An Adventure Game – Part 3

I’ve been procrastinating too much. It’s time to roll up our sleeves and get dirty with the nitty-gritty core of Bacchus-Bosch. We’re currently able to read input from the player and parse commands. Before even considering how commands should be executed we have to take a step back and make some ontological commitments. We know that the game will consists of interaction between game objects, the entities, but haven’t specified how these should be represented or how they are created. It might be tempting to simply use a set of unit clauses and statically recite everything of interest:

entity(key).
entity(room_1).
entity(room_2).
entity(door).

contains(room_1, key).
contains(room_1, door).

connected(room_1, door, room_2).
.
.
.
%And so on for the whole level.

At a first glance this might look like an acceptable solution. It wouldn’t be very hard to add e.g. properties to the entities by adding more facts, but everything breaks down in the moment when the player changes the state of the world by picking up the key. Then the key is no longer directly contained by the room and should instead be added to the inventory of the player. The quick and easy hack is to use assert/retract in order to add and remove facts according to the fluctuating state of the game world, but in the long run that’s a terrible idea. To be forced to use impure features for such an integral part of the game is either a sign that we’re using the wrong data structures or the wrong language. I’ll go with the former theory!

Let’s begin by considering a simplified problem where the game only consists of a player with the property of having health in the form of an integer: “hit points”. It should be possible to both increase and decrease this value according to events from other entities. What’s an entity? A common and simple solution is to create a class hierarchy with an abstract class in the top and concrete classes such as items and monsters in the lower layers. While there’s nothing inherently wrong with this I think we can do better, for two reasons:

  • The class hierarchy will be huge and hard to make sense of.
  • It’s not (at least not in most languages, Logtalk happens to be an exception) possible to create new classes during runtime, with the effect that behaviour can’t be modified or extended once the game has started.

An often cited design principle is to favor composition over inheritance. We want the entities to be entirely composed of smaller objects, of properties. The sum of all the properties constitute the entity – nothing more, nothing less (followers of holism should stop reading here!). It should of course be possible to both add, remove and update properties during runtime. It’s easiest to start with a basic definition of a property, which we’ll do with a prototypical object. A property should be able to:

  • Give the initial state of the property. For the health property, this might simply be a positive integer.
  • Update the entity to which the property belong. If the property doesn’t directly influence the entity, it’s left unchanged.
  • Execute an action that changes the state of the property, e.g. increasing or decreasing the health.

With these considerations the object becomes:

:- object(property).
    :- info([
        version is 1.0,
        author is 'Victor Lagerkvist',
        date is 2011/03/19,
        comment is 'A property constitutes the basic behaviours of the objects in Bacchus-Bosch.']).

    :- public(update/2).
    :- mode(update(+entity, -entity), zero_or_more).
    :- info(update/2, [
        comment is 'Update the entity to which the property belong.',
        argnames is ['Entity', 'Entity1']]).

   :- public(action/6).
    :- mode(action(+atom, +list, +entity, +state, -state, -entity), zero_or_more).
    :- info(action/6, [
        comment is 'Execute the action Name with respects to Args, State and Entity, and store the resulting new state and entity in State1 and Entity1.',
        argnames is ['Name', 'Args', 'Entity', 'State', 'Entity1', 'State1']]).

    :- public(new/1).
    :- mode(new(-term), zero_or_more).
    :- info(new/1, [
        comment is 'Unify State with the initial state of the property.',
        argnames is ['State']]).

    %% Basic definition: do nothing!
    update(E, E).

    %% Basic definition, overloaded in almost every descendant prototype.
    new(void).

:- end_object.

Exactly how these predicates should be implemented will be clearer with an example.

:- object(health_property,
    extends(property)).

    new(10).

    action(decrease_health, [], Owner, H0, Owner, H) :-
        H is H0 - 1.
    action(increase_health, [], Owner, H0, Owner, H) :-
        H is H0 + 1.

:- end_object.

Hence, the initial state for the health property is 10. To decrease the value one calls action/6 with the correct action name, an empty argument list, the owner of the property and the old state, and in return obtains the updated owner and new state with the fifth and sixth parameters. If a property doesn’t support a specific action it’ll simply fail. Let’s see how update/2 can be used together with the health property. It should be called in each tick of the game. For example: the health should be decreased if the player is poisoned or have somehow been ignited. Such a property won’t support any actions and only overload the definition of update/2 from property.

:- object(on_fire_property,
    extends(property)).

    update(E0, E) :-
        entity::action(decrease_health, [], E0, E).

:- end_object.

Since update/2 takes an entity as argument, the on_fire property simply asks this entity to decrease its health. Now we have to decide how to represent entities and how to delegate action messages. As mentioned earlier an entity is simply the sum of its properties, hence we can simply represent it as a list that contains properties and their states. The formal definition reads:

  • [] is an entity.
  • [P|Ps] is an entity if Ps is an entity and P is a tuple of the form: N - S, where N is the name of a property and S is a state of that property.

Continuing on our example, [health\_property - 10] is an entity. If we decide to have some fun and for a moment indulge ourselves with arson, [health\_property - 10, on\_fire\_property - void] is also an entity. A few basic definitions remain before we can actually do anything with these entities. First we need a predicate update/2 that iterates through the list of properties that constitute the entity and calls update/2 for every property.

update(E0, E) :-
    update(E0, E0, E).

update(E, [], E).
update(E0, [P|Ps], E) :-
    P = Name - _,
    Name::update(E0, E1),
    update(E1, Ps, E).

The effect is that each property is updated with respect to the current state of the entity. Next up is action/4. It takes the name of the action that shall be performed, its arguments, the entity in question and returns the updated entity if the action could be performed or simply fails otherwise. Since an entity can’t perform any actions it has no choice but to try to delegate the action to its properties.

action(A, Args, E0, E) :-
    %% Select a property from the list such that the action A can
    %% be performed with the arguments Args.
    list::select(P, E0, E1),
    P = PropertyName - State,
    PropertyName::action(A, Args, E1, State, E2, State1),
    %% Add the property with the updated state to E2.
    P1 = PropertyName - State1,
    E = [P1|E2].

As seen from the code action/4 uses select/3 to find the first property that supports the operation. If there’s more than one potential property it will simply choose the first one but leave choice point for the others. If we put it together into one massive object we get:

:- object(entity).

    :- info([
        version is 1.0,
        author is 'Victor Lagerkvist',
        date is 2011/03/20,
        comment is 'The entity operations of Bacchus-Bosch.']).

    :- public(update/2).
    :- public(action/4).
    :- public(get_property/2).
    :- public(select_property/3).

    update(E0, E) :-
        %% As before.

    action(A, Args, E0, E) :-
        %% As before.

    get_property(E, P) :-
        list::member(P, E).

    select_property(P, E, E1) :-
        list::select(P, E, E1).

This is more or less the basic building blocks, the engine, but of course quite a lot of work remains before we have an actual game in our hands. For the remainder of the post we shall implement enough properties so that it’s possible to construct a room with some entities in it. A room is an entity that contains other entities. We can model this as a property, namely the property of being a container. This property at least has to support the following operations:

  • Being able to update the state of its children, i.e. overload update/2 from its parent.
  • Add and remove items with actions.

For simplicity the data structure is going to be a list of entities. This makes it almost trivial to write the actions.

:- object(container_property,
    extends(property)).

    new([]).

    update(E0, E) :-
        entity::select_property(container_property-Items, E0, E1),
        update_children(Items, Items1),
        E = [container_property-Items1|E1].

    update_children([], []).
    update_children([E|Es], [E1|E1s]) :-
        entity::update(E, E1),
        update_children(Es, E1s).

    action(add_item, [E], Owner, Items, Owner, [E|Items]).

    action(remove_item, [E], Owner, Items, Owner, Items1) :-
        list::select(E, Items, Items1).
:- end_object.

Then we of course need a door. Doors are awesome: you can walk through them, open them, close them, lock them – the only limit is the imagination! Our particular door will only have one property though, that of being openable. But it’s only possible to open a door if it happens to be unlocked, hence the openable property in turn depends on whether or not the lock can be unlocked with the key in question. For simplicity we’re going to assume that the player uses the key every time he/she wishes to open the door, but extending these properties so that the lock has an internal state would not be terribly difficult.

:- object(openable_property,
    extends(property)).

    new(closed-[lock_property- Lock]) :-
        lock_property::new(Lock).

    action(open, Key, Owner, closed-Lock, Owner, open-Lock) :-
        entity::action(unlock, Key, Lock, _).
    action(close, [], Owner, _-Lock, Owner, closed-Lock).

:- end_object.

The action/6 command should be read as: change the state of the door from closed to open if the key can unlock the lock. To complete the door we of course need to define the lock and key properties.

 :- object(key_property,
    extends(property)).

    %% The default key.
    new(key).

:- end_object.

:- object(lock_property,
    extends(property)).

    %% The (default) key that opens the lock.
    new(key).

    valid_key(E, Key) :-
        entity::get_property(E, key_property-Key).

    action(lock, [Entity], Owner, Key, Owner, Key) :-
        valid_key(Entity, Key).
    action(unlock, [Entity], Owner, Key, Owner, Key) :-
        valid_key(Entity, Key).

:- end_object.

The two actions specifies that the entity can lock or unlock the lock if it has the property of being a key with the correct state. Note that the entity in question doesn’t just have to be a key, it can still support other properties as long as the key property is fulfilled.

We’re now able to create a room with a door and a key. What remains is the player. For the moment that’s just an entity that has the property of having health, which we defined earlier, and that of having an inventory. The Argus-eyed reader will probably realize that having an inventory is quite similar to that of being a container. The only difference is that the player can only pick up items that are carriable, so instead of defining everything from the ground up we’re going to inherit some definitions from container\_property. What happened to the principle of favoring composition over inheritance? Bah!

:- object(inventory_property,
    extends(container_property)).

    valid_item(E) :-
        entity::get_property(E, carriable_property-_).

    action(add_item, [E], Owner, Items, Owner, [E|Items]) :-
        valid_item(E).

    action(drop_item, [E], Owner, Items, Owner, Items1) :-
        valid_item(E), % This shouldn't be necessary since only valid item are added in the first place.
        list::select(E, Items, Items1).

:- end_object.

:- object(carriable_property,
    extends(property)).

    %% Perhaps not the most interesting property in the game.

:- end_object.

It should be noted that I haven’t actually tested most of these properties, but it should/could work! So do we now have a functional but simplistic game? Not really. We have the basic entities, but there’s a quite glaring omission: everything is invisible since nothing can be printed. For a text game this is a distinct disadvantage! The simplest solution is to add another property, that of being printable, where the state is the string that shall be displayed.

:- object(printable_property,
    extends(property)).

    new(Description) :-
        list::valid(Description).

    action(print, [], Owner, Description, Owner, Description) :-
        format(Description).
:- end_object.

But since we also want the possibility to print items in a container, e.g. a room, we’re going to define an additional action in container\_property.

:- object(container_property,
    extends(property)).

    new([]).
    .
    . % As before.
    .

    action(print_children, Args, Owner, Items, Owner, Items) :-
        % Print all children that are printable.
        meta::include([E] >>
                       (entity::action(print, Args, E, _)),
                      Items,
                      _).
:- end_object.

Putting everything together

Let’s use the properties and construct some entities. The test program is going to build a room and then print its description together with its children.

    init :-
        write('Welcome to Bacchus-Bosch!'), nl,
        build_test_room(Room),
        entity::action(print, [], Room, Room1),
        write('You see: '), nl,
        entity::action(print_children, [], Room1, _Room).

Building the room and its components is not hard, just a bit tedious.

    build_test_room(Room) :-
        build_test_door(Door),
        build_test_player(Player),
        build_test_key(Key),
        Room0 = [container_property - State1, printable_property - State2],
        container_property::new(State1),
        room_description(State2),
        entity::action(add_item, [Door], Room0, Room1),
        entity::action(add_item, [Player], Room1, Room2),
        entity::action(add_item, [Key], Room2, Room).
    build_test_door(Door) :-
        Door = [openable_property-State1, printable_property-State2],
        openable_property::new(State1),
        door_description(State2).

    build_test_key(Key) :-
         Key = [key_property-State1, printable_property-State2],
         key_property::new(State1),
         key_description(State2).

    build_test_player(Player) :-
        Player = [inventory_property-State1, health_property-State2],
        inventory_property::new(State1),
        health_property::new(State2).

The string descriptions are simply given as facts.

    room_description("A rather unremarkable room.\n").
    door_description("A wooden door with a small and rusty lock.\n").
    key_description("A slightly bent key.\n").

When running the test program we get the following output:

Welcome to Bacchus-Bosch!
A rather unremarkable room.
You see:
A slightly bent key.
A wooden door with a small and rusty lock.
true

Only three major obstacles remain before we have a game in our hands:

  • More properties.
  • Conjoin the parsed commands with the game world so that it’s possible for the user to interact with entities. It shouldn’t be too hard to see that commands will be executed by sending action commands to the entities in question.
  • Abstract the creation of game objects, e.g. with the help of an embedded language.

Which will be the topics of the next post. Stay tuned!

Source code

The source code is available at https://gist.github.com/878277.

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4 responses to “An Adventure Game – Part 3”

  1. victorlagerkvist says :

    By the way: I have no idea whether everything will blend together nicely or crumble like a house of cards. I’m just making stuff up as I go! Wait, did I say that last part out loud?

  2. Mikael Lennartsson says :

    I cannot find the monkey.

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