Topics in Programming Reactive Systems Prof. Ronen Brafman and Dr. Gera Weiss

Goal Learn, and help us refine a new methodology for developing reactive systems and autonomous systems – This means that what we describe are guidelines, and that you are free to structure your code as you wish, provided you keep the basic ideas Use this methodology to develop players for robocode

Robocode information, tutorials, code, suggested strategies, etc. file://localhost/Users/Brafman/robocode/rob ocode.command file://localhost/Users/Brafman/robocode/rob ocode.command

Course Schedule and Work Load 14/3: Lecture a. Description of course goal b. Description of robocode c. Description of the programming methodology. 21/3: Assignment due: simple, hand coded robocode agent 28/3: Lecture + Assignment due: List of b-threads, their description, and their place in the hierarchy a. 18/4: Student Lectures Describe agent design and methodology support. 10 min each pair – Assignment due: Methodology support and critiquing document – max 2 pages 2/5: Assignment due: Player code + documentation + time report 23/5: Student Lecture. Students describe agent team design 10 minutes each. 1/7: Assignment due: a. Team code b. Architecture document – max 3 pages c. Methodology support and critiquing – max 2 pages d. Time report.

Requirements and Grade Attend all lectures – Both students should be prepared to present their architecture in both cases Submit all assignments in time and receive a passing grade on each assignment – Submission in pairs Grade: assuming you meet all above requirements, your grade will be: – Simple player: 5 pts – Agent architecture description: 10 pts – Agent architecture document: 10 pts – Agent code submitted on time with proper documentation and using proper methodology 15 – Agent performance 5 – Team architecture description: 10 pts – Team architecture document: 10 pts – Team code submitted on time with proper documentation and using proper methodology 20 pts – Team performance 15 pts – Time logs submitted 5 pts

(flat) Behavioral Programming Basic idea: write behaviors (called b-threads) – almost standalone threads that deal with one aspect of the problem you care about – each behavior can ask for “actions” to be done, and for other actions not to be done “blocked” – a coordinator receives all requests, and selects an action that was requested but not blocked, and informs the thread on its selection – Each thread continues as it code specifies. The code will say how to react to some changes/actions It does nothing when others occur – behaviors can only change their own state based on events, and make a request to execute/block – they cannot actually carry out an action, only the coordinator can When the overall behavior is missing something, add a new behavior – It will either request something that was not requested or block some undesirable behavior Motivation: – We never get a complex system right the first time – We want an incremental modification process -- add new behaviors that handle the problem Example: quad-rotor

Important Principle – Try to Focus Try to write behaviors that achieve, or maintain something concrete – Prevent collision – Save fuel – Reach target quickly We will soon see how to combine them – Some b-threads will be in charge of “orchestrating” these behaviors

Weights Actually, rather than ask for an event/action, b- threads vote. Each b-thread votes for/against each behavior it cares about, by assigning it some weight. – Positive weight – wants to see it take place – Negative weight – prefers not to see it – 0 – don’t care – - ∞ -- absolutely forbiden The coordinator will combine these “votes” and select the action with highest value

Important Principle: Consider/ Program Many Options In standard code, a thread/procedure etc. can request (or directly activate) some function/actuator/action In BP, a thread does not know whether its favorite choice will be executed It needs to consider many actions: either ask for them, or ask that they will not take place – You like Bibi, but maybe he won’t be selected. So let the coordinator know what you think about Liberman, or Mofaz So you should program many ways to implement things, so that in case one is blocked, the others may be acceptible Remember – the worse thing is for the coordinator not to have any valid choice

Structure of a b-thread Regular code that keeps track of state variables and external events in order to achieve (or maintain) some goal: - Keep vehicle stable, or – Prevent collisions, or – Pickup an object Whenever it wants something done, uses a b- synch statement to vote for behaviors/actions that achieve what it needs now – Need to define a “vote” object that is basically the value of a b-synch statement

Synchronization To support this, we need a synchronization thread – Waits for all relevant requests (i.e., b-synchs) – Decides what to do based on the votes – Informs the b-threads about the action that was selected (so they can update their state, and continue along the correct branch)

Synchronization Many options! Many ways of implementing: – Via continuations (you store the state of the procedure before a b-synch, and continue later after decision) – Via real threads Many design choices: – How much time to wait for a decision? – Do we need to wait for all relevant b-threads? – Various “standing” requests” – don’t do X until I say so

Hierarchy It is impossible to build large systems without some hierarchy and structure Our methodology has some clear guidelines on this, which also help decompose the problem – We focus on a linear hierarchy – Each level has a set of b-threads + own coordinator – Each level defines a set “actions” for the level above One typical action allows us to set the priority of b-threads Others allow us to set other parameters Others allow us to activate a b-thread

Top-Down Design Top level (0): single b-thread called “success” Level 1: one b-thread for every goal/success criteria – Robocode: kill, stay alive “Success” b-thread sets the priority of level 1 criteria – Can be static (fixed weight) – Can be dynamic (update weight based on events)

Top-Down Design Threads at level i+1 describe behaviors that are relevant to some (possibly many) threads at level i. – “Kill” will vote for various behaviors that can destroy other tanks Various techniques for shooting other tanks – “Stay alive” will vote for various behaviors that make it hard to hit our tank – Various strategies for moving or hiding can used by b-threads that support kill/stay alive