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, right?
However, one of their design goals is that people playing on the same seed should have roughly the same game, it should feel "fair". Some things you probably want to be fairly random, for instance your card choices can depend on what cards you chose before. But it's also important that people choosing the exact same cards (and taking the same path, maybe?) should be offered the same options.
In STS1, the order of relics was fixed from the start as I recall. So if you skipped a shop, you'd get exactly the same relics in the next shop as you would have in the one you skipped. Good for seed fairness, but a little odd.
Imagine the game of two players having the same state X. While combat, one player would trigger a random action, the other doesn't. After the combat, both should still get the same randomized reward options. This wouldn't work with just a singular RNG.
This way, you can see how e.g. players of different skill level navigate the "same" run (same seed), without everything diverging completely on the very first (meaninglessly small) combat choice.
That requirement is what made this problem difficult for the devs to solve.
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.
It's also more robust than calling RNG 10 times since if you use the same algorithm to seed as for the RNG proper then you will get the same sequences in each instance, just offset.
Point being, the current problematic state of the game is trivially fixable in multiple ways that require half a second's thought (once being aware of the problem).
I think you're overlooking the distinction between seeded and unseeded runs. An unseeded run is a run in which the player enters the game not knowing what the seed of the RNG is and not being able to pick it (this is the default mode). A seeded run is where the player provides the RNG seed. Generally, things like unlocks and steam achievements can only be earned on unseeded runs, but players want to be able to play seeded runs anyway. Of course all runs have an RNG seed: an unseeded run is when the seed is itself chosen at random, a seeded run is when the player specifies the seed.
Imagine a game with a standard deck of cards played over several rounds, where your opponent performs actions in response to your actions. The deck of cards is shuffled pseudo-randomly between every round. Every time you make a move, your opponent has N valid moves, and picks between them pseudo-randomly.
Players play a seeded run because they want to retry the same set of challenges, because they are asking themselves "if I had done this, would I have won" or "if I had done this, what would have happened".
So in this example, given a known seed: in round 1, my cards are shuffled this way, and in round 2, my cards a shuffled this other way, regardless of which moves I make in round 1.
If the opponent picks its response using the same RNG that shuffles the deck, the players actions in round 1 would change the shuffling of the deck in round 2. This would change the design parameters of what a seeded run means: it's no longer giving the guarantee "the deck is shuffled in the same way in round 2 regardless of what you do in round 1", which is the experience the designer wants to create and what the players want to play. Players might, e.g., say "who can get the highest score on this seed", they might search for the easiest or most difficult seeds, or they might search for seeds where particularly unlikely sequences of events are guaranteed to occur, because perhaps this sequence of events is so unlikely that a human would have to play a hundred million games to witness that sequence organically, and people want to see that sequence of events because it's interesting in some way. It's designed this way so that if you play the same seed, certain random events play out the same way, i.e., non-randomly.
>The way Slay the Spire allows you to save and resume runs is by storing the total number of times each RNG has been called, and then calling each RNG that many times (throwing away the result) whenever a save file is loaded.
Depending on what the game is like (I know nothing about it), that could make sense, even if it is inelegant.
This doesn't really have any impact on the gameplay, and isn't related to the correlation problem, it's just a constraint on the class of RNG algorithms in use, they need to be deterministic with recoverable state.
You can be safe from RNG manipulation while still suffering from RNG prediction. Players can't modify the events that are going to happen, but if they can predict them, it's still a problem.
The situation is like there's a bug in the blackjack table where, instead of shuffling the whole shoe together, each deck in it was shuffled on its own in the same way and then the identical decks are stacked together. Once you've seen 52 cards, you know the repeating pattern and can play with perfect or near-perfect knowledge of what's about to be dealt.
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