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A legal player, is one of the simplest
types of game players.

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You need to stay.

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A legal player selects an action based
solely

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on its legality, without consideration of
the consequences whatsoever.

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Typically, the choice of action is
consistent, and selects the

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same action every time it finds itself in
the same state.

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Using the basic sub-routines provided in
the GDP

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starter pack, building a legal player is
simple.

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Here we see the entire implementation.

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And throughout this course, we use
JavaScript

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for computer code, as I mentioned earlier.

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This is a compromise between Lisp and
Java.

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Lisp is a powerful language, it's highly
efficient

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and it's easy to use in building players.

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However, it's not very

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widely known.

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Building programs in Java is slightly more
difficult, but

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the language is familiar to more people
these days.

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Java script has a flexibility of Lisp, and

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at the same time it's easily converted to
Java.

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It can also be used directly to implement
players,

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either in web browsers or in stand alone
applications.

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Let's walk through this implementation.

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We start by setting up some global
variables

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to maintain state, while a match is in
progress.

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That's a variable to hold the game rule
set, a

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variable to hold the player's role in that
game, a

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variable to hold a list of all the roles
in

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the game, and a variable to hold the
current state.

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And now, properly we should create a data
structure

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for each separate match, and we should
attach these values

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to this data structure, so the player can

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actually play multiple matches at the same
time.

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However, in this presentation we're
striving for simplicity, and so

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we'll be treating these as global
variables and what follows.

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Okay, next we define a handler for each
type of message.

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The info handler, does nothing at all and
simply returns ready.

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The start event handler, assigns values to
the rule

39
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set and role variables, based on the
incoming start message.

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It, it initializes roles and the state
variable, and then returns ready.

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The play event handler, takes a match
identifier and a move his arguments.

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If the move is not nil, it uses nexts to
find nexts, to compute

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the state resulting from them preceding
state and the action supplied in the move.

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Once the player has the latest state, he

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uses legalx, findlegalx, to compute a
legal move.

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The stop event handler for a legal player
does nothing.

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Ignores the inputs, and simply returns
done, as required by the GGP protocol.

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And like the stop message handler, the
abort message handler

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for a player, also does nothing and it
simply returns done.

50
00:03:03,862 --> 00:03:06,730
Okay, just to be clear on how this

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works, lets work through a short
tic-tac-toe match.

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When the players initialized, it sets up
data structures to hold

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the game description, the role, roles, and
the state.

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These are all initially empty.

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Players not retain start clock and play
clock, since it does not need to do

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extensive computation for those li, how
limits are irrelevant.

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[BLANK_AUDIO]

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Okay, let's assume that player reads the
start, receives the

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start message from the game manager, as
sort shown here.

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Match identifiers in 23.

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The player is told to be the white player.
There are the usual axioms of tic-tac-toe.

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The start clock and play clock are both
set to ten seconds.

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On receipt of the message, the player code
calls start handler.

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This records the game description and the
rule set

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variable, and the player's role in the
role variable.

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Then computes roles from the game
description,and the initial state.

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Returned values ready, which is then sent
back

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to the game manager by the listener loop.

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Okay, now play begins after all the
players have responded,

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or after the start clock has expired,
whichever comes first.

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Once the game manager's ready, it sends a

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suitable play message to all of the
players.

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Here we see a request for the player to
choose an action from match m 23.

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Excuse me.

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The value nil signifies that this is the
first step of the match.

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On receipt of this message, the player
code invokes the play

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handler with the arguments passed to it by
the game manager.

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Since the move argument is nil, no changes
are made to the data structures at all.

79
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And using the state together with the role
in the role description,

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associated with this match, the player
then computes the first legal move.

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In this case mark (1,1), and it returns
that as the answer.

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Game manager checks that the actions of
all the players are

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legal, and simulates their effects, and
updates the state of the game.

84
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And then sends play messages to the
players to solicit their next actions.

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In this case, the player will receive the
message shown here.

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Mark (1, 1), from the white player, and no

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opt the only legal move for the black
player.

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Again, the player invokes its play handler
with

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the arguments passed to it by the game
manager.

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And this time the move is not nil, and so

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the player uses find next, to compute the
next state.

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There are two differences.

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There's now an x in cell (1, 1), and
control is switched from white to black.

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Using this state, the player then computes
the first legal action in this state.

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Its, its only legal action, there's no up,
and returns that as answer.

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Okay, this processing repeats until the
end of the game.

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When the game's over the player receives a
message like the one shown here.

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While some players are able to make use of
the information in a stop message,

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our legal player simply ignores this
information,

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returns done, terminating its activity on
this match.

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[SOUND]