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Correlated randomness in Slay the Spire 2

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Re: Correlated randomness in Slay the Spire 2

#41

Post by godwinson__4-8 »

account42 wrote: post #27

> Implementing a PRNG within the codebase instead of calling the C# standard library has an additional advantage: seeds are guaranteed to be the same on all platforms. In Spire 1, seeds on the desktop version of the game were different from seeds on the mobile version of the game, because the standard library implementation of PRNG differed between platforms. It is also worth mentioning that the standard library impl…

Sometimes it is useful to deal with a platform where such things are not even available, never mind platform dependent. Then see how quickly your code breaks.

Standard library invocations - including random number generation - often break entirely when targeting wasm freestanding for instance, as in that case there is really very little "platform" to speak of.

Re: Correlated randomness in Slay the Spire 2

#42

Post by rcxdude »

xdertz wrote: post #2

> the game used several distinct pseudorandom number generators, to prevent e.g. randomness within a combat from influencing future card rewards. Why is this important? Feels like fixing what seems to be a non-issue lead to a bunch of real issues. With a good RNG it should not be possible to predict future numbers based on past numbers so players cannot manipulate card rewards in their favour based on combat actions,…

Some degree of stability in seeds is desirable, partly because of the compatitive elements (multiple players playing the same seed should have roughly the same game), but also when updating the game is means that if they tweak one area the rest of the seed will stay roughly the same. (Maybe less important for games with short runs compared to sandbox games like Minecraft where the world might be generated by very different versions if the game and you don't want blatent seams when it happens)

Re: Correlated randomness in Slay the Spire 2

#43

Post by gpm »

Lalabadie wrote: post #40
jszymborski wrote: post #33

> The phenomenon of "correlated RNG" (or "CRNG") This is a pretty funny abbreviation since CRNG is sometimes "cryptographic random number generator", which would not be susceptible to this correlation. Albeit I think CSRNG is more common.

The criteria for calling a RNG "cryptographically secure" are incompatible with the game design goals here. The game needs a RNG that's stable when seeded, for reproducible runs. I look for the same kind of qualities when doing generative art. In comparison, a CSPRNG should be safe from oracle attacks, which is essentially the opposite goal.

CSPRNGs are absolutely seedable deterministic functions that will result in entirety reprodible runs.

The only difference is that if you don't know the seed it is computationally difficult to predict the next value given the previous ones. But that's not something any game dev is ever going to want to do (or waste time trying to do)

Re: Correlated randomness in Slay the Spire 2

#44

Post by HansHamster »

Lalabadie wrote: post #40
jszymborski wrote: post #33

> The phenomenon of "correlated RNG" (or "CRNG") This is a pretty funny abbreviation since CRNG is sometimes "cryptographic random number generator", which would not be susceptible to this correlation. Albeit I think CSRNG is more common.

The criteria for calling a RNG "cryptographically secure" are incompatible with the game design goals here. The game needs a RNG that's stable when seeded, for reproducible runs. I look for the same kind of qualities when doing generative art. In comparison, a CSPRNG should be safe from oracle attacks, which is essentially the opposite goal.

I would expect all RNG algorithms to be deterministic and stable with their seed, but the cryptographically secure ones to have some additional properties like making it unfeasible to reverse the seed from the output, having a very long period or strong guarantees on the distribution of the output. It's just that using a 'secure' algorithm is often overkill for a game when you don't really need those extra guarantees.

Re: Correlated randomness in Slay the Spire 2

#45

Post by cevn »

I wonder if this can explain something happening to me. If I select "random" at character select, I had a run of 30 or 40 where I never received the Silent. Defect seem to come up more often than it should, and Ironclad less often.

Re: Correlated randomness in Slay the Spire 2

#46

Post by AlotOfReading »

margalabargala wrote: post #18
eig wrote: post #7

Earlier quoted context omitted.

The game stores and allows you to see the RNG seed that controls the run's events and layout. The developers want players to be able to share seeds that produce interesting runs. That requirement is what made this problem difficult for the devs to solve.

This shouldn't actually be difficult to solve though. The issue is that knowing the offset of seeds helps predict outputs. Instead of calling RNG(seed+hash(string)) 10x, make one RNG(seed) and call that 10 times to get random seeds for your 10 rngs. Now you have perfect determinism and no correlation.

That's assuming the game initialization order is deterministic. Using the hash of the combined state of seed and string avoids that assumption without giving up determinism.

Re: Correlated randomness in Slay the Spire 2

#47

Post by swiftcoder »

bilekas wrote: post #30
account42 wrote: post #27

> Implementing a PRNG within the codebase instead of calling the C# standard library has an additional advantage: seeds are guaranteed to be the same on all platforms. In Spire 1, seeds on the desktop version of the game were different from seeds on the mobile version of the game, because the standard library implementation of PRNG differed between platforms. It is also worth mentioning that the standard library impl…

> It is also worth mentioning that the standard library implementation might change over time, which would break all past seeds. If the stdLib changes and you need to use the same, then you're unfortunately going to be suck with porting the previous version into your own library. It's pretty forward thinking from the devs here, I would love to see my boss' face if I told him we need time to port some of the stdLib in…

Be glad you work on top of a relatively standardised platform! The C standard doesn't specify any details of the implementation backing rand(), so a bunch of platforms have wildly different implementations, and they change over time (FreeBSD swapped theirs out in 202, for example)

Re: Correlated randomness in Slay the Spire 2

#48

Post by swiftcoder »

xdertz wrote: post #2

> the game used several distinct pseudorandom number generators, to prevent e.g. randomness within a combat from influencing future card rewards. Why is this important? Feels like fixing what seems to be a non-issue lead to a bunch of real issues. With a good RNG it should not be possible to predict future numbers based on past numbers so players cannot manipulate card rewards in their favour based on combat actions,…

> With a good RNG it should not be possible to predict future numbers based on past numbers

Since they are using the built-in RNG, it is trivial to predict if you know (or can guess) the seed: just run the same RNG a few steps ahead.

For something like a tool-assisted speed run, this is very exploitable to setup optimal runs

Re: Correlated randomness in Slay the Spire 2

#50

Post by jcalx »

Maybe turn-based roguelike deckbuilders aren't the best for this, but I actually like some correlated randomness in some games, as it adds a new hidden mechanic to explore. In Hades 1 there are some (presumably unintentional) RNG manipulations that open up high-level techs for seeded speedruns:

- Hades 1 is a series of "chambers", or enemy encounters, where some layouts are faster than others [0]

- chambers (and other things like enemy spawns, boons, etc.) are "randomly" picked by an RNG with its seed normally unknown to the player (well that, and other factors [1])

- you can see the per-chamber RNG seed using mods [2], and manipulate it with seemingly meaningless actions [3] — e.g. breaking a pot (a mundane, cosmetic environmental item) increments the RNG seed by 1

- this leads to the existence of "routed runs" [4] — very fast speedruns enabled by very deliberate actions that can be replicated by a skilled player [5].

- anecdotally, with enough practice, skilled players can also recognize chamber patterns in unseeded speedruns and give themselves better odds at more favorable chambers by manipulating the RNG (although tbh the ability to recognize this on the fly is a little dubious)

So the invisible correlated RNG seeding adds in a higher skill ceiling for experienced players, while not really taking anything away from casual players.

Another game with this kind of RNG mechanic is Super Mario Bros. 3 — there's an excellent (86-minute, fyi) Summoning Salt video about the history of speedrunning this game and dealing with the "random" Hammer Bros movement (@27:15 to skip to that part).

[0] https://docs.google.com/document/d/e/2PACX-1vR6NaU9v1-raeibk...

[1] https://docs.google.com/document/d/e/2PACX-1vSl9RGGyPbNqCnTL...

[2] https://www.youtube.com/watch?v=AHdt35TDvNY

[3] https://www.speedrun.com/hades/guides/jxpkj

[4] https://www.youtube.com/watch?v=CBRTQkoOZ4k

[5] https://docs.google.com/spreadsheets/d/1fNlBhBOsCz6092GUnsIt...

[6] https://www.youtube.com/watch?v=_EsFyogVvkw

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