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Having a formal description of a game is
one thing.

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Being able to use that description to

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play the game effectively is something
else entirely.

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In this lesson, we discuss strategies for
building general game

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players and some of the difficulties that
need to be handled.

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Since game descriptions are written in
logic, it's obviously necessary

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for a game player to do some amount of
automated reasoning.

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Now, there are two extremes here.

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One possibility is for the game player

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to process the description interpretively
throughout a game.

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The second possibility is for a player to
use the description to

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devise a specialized program and then use
that program to play the game.

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This is effectively automatic programming.

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In this lesson, we're going to discuss the
first possibility and leave it

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to you to think about the second possibil,
bility and various hybrid approaches.

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We discuss this second approach later in
the course.

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We begin the lesson by talking about

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some infrastructure that frames the
problem more precisely.

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We then consider a couple of search free
uses

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of this instra, infrastructure, namely
legal players and random players.

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In lessons five through eight, we look at
complete search strategies which are

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appropriate for small game trees and
incomplete search strategies which are

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necessary for large game trees.

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In lessons nine through 12, we examine
some game playing techniques based on

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the properties of states rather than

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properties of states rather than
monolithic states.

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And finally, in lessons 13 through 16, we
show ways that game descriptions can

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be used to deduce general properties of

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games without enumerating states or
properties of those.

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Game players typically implement

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it as a web service.

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As soon as it begins running, the player
enters

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a loop, listening for messages from the
game manager.

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Upon receipt of a message, the player
calls

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an appropriate handler for that type of
message.

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When the handler is finished, player sends
the return value to the game manager.

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Given this architecture, building a game
player

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means writing event handlers for the
different types

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of messages in the GGP communication
protocol.

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There's typically one handler for each
type of message.

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Info, start, play, stop and abort.

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So, that means writing five event
handlers.

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In order to make the implementation of

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these message handlers a little easier,
the GGP

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a code base available in the course

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website contains definitions for the
subroutines shown here.

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there's subroutines for computing the key
components of a game.

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For example, findrules produces a list of
role find

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findroles, produces a list of roles in the
game.

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Findinits gives a list of sentences true
in the initial state.

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There are also some utility routines to
make our job easier.

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For example, doesify takes a list of roles
and

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a list of actions as argument, produces a
list of

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sentences stating that each role specified
in the roles

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argument executes the corresponding action
specified in the actions argument.

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And then undoesify reverses this process.

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You'll see how that plays into the
definitions a little later.

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Now,

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your job in building a player as I
mentioned earlier is to

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use the subroutines provided to write the
handlers called by the message handler.

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In these next two segments, we're going to
look

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at a couple of simple approaches for doing
this.

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Now, we're going to present the
definitions in JavaScript.

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Yeah, you can use these yourself if

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you're building your player directly in
JavaScript or

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you can adjust them as appropriate to

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implement in a Java based version for
example.

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Alternatively,

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you can click the appropriate box in the

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parametric player to select from among
different approaches.

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In the slider case you don't need to type
in any code at all, but you'd look

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at the code that we present so that you

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know what's going on when you make your
selections.

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[SOUND]