends an event to each child until one of them returns SUCCESS or RUNNING, then returns that value.
    • If we exhaust all the children, return FAILURE.

• LatchedSelector

  • Sends an event to each child until one of them returns SUCCESS or RUNNING, then returns that value.
    • If we exhaust all the children, return FAILURE.
    • If a child returns RUNNING, subsequent events start at that child.

• Sequence

  • Sends an event to each child until one of the returns FAILURE, or RUNNING, then returns that value.
  • If all children return SUCCESS, return SUCCESS.

• LatchedSequence

  • Sends an event to each child until one of the returns FAILURE, or RUNNING, then returns that value.
  • If all children return SUCCESS, return SUCCESS.
  • If a child returns RUNNING, subsequent events start at that child.

• IfElse

  • Accepts a conditional function a consequent node, and an optional alternative node.
  • If conditional(state, event) returns true, will return the result of activating the consequent node.
  • If conditional(state, event) returns false, will return the result of activating the alternative node, if one is provided.
  • If conditional(state, event) returns false, will return FAILURE if no alternative node is provided.

Decorators

Decorators intercept and can modify the event sent to or the result from the child.

• Not

  • Returns 'FAILURE' when the child returns 'SUCCESS', and vice-versa

• RepeatWhen

  • Repeats the child when an evaluation function returns true.

• RepeatOnResult

  • Repeats the child if it returns the specified status.

• ResultSwap

  • Allows you to swap one result of a child node for another.
  • For example, you can use this to mask FAILURE as SUCCESS.

Base Classes

• Base

  • The base of all nodes.

• Composite

  • The base class for all nodes which have children.

• Decorator

  • The base class for decorators.
  • Can only have one child.

Inspiration and Further Reading

The following are sources used when designing this library

• Handling Node Traversal
• Difference between a Decision Tree and a Behavior Tree
• Behavior Trees for AI
• Wikipedia Article

Unread

• What is a Behavior Tree?
• Halo 2's AI
  • Halo 2 was the first game to popularize Behavior Trees.

Name

Blueshell is named for a Skroderider from the novel A Fire Upon the Deep by Vernor Vinge. Skroderiders are intelligent plants (trees) that use mechanical constructs to give them locomotion and short-term memory.

In other words, Blueshell is an intelligent tree. Or a tree with behavior. Get it?