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== Introduction ==

We would like a system that is more general the FSMs,more structured than
programs, and lighter weight than planners. Behavior trees were developed by Geoff Dromey in the mid-2000s in the field of software engineering, which provides a modular way to define software in terms of actions and preconditions. They were first used in Halo 2 and were adopted by a number of other games such as Spore.

== HTN + HSM = Behavior tree ==

 * HTN - Hierarchical task network (planning)
  * Goal directed but ignores execution
 * HSM - Hierarchical finite state machines
  * Reactive (state oriented) but not goal oriented
  * Intuitive but worksome
 * Behavior tree
  * Task-oriented rather than state-oriented
  * Modular, reusable behaviours
  * Can easily be built up into hierarchies


{{attachment:bt_snippet.png|Behavior Tree Snippet|width="500"}}

== Behavior tree building blocks ==
 * Tasks [boxes] - A preemptive task. Can be a local function or an actionLib call, will return true, false or error code.
 * Sequencer [right arrow] - all tasks until one fails
 * Selector [circle] - all tasks until one succeed
 * Parallel [parallelogram] - do in parallel all tasks connected
 * Decorator [text on the edges] - filters on return values (and execution)
 
The behavior tree snippet above reads:
 * Goto car and in parallel plan paths while doing the mission
 * During the mission first stand up and then in parallel monitor not to fall while searching for an object and going towards it
 * The object is first validated by an operator, and then tracked while the robot approaches it... etc...
 * !L! is a while decorator. It continues as long as the task returns false. It is used in the scenario above to assure that if the mission fails if will be re-run, if the object serch and go failed it will be re-run, and if the operator did not respond the rutine will be executed again. 

== Reading material ==
 1. FSM, HSM and Behavior trees[[http://www.cs.umd.edu/class/spring2013/cmsc425/Lects/lect20.pdf]]
 1. Behavior trees [[http://aigamedev.com/insider/-presentations/behavior-trees/]]

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