GoKawiilBroken in Factorio 2.1 – Version 2.0 only! Factorio 2.1 changes the way the RNG is used. This breaks my in-game implementations. The theoretical aspects of how the RNG works still apply, as they still use the same RNG. For more info see section 6.1. Factorio 2.1.
Introduction
With the release of the Space-Age DLC in Factorio several new mechanics were introduced. One major mechanic was the new concept of different items and building qualities. By default, items are created with common quality. If quality modules are used in the crafting machine we gain a small chance to obtain items of higher quality.
What does that entail? In short: Items and buildings gain improved stats, such as faster crafting speeds, modules providing stronger buffs, power poles having an increased range and inserters swinging faster. Thats pretty neat – hence we are interested in obtaining the highest possible quality on our items and buildings. To source a large number of such items the devs essentially said that this randomness boils down to “basically statistics”, i.e. if the volume high quality items one obtains is sufficiently large, then the observed distribution of qualities will be close to the expected distribution.
But is it the only way to scale? Thinking about it, one might ponder:
How is it possible for a deterministic game like Factorio to have a random mechanic?
The short answer is: It isn’t random.
Instead – as is common in computing – the simulation makes use of a pseudo-random number generator (PRNG). A PRNG is a deterministic algorithm which produces a sequence of numbers which for all intents and purposes appears to be random. In particular this means properties like it following a well defined distribution of outputs, which contains no discernable patterns. Normally in computer science one can get away with treating the PRNG as just a black box function which can yield random numbers, without concerning oneself with how it actually works. Yet by taking a look under the hood we can do something funny.
The Funny: What happens if we know the exact algorithm and its internal state? Then we could just run the same algorithm on the state and obtain the same outputs, which will also be seen by the game internally. Its necessarily always the same outputs, as otherwise the chosen algorithm would not be deterministic. As such we can run the same computations simultaneously to the game and predict the future outputs of the PRNG, allowing us to predict the future “random” events which will occur in the game, such as which crafts will observe an increase in quality.
In the following sections I’ll build up to that point, starting from what RNG the game uses, how it is breakable and how it can be ab used ingame. The entire background should be understandable if you have a rudimentary understanding of linear algebra. That should be the only prerequisite.
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