Zupplements

For my last post, I wanted to analyze some of the more data-oriented aspects of Zork.

Zork is a heavily data-driven game, programmed in a language derivative of LISP, and with a lot of conventions reminiscent of object oriented programming.

The MDL source code is therefore very rich in information, but trying to extract all of the data from the game would not be an interesting exercise. Almost the entire game is defined in the in-line data tables – they define the rooms, their descriptions, the objects, where they can be used, and what commands can be used where. The tables would be overwhelming to browse, and would mostly reveal what I already figured out by solving the game.

But there were a number of mysteries of Zork that I wanted to solve, by way of examining the source code.

First, here’s a list of things that score points:

Enter Cellar	25
Enter East-West passage	5
Enter Kitchen	10
Enter Land of Dead	30
Enter Lower Shaft	10
Enter Strange Passage	10
Enter Treasure Room	25
Enter Top of Well	10
Get bauble	1
Get bills	10
Get blue sphere	10
Get bracelet	5
Get canary	6
Get chalice	10
Get coffin	3
Get coins	10
Get crown	15
Get diamond	10
Get egg	5
Get emerald	5
Get gold card	10
Get grail	2
Get jade figurine	5
Get necklace	9
Get painting	4
Get platinum bar	12
Get portrait	10
Get pot of gold	10
Get red sphere	10
Get ruby	15
Get spices	5
Get stamp	4
Get statue	10
Get torch	14
Get trident	4
Get trunk	15
Get violin	10
Get white sphere	6
Get zorkmid	10
Deposit bauble	1
Deposit bills	15
Deposit blue sphere	5
Deposit bracelet	3
Deposit canary	2
Deposit chalice	10
Deposit coffin	7
Deposit coins	5
Deposit crown	10
Deposit diamond	6
Deposit egg	5
Deposit emerald	10
Deposit gold card	15
Deposit grail	5
Deposit jade figurine	5
Deposit necklace	5
Deposit painting	7
Deposit platinum bar	10
Deposit portrait	5
Deposit pot of gold	10
Deposit red sphere	5
Deposit ruby	8
Deposit spices	5
Deposit stamp	10
Deposit statue	13
Deposit torch	6
Deposit trident	11
Deposit trunk	8
Deposit violin	10
Deposit white sphere	6
Deposit Woods stamp	1
Deposit zorkmid	12

And here are point-scoring events in the endgame:

Enter Crypt	5
Enter Top of Stairs	10
Enter Inside Mirror	15
Enter Dungeon Entrance	15
Enter Carrow Corridor	20
Enter Treasury of Zork	35

One of the mysteries of Zork that I pondered was, how does the combat actually work? There’s clearly a rather in-depth combat engine, offering much more complexity than what seems warranted. There are only three enemies who can be fought in the whole game – not enemy types, three enemies total, and only two of them can be beaten through combat. There’s the troll, a pushover, and the thief, who is very tough, but becomes easier late in the game, as your strength correlates to your score. And the cyclops, who is impossible to beat in combat.

I studied the code, but I don’t know MDL, and so a lot of my conclusions are guesses.

First of all, each fighting-capable entity – you, the troll, the thief, and the cyclops, have a base strength rating.

Your base strength is determined by this formula:
2 + int(5 * ([score]/616) + 0.5)

It translates to this, table-wise:

Score	Strength
0-61	2
62-184	3
185-307	4
308-431	5
432-554	6
555-616	7

The troll has a base strength of 2.

The thief has a base strength of 5.

The cyclops has a base strength of 10,000. That’s just a formality, possibly a leftover from an early build. Attacking him doesn’t even invoke any combat functions that would take his strength into account.

HP is determined by base strength, subtracted by 1 for every light wound sustained, and subtracted by 2 for every serious wound sustained. This also affects carrying capacity. Wounds heal over time.

Fighting strength is equivalent to HP remaining. The thief has a special case; if “engrossed,” fighting strength will be lowered to 2 (unless it was already 2 or lower).

Attack and defense are both equal to fighting strength, though the troll gets a -1 defense penalty if attacked with the sword, and the thief gets a -1 defense penalty if attacked with the knife.

When one entity attacks another, a lookup table for a result is selected depending on the attacked target's defense level.

A big thanks to Viila for helping me better understand how the tables work!

If the defense is 1, this table is used:

Attack = 1	Attack = 2	Attack >= 3
Miss	Miss	Miss
Miss	Miss	Miss
Miss	Miss	Stagger
Miss	Stagger	Stagger
Stagger	Stagger	Knockout
Stagger	Knockout	Knockout
Knockout	Knockout	Kill
Knockout	Kill	Kill
Kill	Kill	Kill

Then, one of these columns of possible results is selected, based on the attacker’s attack value. From the nine cells in the column, a result is randomly selected.

So, if your opponent’s defense is 1, and your attack is 1, then these will be the possible results when you attack:

Miss

Miss

Miss

Miss

Stagger

Stagger

Knockout

Knockout

Kill

And if your attack is 3 or higher, then these will be the possible results:

Miss

Miss

Stagger

Stagger

Knockout

Knockout

Kill

Kill

Kill

Also, if the result is “stagger,” then there will be a 25% chance of the staggered victim being disarmed. Unless you broke a mirror! In that case, there’s a 50% chance of disarming if you’re the staggered victim, and a 10% chance of disarming if you’re the attacker. I love little touches like that. That said, I’m not sure if there’s a meaningful difference between staggering and disarming. Either one, in practice, seems to just cost the victim their turn without doing damage.

If the target’s defense is 2, then the table looks like this:

Attack = 1	Attack = 2	Attack = 3	Attack >= 4
Miss	Miss	Miss	Miss
Miss	Miss	Miss	Stagger
Miss	Miss	Stagger	Stagger
Miss	Stagger	Stagger	Light wound
Miss	Stagger	Light wound	Light wound
Stagger	Light wound	Light wound	Light wound
Stagger	Light wound	Light wound	Knockout
Light wound	Light wound	Knockout	Kill
Light wound	Knockout	Kill	Kill

And if the target’s defense is 3 or higher, then this is the result table. You must subtract the target’s defense from the attacker’s attack to get the correct result column. E.g. - if you attack with strength 2 against the thief's 4, the difference is -2, and you select the leftmost column.

A – D <= -2	A – D = -1	A – D = 0	A – D = 1	A – D >= 2
Miss	Miss	Miss	Miss	Miss
Miss	Miss	Miss	Miss	Stagger
Miss	Miss	Miss	Stagger	Stagger
Miss	Miss	Stagger	Stagger	Light wound
Miss	Stagger	Stagger	Light wound	Light wound
Stagger	Stagger	Light wound	Light wound	Light wound
Stagger	Light wound	Light wound	Light wound	Light wound
Light wound	Light wound	Light wound	Serious wound	Serious wound
Light wound	Serious wound	Serious wound	Serious wound	Serious wound

So let’s say you are starting out, and attack the troll with your sword. Your attack value will be 2, since you are at perfect health but have no experience. His base defense is 2, and will be reduced to 1 because of his defense penalty. This lookup table is used:

Attack = 2, Defense 1

Miss

Miss

Miss

Stagger

Stagger

Knockout

Knockout

Kill

Kill

That’s essentially a 3/7 chance to do nothing and a 4/7 chance to win the fight, since staggering is pretty much just a free turn for you, and a knockout gives you a guaranteed kill on the next round. This is pretty good odds.

If you fail, then the troll attacks with 2 strength against your defense of 2.

Attack = 2, Defense 2

Miss

Miss

Miss

Stagger

Stagger

Light wound

Light wound

Light wound

Knockout

Odds are 3/7 he misses and the ball is in your court again, everything back as the combat started, 3/7 that he lightly wounds you, and 1/7 that he knocks you out (and will probably kill you next round).

Assuming he lightly wounds you, your strength becomes 1, and now this becomes your new lookup table when you attack:

Attack = 1, Defense 1

Miss

Miss

Miss

Miss

Stagger

Stagger

Knockout

Knockout

Kill

4/7 odds of missing and reverting control to the troll, and 3/7 odds of winning. Miss, and the troll attacks with 2 against your defense of 1, the same sub-table that you started with.

I did some math, and estimate you have about an 83% chance of beating the troll at the start of the game.

Then I did more math to estimate the odds of beating the thief based on your experience level. This assumes that he gets first strike, that you use the knife, and that he will kill you if you get knocked out. Take this with a grain of salt, because I got really bogged down in the algebra, and I don’t feel like checking it again.

Your level	Approximate odds
2	0.25%
3 (62 points)	16%
4 (185 points)	23%
5 (308 points)	46%
6 (432 points)	67%
7 (555 points)	89%

I also did an experiment to see how early in the game I could kill the thief. I got a few early treasures and also robbed the Bank of Zork, giving me a score of 64, and a combat strength of 3. I went to visit the thief, and this happened:

Treasure Room
This is a large room, whose north wall is solid granite.  A number
of discarded bags, which crumble at your touch, are scattered about
on the floor.  There is an exit down and what appears to be a newly
created passage to the east.
There is a silver chalice, intricately engraved, here.
You hear a scream of anguish as you violate the robber's hideaway.
Using passages unknown to you, he rushes to its defense.
The stiletto flashes faster than you can follow, and blood wells
from your leg.

So, the thief gets first strike when you intrude on his lair. He had a combat strength of 5, I had a combat strength of 3, so he was going to get the most favorable combat table possible, and “light wound” was the result. That left me with a combat strength of 2, but I figured if I gave him treasure, his defense would be only 1 – a base value of 2 for being engrossed, and a -1 penalty for being attacked with the knife.

> give egg to thief
The thief is taken aback by your unexpected generosity, but accepts
the jewel-encrusted egg and stops to admire its beauty.
> kill thief with knife
The quickness of your thrust knocks the thief back, stunned.
The thief slowly regains his feet.

Would he accept more treasure?

> give portrait to thief
The thief is taken aback by your unexpected generosity, but accepts
the portrait of J. Pierpont Flathead and stops to admire its beauty.
> kill thief with knife
The thief is disarmed by a subtle feint past his guard.
The robber, somewhat surprised at this turn of events, nimbly
retrieves his stilletto.

Yep! Continuing:

> give bills to thief
The thief is taken aback by your unexpected generosity, but accepts
the stack of zorkmid bills and stops to admire its beauty.
> kill thief with knife
The thief drops to his knees, staggered.
The thief slowly regains his feet.
> give painting to thief
The thief is taken aback by your unexpected generosity, but accepts
the painting and stops to admire its beauty.
> kill thief with knife
The fatal thrust strikes the thief square in the heart:  He dies.
Almost as soon as the thief breathes his last breath, a cloud
of sinister black fog envelops him, and when the fog lifts, the
carcass has disappeared.  His booty remains.

As the thief dies, the power of his magic decreases, and his
treasures reappear:
  A jewel-encrusted egg
  A portrait of J. Pierpont Flathead
  A stack of zorkmid bills
  A painting
>

And so, I killed the thief pretty early in the game. Of course I made it unwinnable in the process, since you need him alive to open the egg for you, but it shows that if you’re willing to exploit his behavior, you don’t need to put this off for very long, and it your life will be easier without him wandering around the dungeon messing things up.

I had also wondered how the thief behaves when wandering, but couldn’t reach as many conclusions as I wanted to. The thief traverses most of the rooms in Zork, even outdoor rooms like the climbable tree in the forest. He ignores the normal passageways, and instead just moves from room to room in a completely predetermined pattern, based on the room’s internal ID numbers. A large number of rooms are marked “sacred,” including the living room, and I believe this flag prevents the thief from entering.

One last mystery was determining how the inventory limit works, and the data tables proved invaluable for this.

Almost all objects, whether takeable or not, have a “size” parameter. Those that don’t effectively have a “size” of zero. You can hold up to 100 units in your inventory.

Some objects also have a “capacity” parameter, and can hold other objects, with a combined size of up to the container’s capacity. Sometimes they need to be “opened” first, such as the grail, which apparently is more like a stein.

Here is a list of container objects:

	Size	Capacity
Brochure	30	1
Woods Stamp	1	1
Blue book	10	2
Brick	9	2
Green book	10	2
Purple book	10	2
White book	10	2
Glass bottle	0	4
Silver chalice	10	5
Flask	10	5
Grail	10	5
Egg	0	6
Sack	3	15
Red buoy	10	20
Steel box	40	20
Nest	0	20
Coffin	55	35
Wooden barrel	70	100
Balloon basket	70	100
Boat	20	100

And here is a list of non-container objects and their sizes. Not all can be taken.

Bag of coins	15
Bloody axe	25
Blue label	1
Black book	10
Broken balloon	40
Stick	3
Timber	50
Coke bottles	15
Useless lantern	20
Card	1
Crown	10
White sphere	10
Trident	20
Violin	10
Green paper	3
Pump	0
Diamond	0
Guano	20
Jade figurine	10
Lamp	15
Emerald	0
Leaflet	2
Cage	60
Matchbook	2
Newspaper	2
Painting	15
Candles	10
Necklace	10
Eatme cake	10
Blue cake	4
Orange cake	4
Red cake	4
Vitreous slag	10
Leaves	25
Plastic	20
Platinum bar	20
Pot of gold	15
Zorkmid	10
Water	4
Rope	10
Ruby	0
Rusty knife	20
Bracelet	10
Skeleton Keys	10
Shovel	15
Coal	20
Printer paper	70
Flathead stamp	1
Statue	8
Stiletto	10
Sword	30
Tan label	2
Rare spices	8
Torch	20
Trunk of jewels	35
Tube	10
Gunk	6
Wire	1
Wrench	10
Zorkmid bills	10
Portrait	25
Bauble	0
Clockwork canary	0
Gold card	4
Blue sphere	0
Mat	12
Red sphere	0