objectoriented simulation games

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object-oriented simulation(games!)
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This is a simplified version of the game code

• It makes it a lot easier to read and explain
• But its missing a lot of features
• Most of which you wont want to use anyway
• A few of which some of you will
• So were just covering the most important
  features
• So dont worry if you look at the real code and
  see something different

3
Adventure games

• One of the oldestcomputer game genres
• Based on
• wandering througha set of rooms
• interacting withobjects and characters
• Solving puzzlesor mysteries
• Subgenres
• Point-and-click adventures (Myst, Syberia)
• Text adventure games (Zork, Infocom games)
• RPGs (Final Fantasy, Everquest, Baldurs gate)

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Class hierarchy of adventure games
Things
Persons
Items
Key items
Player
Monsters
Supplies
Portals
NPCs
Equipment
Places

• Game state
• Spatiality
• Things have locations (places)
• Places have contents (sets of things)
• Inventory
• Equipment
• Consumables (health, gold, potions, etc.)
• Key items
• Unlock parts of the game
• Allowing the narrative to progress

Rooms
Inventory
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An adventure game in Meta

• Supports
• Pictures (for point-and-click style)
• Text output
• Sound
• You can decide which of these you want to use
• Basic ontology
• Things (have name, location)
• Places (have name, contents)
• Player
• Portals (have name, location, destination)

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The Thing class

• Things have these fields
• Name
• A string describing the object
• May be null
• Location
• Place in which the Thing can be found
• May be null
• And optionally adjectives
• List of strings describing the object
• May be null

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The code

• Things have
• Name
• A string describing the object
• May be null
• Location
• Place in which the Thing can be found
• May be null
• And optionally adjectives
• List of strings describing the object
• May be null

• define Thing class Thing name
  location Object
  adjectives

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Make a class (type) called Thing

• define Thing class Thing name
  location Object
  adjectives

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Thats a kind of Object

• define Thing class Thing name
  location Object
  adjectives

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That has these fields

• define Thing class Thing name
  location Object
  adjectives

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But you only specify values for these when you
make one with new

• define Thing class Thing name
  location Object
  adjectives

12
The Place class

• Places should have these fields
• Name (like things)
• ContentsA list of Things inside the place
• And a bunch of optional fields
• Sound
• Sound to play when the user enters the room
• Can be null if no sound
• Sound-loop?
• If true, the sound plays continuously
• Image
• Bitmap to display when the user enters the room
• Hotspots
• A list of places in the image the player can
  click and handlers (procedures to call) for when
  theyre clicked
• Adjectives (like Things)
• Location (like Things)

13
The code

• Places have
• Name
• ContentsA list of Things inside them
• And
• Sound
• Buffer to play when the user enters the room
• Can be null if no sound
• Sound-loop?
• If true, the sound plays continuously
• Image
• Bitmap to display when the user enters the room
• Hotspots
• A list of places in the image the player can
  click and handlers (procedures to call) for when
  theyre clicked
• Adjectives (inherited from Thing)
• Location (inherited from Thing)

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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Make a class (type) called Place

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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Thats a kind of Thing

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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Thats a kind of Thing and so has all of
Things fields

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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That also has these fields

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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That also has these fields (well,
okay, Thing already has a name)

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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But you only specify a value for name when you
make one with new

• define Place class Place name
  Thing contents hotspots
  sound sound-loop?
  image

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Trivial example

• (Based on a randomly chosen picture from my hard
  drive)
• One room
• Two Things in it
• The Player
• A group of Nerds

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Trivial example

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

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Trivial example
Make the Place object

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

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Trivial example
Set up its attributes

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

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Trivial example
Make Player object

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

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Trivial example
Make Nerds object

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

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Wait a minute

• Were creating in-game objects in three different
  ways

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

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Wait a minute

• Were creating in-game objects in three different
  ways
• Putting it in a local variable

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

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Wait a minute

• Were creating in-game objects in three different
  ways
• Putting it in a local variable
• Putting it in a previously defined global variable

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

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Wait a minute

• Were creating in-game objects in three different
  ways
• Putting it in a local variable
• Putting it in a previously defined global
  variable
• And not even bothering to name it
• How can this work?

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

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Wait a minute

• Variables are just a mechanism for naming objects
• Objects exist independently of variables
• An object can have 0, 1, or many names
• The names an object has change over time

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

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So how do I know how/whether to use a variable to
name my object?

• You name it if your code needs to refer to it
  elsewhere
• If you only need to refer to it in the procedure
  that creates it, its best to use a local
  variable
• If other parts of the program need to refer to
  it, you need to use a global variable
• Because a procedures local variables are only
  usable by that procedure.
• Otherwise, theres no need to name it

• define great-hall make Place the Great
  Hall adjectives list big
  pink image image-file
  Aaron talk.jpg sound
  sound-file dog-bark4.wav
  hotspots list list 640 320
  760 450 Aaron
  says
• the-player ? new Player Emily great-hall
• make Nerds null great-hall adjectives
  list many

32
Trivial example
Open window and start game

• define trivial-exampledefine great-hall
  make Place the Great Hall
  adjectives list big
  pink
  image image-file Aaron talk.jpg
  sound sound-file
  dog-bark4.wav
  hotspots list list 640 320 760 450
  
  Aaron says the-player ? new
  Player Emily great-hallmake Nerds null
  great-hall adjectives list
  manystart-game Sample game
  800 600 100 100

33
Moving objects

• We need to make sure that
• If a thing T says place P is its location
• Then P lists T in its contents
• And vice-versa
• Note however, some Things can have null (no
  object) as their location
• Then we have to make sure we dont try to use the
  locations fields (since the location isnt
  really an object)

• define move thing placeremove thing
  thing.locationadd thing place
• define remove thing placething.location ?
  nullunless null? place
  place.contents.Remove thing
• define add thing placething.location ?
  placeunless null? place
  place.contents.Add thing

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Initializing the fields

• When we create Things and Places we need to make
  sure their location and contents fields are set
  properly
• This is called initializing them

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Initializing the fields

• When we make a thing, tell its location to add
  the thing to its contents list

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Initializing the fields

• And when we make a place, make sure it has a
  contents list!

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Initializing the fields

• Oh, but remember that Places are Things too!

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Initializing the fields

• Now theres just one problem
• How does the system know to call these procedures
  when you make a Thing or a Place?

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Initializing the fields

• It doesnt
• It calls one procedure called initialize,
  regardless of what kind of object its making

• define initialize xif is? x Place
  initialize-place x if is? x Thing
• initialize-thing x
• etc

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Yuk! What a ing mess!

• Its completely crazy to have to have
• One big, huge initialize procedure
• That you have to modify every time you make a new
  class

• define initialize xif is? x Place
  initialize-place x if is? x Thing
• initialize-thing x
• etc

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Generic procedures to the rescue!

• Initialize is a special kind of procedure called
  a generic procedure, that
• Runs different code
• Depending on what kind of inputs you give it
• Generic procedure have a little table of methods
  to run for different types of inputs.
• You can add methods (code) to the table using
  define-method

• define initializegeneric-procedure
• define-method initialize Type1 x code for
  Type1
• define-method initialize Type2 x code for
  Type2
• define-method initialize Type3 x code for
  Type3

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Defining initialize methods

• We just change our original initialization
  procedures

• define initialize-thing xadd x x.location
• define initialize-place pinitialize-thing
  pp.contents ? new ArrayList

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Defining initialize methods

• We just change our original initialization
  procedures
• Into method declarations

• define-method initialize Thing xadd x
  x.location
• define initialize Place pcall-next-method
  p.contents ? new ArrayList

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How to read a method definition

• define-method initialize Thing xadd x
  x.location

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How to read a method definition
Whenever initialize is called

• define-method initialize Thing xadd x
  x.location

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How to read a method definition
And its input is a Thing

• define-method initialize Thing xadd x
  x.location

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How to read a method definition
Name that input x

• define-method initialize Thing xadd x
  x.location

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How to read a method definition

• define-method initialize Thing xadd x
  x.location

And run this code
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How to read a method definition

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

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How to read a method definition
Whenever initialize is called

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

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How to read a method definition
And its input is a Place

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

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How to read a method definition
Name that input p

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

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How to read a method definition

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

And run this code
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How to read a method definition

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

In this case, starting with whatever code
Places parent class would run
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How to read a method definition

• define-method initialize Place
  pcall-next-methodp.contents ? new
  ArrayList

Places parent class is Thing, so this
runs Things initialize code
56
Describing scenes

• ? view-description great-hall
• You are in a big, pink place named the Great
  Hall. You see a many nerds.
• The describe procedure is used to generate the
  text the system prints to describe a place

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Describing scenes

• define view-description p concat You are in
  description p .
  You see comma-separate map
  description p.contents
  true .
• The description procedure makes a string that
  describes a Thing
• So we get
• The description of the room
• The descriptions of all the objects in the room
• (which we get by mapping description over
  p.contents)
• And glue them together using concat
• And comma-separate, which takes a list of strings
  and glues them together with commas

58
Describing scenes

• define view-description pconcat You are in
  description p .
  You see comma-separate map
  description
• 
  filter t ? ? t the-player
  
  p.contents
  true .
• Actually, that has some bugs
• First of all, the player is one of the objects in
  the room so wherever the player goes, it will
  tell them that they see themselves
• Kind of stupid
• So we filter the player out from the list of
  objects in the room before we map description
  over it.

59
Describing scenes

• define view-description pwith stuff-in-room
  filter t ? ? t the-player Everything
  but the player
  p.contents concat You are in
  description p
  . if empty? stuff-in-room
  
  concat You see
  comma-separate map description
  stuff-in-room
  true
  .
• Also, if the player goes into a room with no
  objects (other than the player), it says You are
  in a room. You see .
• Looks stupid
• So we have to check the number of objects in the
  room and not print the You see part if there
  isnt anything to see.

60
Describing objects

• Okay, how do we describe anindividual Thing?
• Print out
• a
• Its adjectives (if any)
• Its type
• named its name (if any)
• Examples
• description new Cat
• a cat
• description new Cat Harry null
• a cat named Harry
• with h new Cat Harry null
  h.adjectives ? list big black
  describe h
• a big, black cat named Harry
• Note
• Stuff in gray is black magic we havent taught
  you about yet

• define description twith type-name
  type-of t.Name.ToLower modifiers
  maybe-pad
  comma-separate t.adjectives
  false
  name-string if null? t.name
  
  concat named t.name concat
  a modifiers
  type-name name-string

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Sound and vision

• You can tell the system what to show/play when
  the player enters a room by setting its fields
• make Place image image-file
  filename.jpg
• Tells the system to display the image
  filename.jpg from the images directory of the
  game.
• make Place sound sound-file
  filename.wav
• Tells the system to play the sound filename.wav
  from the sounds directory.

62
Handling clicks of the mouse

• make Place image image-file
  filename.jpg hotspots list hotspot1
  hotspot2
• A hotspot specifies an area of the screen and
  what do to when the user clicks there
• Its specified as list
• list left top right bottom string
• Where
• Left and top give the coordinates of the
  upper-left corner of an area of the image
• Right and bottom give the coordinates of its
  lower-right corner
• When the user clicks within the hotspot, the
  system prints string in the game window.

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Other kinds of hotspots

• make Place image image-file
  filename.jpg hotspots list hotspot1
  hotspot2
• Besides strings to print, you can use a hotspot
  to specify a procedure to run or a Thing object
  to click
• list left top right bottom procedure
• When the user clicks the hotspot, the system runs
  procedure (calls it with no inputs)
• list left top right bottom Thing
• When the user clicks the hotspot, the system runs
  the click procedure on Thing.
• click is a generic-procedure, so you can add
  methods to it
• list left top right bottom string
• When the user clicks within the hotspot, the
  system prints string in the game window.

64
The Portal class

• Portals are objects that, when clicked on, move
  the player to a specified destination (a Place
  object)
• However, it looks funny for the system to print
  You see a portal
• So theres a subclass called Door
• That just forces it to say you see a door named
  blah rather than a portal named blah.

• define Portalclass Portal name location
  destination Thing
• define Doorclass Door name location
  destination Portal

65
Going through portals

• When you put a portal in a hotspot, clicking the
  hotspot causes the system to run the click
  generic procedure
• The click method for Portal just
• Moves the player to the portals destination
• Tells the game to update the display

• define-method click Portal pmove
  the-player p.destinationupdate-disp
  lay!

66
Handling text input

• define-pattern procedurepattern
• Tells the system to run procedure when the user
  types something that looks like pattern

• define quit current-window.Close
• define-pattern quit quit
• Tells the system to run the quit procedure when
  the player types quit

67
Text patterns

• define-pattern procedurepattern
• Elements of pattern can be
• Strings (must be matched exactly)
• Type names
• String (the type)Matches whatever the next word
  is
• Thing, Place, etc.System searches the users
  current location for an object of this type
• Integer, Float, or Number

• define-pattern pick-uppick up Thing
• Example
• Suppose
• You wrote a procedure called pick-up that lets
  the player collect objects
• Takes a Thing as its input
• The player is in a room that has an object called
  screwdriver
• Then when the user types
• pick up screwdriver
• The system will call pick-up on that object

68
Text patterns

• define-pattern procedurepattern
• Elements of pattern can be
• Strings (must be matched exactly)
• Type names
• String (the type)Matches whatever the next word
  is
• Thing, Place, etc.System searches the users
  current location for an object of this type
• Integer, Float, or Number

• define-pattern buybuy Thing for Number
  dollars
• Example
• Suppose
• You wrote a procedure called buy that lets the
  player buy objects
• Takes a Thing as its first input
• Takes a Number as its second input
• The player is in a room that has an object called
  screwdriver
• Then when the user types
• buy screwdriver for 10 dollars
• The system will execute
• buy screwdriver 10
• Where screwdriver is whatever the object is whose
  name field says screwdriver.

69
A fancier example

• This example sets the variable mouse-debug-mode?
• To true when the user types mouse debug on
• Or false if they type mouse debug off
• When mouse-debug-mode? is true
• The system prints the image coordinates of the
  point you click on
• Rather than running the click procedure
• This lets you easily find coordinates when youre
  making hotspots in your images

• define set-mouse-mode mode ?
  mouse-debug-mode? ? ? mode
  off print-line if
  mouse-debug-mode? Mouse hits
  will display coordinates Mouse
  hits will call click methods
• define-pattern set-mouse-modemouse debug
  String

70
Other built-in handlers

• debug on/off
• Turns the variable debug-mode? true or false
• Not used by the system but might be useful for
  you.
• go back
• Brings the user to the place they were last

71
A more interesting example

• Typing go to Thing will run click on the Thing,
  as if you had put it in a hotspot and the user
  had clicked the hotspot
• You can customize the behavior of go to by adding
  new methods to click

• define-pattern clickgo to Thing
• define-pattern clickgo Thing

72
The sample game

• Brief highlights of my trip to AAAI last year
• AAAI is an Artificial Intelligence Conference
• Yes, I know this is a lame example
• To run it
• Load Sample game.meta
• Run sample-game

• define great-hall null
• define robot-room null
• define art-room null
• define origami-room null
• define sample-gameset-up-hallset-up-robot-
  roomset-up-origami-roomset-up-art-roomset
  -up-doorsstart-game Sample game
• 800
  600 100 100

73
Example room the great hall

• Its best to write a separate procedure to
  initialize each room
• That keeps the top-level procedure from being too
  big and hard to read
• Set up sound and image
• Set misc-text field
• Which we didnt talk about, but which prints when
  you enter the room
• Set hotspots to various strings and procedures to
  run
• Make the player
• And put it in the magic variable the-player
• Make the nerds

• define set-up-hall
• Set up the great hall
• great-hall ? make Place the Video Games
  Symposium
• image image-file Aaron
  talk.jpg
• sound sound-file clap.wav
• misc-text Aaron Khoo is
  giving a talk on Mythica.
• hotspots list list 174 392
  238 471 ouch
• list 466
  346 585 451 ouch
• list 46
  87 513 440 Stop that...
• list 640
  320 760 450 Aaron says ...
• Objects within the great hall
• the-player ? new Player Emily great-hall
• make Nerds null great-hall
• determiner many

74
The ouch procedure

• define ouch-counter 0
• define ouch-statementslist Ouch
  Please don't do that. Ouch! Are
  you having a good time? This is getting a
  little infantile, isn't it?
• define ouchprint-line item ouch-statements
  ouch-counterwhen - length ouch-statements 1
  ouch-counter ? ouch-counter 1

75
The Eat procedure

• Lets you type Eat
• Simulates destroying the Thing by moving it to
  null (i.e. nowhere)
• Now its not in the room
• And so it appears to be destroyed
• Prints Mmmm, yumming Things
• Returns false
• Which tells the system not to redraw the display

• define eat thingmove thing
  nullprint-line Mmmm, yummy
  type-of thing.Name.ToLower ...
• define-pattern eateat Thing

76
The art and origami rooms

• The art room brings you to the origami room when
  you click on it

• define set-up-art-room
• Set up the art room
• art-room ? make Place the art installation
• image
  image-file Art installation.JPG
• misc-text It
  was make by art and
  robotics students
  from the
  University of New Orleans.
• hotspots list
  list 130 50 323 360
• 
  origami-room
• define set-up-origami-room
• Set up the origami room
• origami-room ? make Place null
• image
  image-file Origami room.jpg
• sound
  sound-file dog-bark4.wav
• 
  adjectives list origami filled

77
Making doors

• Doors in this system are only one-way
• They live in one room
• And lead to the other
• But arent visible in the other
• So we need to make doors in pairs
• One from room A to room B
• One from room B to room A
• So its easiest to make a procedure to do this
  for us

• define make-door-pairplace1 name1 place2 name2
  ? new Door name1 place1 place2 new Door
  name2 place2 place1
• define set-up-doorsConnect the
  roomsmake-door-pair great-hall null
  robot-room symposium
• make-door-pair art-room null
  robot-room art

78
Working with the code

• The code is divided into several files
• It is distributed as a .zip file
• Download it from the web
• Unzip it into the directory of your choice
• To load the code
• Click the file Adventure.meta
• Choose Execute All from the Execute menu

• The list of files is stored in the variable
  source-files in Adventure.meta
• To add a new file
• Add its name to the end of source-files
• Type Control-E to update the definition
• Run reload

79
A tour of the code

• Adventure.meta
• List of source files
• Code to load other source files
• Code to find art assets (sound and image files)
• Parsing.meta
• Basic code for handling text input
• Basic class files
• Things.meta, Places.meta, Actors.meta,
  Player.meta, Portals.meta

• Sound stuff.meta
• OS-dependent code for loading and playing .wav
  files
• User interface.meta
• OS-dependent code for window updating, and
  handling of user events (clicks and text input)
• String utilities.meta
• Stuff for handling text
• Sample game.meta
• Example game

You only need to worry about the basic class
files and the sample game
80
Subdirectories

• Sounds
• Holds any sound files you want (.wav only, sorry)
• Files can be accessed using the sound-file
  filename procedure
• Images
• Holds any images files (.jpg, .gif, .bmp, or
  .png) you want to use
• Files can be accessed using the image-file
  filename procedure
• Note
• You dont have to include the directory names in
  the filenames for sound-file and image-file.
  They know to search in the Sounds and Images
  directories, respectively.