Blueshell

Blueshell is a Behavior Tree implementation written in Typescript.

It is synchronous for performance - asynchronous operations can be handled by returning async operations to your framework for evaluation, these can then generate new events to drive the behavior tree.


Node Types

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#### Action

- Handles the event normally and must return a result

#### Predicate

- Executes a provided synchronous function ((state, event) => boolean) and returns SUCCESS or FAILURE based upon the boolean result of such function.

#### SideEffect

- Executes a provided synchronous function ((state, event) => void) and always returns SUCCESS.

#### Success

- Immediately returns SUCCESS when activated.

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#### 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.

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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.

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#### 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.

Publishing

- The publisher can be registered with the tree which will log each transition.

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>)
- 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?