Import some old documentation if applicable

[Cervator]

Cleaning up the engine wiki. It had a page on behavior trees, which are somewhat in flux - unsure if doc should go in Pathfinding or Behaviors, so making issues for both and linking them. Duped by: Terasology/Behaviors#2 - pinging @synopia :-)

Original doc (make into Javadoc, readme file, or wiki elsewhere?):

TODO, copied from pathfinding module.

Nodes

Nodes are the core elements of a behavior tree. A node is a piece of code, that is run while an Actor is interpreting the behavior tree.

When run, every node must return one of this states: SUCCESS, FAILURE, RUNNING.

SUCCESS and FAILURE are returned, depending on the behavior node's task. If the task takes some time, a node may return RUNNING.

Decorators & Composites

Nodes may have child nodes assigned.
If a node accepts exactly one child, this node is called a Decorator. Such nodes add some functionality to another node, or subtree.
If a node accepts many children, this node is called a Composite. The order of the children is important, since a composite delegates execution to one of its children.
The three basic composite nodes are:

• Sequence - All children run after each other. Failing child will fail the sequence.
• Selector - All children run after each other. Succeeding child will succeed the selector.
• Parallel - All children run in parallel. When the parallel finishes, depends on its policies.
• Monitor - All children are executed in parallel. As soon as any child succeeds or fails, the monitor succeed or fail.

Some basic decorators are:

• Invert - Inverts the child's state.
• Timer - Stops time and finishes with given state after defined duration.
• Repeat - Repeats its decorated behavior forever.
• Delay - Delays the execution of its decorated node some time.

Actor and interpreter

A behavior tree can be assigned to be interpreted by several different minions. So, for each actor the tree has a unique state (state of timers, sequences, etc). This state is realized using a Task class. Instead of handling the actual work itself, a Node delegates its work to a Task class. So a node just create a Task for every actor, where the custom state data can be stored safely.

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[jdrueckert]

Already resolved, got integrated into Behaviors documentation.