Programming Magic Cards is Hard

If you want to play Magic on the computer, you have to program cards. The central problem is how to encode Magic cards into a programming language. I have had e-mail asking, “How do you program Magic cards” and this article aims to answer that question. I wish I could say that my way is the easiest, best way to program Magic cards, but it is just a way.

The Magic Project lets you play against other human players over the Internet and enforces the rules unlike Apprentice. The Magic Project, sourceforge.net/projects/magic-project, uses XML for each card.

!-- {2}{b}{b} search your library for a card and put that card into your hand. then shuffle your library. -->
init>
registers>
register index="colorless" value="2"/>
register index="black" value="2"/>
/registers>
colors>black
idcards>sorcery
/init>
abilities>
activated-ability playable="this" name="" zone="hand">
cost>
pay-mana value="manacost"/>
/cost>
effects>
action ref="search-lib"/>
action ref="return-to-hand"/>
action ref="finish-spell"/>
/effects>
/activated-ability>
/abilities>
/card>

(Sorry the XML is butchered so badly.)

The Magic Project has 2,000+ cards programmed, which is a Herculean task in and of itself. The XML schema, mpvalidator.xsd, is 240kb, which is pretty long, but it has to allow for so many card variations and restrictions, like Terror that can only target non-black creatures.

My project MTG Forge, sourceforge.net/projects/mtgforge, just codes the cards into Java. MTG Forge currently has 365 cards. Let us look at Thought Courier, since he has a simple ability. The computer cannot play Thought Courier’s ability because it cannot evaluate cards, that is the meaning of “public boolean canPlayAI() {return false;}”

//*************** START *********** START **************************
if(cardName.equals("Thought Courier"))
{
final Ability_Tap ability = new Ability_Tap(card)
{
public boolean canPlayAI() {return false;}
public void resolve()
{
AllZone.GameAction.drawCard(card.getController());
AllZone.InputControl.setInput(CardFactoryUtil.input_discard());
}
};//SpellAbility
card.addSpellAbility(ability);
ability.setDescription("tap: Draw a card, then discard a card.");
ability.setStackDescription("Thought Courier - draw a card, then discard a card.");
ability.setBeforePayMana(new Input_NoCost_TapAbility(ability));
}//*************** END ************ END **************************

Demonic Tutor is a little bit longer, but is very straight forward. In case you don’t know, it lets you search your library for a card and then put it into your hand.

//*************** START *********** START **************************
if(cardName.equals("Demonic Tutor"))
{
final SpellAbility spell = new Spell(card)
{
public void resolve()
{
String player = card.getController();
if(player.equals(Constant.Player.Human))
humanResolve();
else
computerResolve();
}
public void humanResolve()
{
Object check = AllZone.Display.getChoiceOptional("Select card", AllZone.Human_Library.getCards());
if(check != null)
{
PlayerZone hand = AllZone.getZone(Constant.Zone.Hand, card.getController());
AllZone.GameAction.moveTo(hand, (Card)check);
}
AllZone.GameAction.shuffle(Constant.Player.Human);
}
public void computerResolve()
{
CardList list = new CardList();
Card[] library = AllZone.Computer_Library.getCards();

//gets cards that Computer has mana to play
for(int i = 0; i < library.length; i++)
if(ComputerUtil.canPayCost(library[i].getSpellAbility()[0])) list.add(library[i]);
//pick best creature
Card c = CardFactoryUtil.AI_getBestCreature(list);
if(c == null) c = library[0]; //System.out.println("comptuer picked - " +c); AllZone.Computer_Library.remove(c);
AllZone.Computer_Hand.add(c);
}

public boolean canPlay()
{ PlayerZone library = AllZone.getZone(Constant.Zone.Library, card.getController());
return library.getCards().length != 0; }

public boolean canPlayAI() {
CardList creature = new CardList();
creature.addAll(AllZone.Computer_Library.getCards());
creature = creature.getType("Creature");
return creature.size() != 0; }

MTG Forge there is a Card class that represents cards both in your hand and in play. Every Card object has one or more spells or abilities that are added to it. The class SpellAbility implements all spells and abilities. Every spell or ability has to implement a resolve() method in the SpellAbility class, resolve() does the main functionality of the card. Thought Courier’s ability is implemented in the resolve() method. The same goes for Demonic Tutor, the resolve() method lets you actually look through your library and choose a card. Since the computer can play cards, you will see that Demonic Tutor’s resolve() method does something different depending on if you or the computer played it. MTG Forge also reads cards from a file called “cards.txt” Cards with simple abilities like haste, fear, flying, vigilance, and mana abilities don’t have to be programmed, they are just added to the text file. No extra programming has to be done for cards like Llanowar Elves and Lightning Angel.

Llanowar Elves
G
Creature Elf Druid
no text
1/1
tap: add G

Lightning Angel
1 R W U
Creature Angel
no text
3/4
Flying
Vigilance
Haste