Author: Nick Gammon
Date: Original text written in 1982
Ever since Will Crowther and Don Woods wrote in 1976 the “original” Adventure game, known as Colossal Cave Adventure, computerised fantasy games have become very popular. These days, of course, you can play in highly sophisticated immersive 3D worlds, such as World of Warcraft, and Final Fantasy XIV.
Have you not wanted to write your own game? To design a dungeon, place treasures, make mazes, and generally be in control? With G-Pascal you can do that with a minimum of fuss.
Pascal makes the job easy because:
The game is available here. Because of its size you need to make the hardware modification described here, to give yourself 24kB of RAM rather than 16kB.
Adventure games generally accept commands from the player in the form of two-word sentences, for example GET ROD or WALK NORTH. Our game expects commands to consist of a verb, followed in some cases by a noun. For the sake of brevity commands such as GO NORTH can be abbreviated to the direction only: NORTH (for example).
Commands are accepted from the player by getline which reads a line of text from the player. This calls getword twice to decode the text into two words. If getline finds more than two words it asks the player to try again. You could easily accept more than two words by changing the constant maxwords.
getword may seem a bit obscure, but it pasically “packs” the first three letters of the next word on the input line into a single integer (G-Pascal uses 3-byte integers so this is a neat way of storing a three-letter word).
A side-effect of this is that our game only regards the first three characters of the word as significant. getword also keeps track of where the whole word starts and ends, so if you take TAKE ANTELOPE it can reply I SEE NO ANTELOPE HERE rather than I SEE NO ANT HERE.
Our adventure player can take one of five general categories of actions: Move (for example, GO NORTH); take something (TAKE ROD); drop something (DROP LAMP); use something (WAVE RING); and other (SCORE, QUIT, INVENTORY).
The player’s current location is stored in room. Their previous location is stored in oldroom. If room and oldroom differ the program prints a description of the current location by using a case statement in describeroom.
write ("You are ");
case room of
1: writeln ("at a plateau near a cliff. A rocky path leads south.");
2: writeln ("on a rocky path leading north and curving to the east.",chr(newline),
"There is a slight breeze.");
3: writeln ("at the entrance to a dark cave. The cave is east of here. A rocky path to the west curves north.");
...describeroom also prints a description of all objects in the same room as the player:
for i := 0 to maxobj do
if inroom (i) then
begin
write ("There is a ");
describeobject (i);
writeln (" here!")
end; { of if in room }
end; { of for }When the player enters a movement command such as NORTH, SOUTH, UP, DOWN and so on, the verb procedure calls the appropriate movement procedure. The movement procedures handle all movement in a single case statement (per verb) — invalid directions are caught by the else clause of the case.
procedure south;
begin
case room of
1: room := 2;
4: room := 5;
5: room := 6;
9: room := 7;
7: room := 11;
16: room := 15;
17: room := 16;
19: if carrying (statue) then
room := 18
else
force
else noway end
end; { of south }Movement can be conditional — in the above example the player can only move from room 19 to 18 if carrying the statue.
To add more rooms to our game we merely have to add their descriptions to describeroom, and make provision for getting to and from them in the movement procedures.
A lot of the fun in adventure games is finding “objects” (such as lamps and rods) and discovering a use for them. An object can either be carried, lying around somewhere, or nonexistent.
In our game we use an array, object, to contain the current location of each object. For example object [lamp] is the location of the lamp. The locations are: -1: being carried, 0: nowhere, room number: lying in that room.
We now define some handy boolean functions which tell us if a given object is being carried, is in the room, is is here (meaing carried or in the room).
function carrying (x);
begin
carrying := object [x] = inhand
end; { carrying }
function inroom (x);
begin
inroom := object [x] = room
end; { inroom }
function here (x);
begin
here := carrying (x) or inroom (x)
end; { here }We can now pick up an object by setting its location to -1, or drop it by setting its location to the current room number, for example:
procedure pickup (x);
begin
if holding >= maxcarry then
writeln ("You can't carry any more!")
else
begin
holding := holding + 1;
object [x] := inhand;
writeln ("Taken.")
end
end; { of pickup }At this point you might ask: “if I say TAKE LAMP, how does the word ‘LAMP’ become a subscript in the object array?”
Good question! This conversion is carried out by convertobject:
function convertobject;
begin
obj := 0;
if word [2] = 0 then
begin
sayword (1);
writeln (" what?")
end else
case word [2] of
"lam" : obj := lamp;
"bun", "cre" : obj := bun;
"rod" : obj := rod;
"rin", "gol" : obj := ring;
"sil", "sta" : obj := statue;
"jew", "cro" : obj := crown
else
obj := 9999
end; { case }
convertobject := obj
end; { convertobject }convertobject uses a case statement to convert the name of any object (or its synonym/s) into a value which is stored in obj. (The actual numbers are stored as constants at the start of the program to avoid confusion and make the program more self-explanatory). convertobject also prints “WHAT?” if the player has not supplied a noun, so if you just take TAKE, it will reply TAKE WHAT?
convertobject is called by the boolean function getobject whose function is to check that the requested object is in the right place.
Since getobject returns true or false, you can write (for example): “if getobject (carried) then statement;”.
In this case the statement is executed if:
If the statement is executed then the the object number is in obj otherwise the appropriate error message will already have been printed. This greatly simplifies programming the rest of the game. For example, here is how we handle the verb EAT:
procedure eat;
begin
if getobject (nearby) then
if obj = bun then
begin
writeln ("Thanks! You were rather hungry!");
destroy (bun)
end else
crazy
end; { eat }Here’s how you add more objects to the game:
To turn this game into a full-scale adventure you would need to:
Your quest is to explore the cave of the evil Ur-Lord, and bring back to
the edge of the cliff the following valuables:
1. The white gold ring;
2. The sacred silver statue;
3. The jewelled crown of the Ur-Lord.
Be careful ...
Hints: LOOK, INVENTORY, HELP, SCORE,
TAKE and DROP, compass directions to move.
(Also: IN, OUT, UP, DOWN, INSPECT, WAVE)
You are at the entrance to a dark cave. The cave is east of here. A rocky path to the west curves north.
west
You are on a rocky path leading north and curving to the east.
There is a slight breeze.
north
You are at a plateau near a cliff. A rocky path leads south.
There is a lamp here!
take lamp
Taken.
inventory
You are carrying:
lampColossal Cave Adventure: Wikipedia
Graphic from Australian Your Computer magazine, August 1982, used with permission.
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