Rule Emergence in Diablo


I love the way that complex systems can produce their own rules, and I just came across an example that made me smile. I was reading this fantastic piece on Gamasutra by Radek Koncewicz, which is a deep dive analysis into some of the properties of the 1996 classic Diablo. It referenced the insanely detailed Jarulf’s Guide, a 150+ page examination of all the game’s mechanics that was put together by fans in the years following the game’s release. I started skimming it, and its a great examination of an interactive system in precise detail. Look at this great stuff:PVP

Something I came across struck me as a concise example of the way systems can emergently produce new rules. So, here are three (simplified versions of) rules in how Diablo generates magical equipment for the player.

1) Equipment is generated with prefixes and suffixes, which confer various effects. In the example pictured, we can see the prefixes “Massive” and “Brutal” which confer increased damage, D1-hellfire-obzod1and the suffix “Of Perfection” which increases dexterity. There are also ‘cursed’ prefixes and suffixes, which lower stats or other values instead of raising them.

2) Prefixes and suffixes change the price of the item, according to their magnitude and quality (positive ones raise it, and ‘cursed’ modifiers lower the price). The price will be what we can buy the item at from the store, and the fraction thereof is what we could sell it for.

3) Items have durability, which decreases with use. Items can be repaired by the blacksmith for a fee. The fee is equal to: [the percent of durability needing to be restored] * [the item’s price] * 15%. If that value is less than 1, no repair will take place, since you can’t pay in fractional amounts.

All of that is pretty straightforward. Now, an interesting new rule arises as a consequence of the ones above: “Cursed items cannot be repaired.” But it’s not an explicit rule, it’s not specifically encoded anywhere or deliberately part of the game. Rather, the way those rules interact produces it. Because cursed prefixes and suffixes are negatively multiplied against an item’s price, cursed items have a negative ‘price’, but the game only allows a minimum price of 1. So, all cursed items have a price of 1. Cursed items can’t be repaired, because 15% of 1 is always less than 1, so the blacksmith won’t ever be willing to repair it, no matter how damaged it is.

It’s a small and largely irrelevant little rule, which the player might even fail to notice. But it’s interesting to me because the system itself produces it. Even with minor examples like this, that quality of emergence, where the way parts interact produces novel outcomes, is something I find fascinating and a little beautiful.




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