Can you explain more what type of game would need a call stack snapshot? I've never developed a game, but it seems like as long as you store like the initial state and prng you could always get the current state by replaying the full history. All the other logic would be stored outside the state, and only added when "committed". As long as prng is stable and you start from the clean state every time, you'd get the same outcome.
I think I have a good example from Magic: the Gathering.
There’s a card called “Fact or Fiction”. You reveal the top five cards of your deck. Then your opponent splits the cards into two piles. Then you pick one of the two piles to take into your hand.
You’ll need to store structures representing the choices that are intermediary steps (split cards, pick stack) in your state, which is basically function calls and their params (a call stack). This example could get hairier - Magic also features cards with branching logic “choose 1: do a or b”. I can imagine designing cards with large and convoluted possible execution paths.
You could have the card define a schema for state transitions/params and represent all these choices as JSON encoded POJOs, but as a developer it sounds a lot nicer to just be able to suspend an async function every time a choice is made.
I had the same issue in AGoT:BG and I solved it by representing the state of the game as a tree. At any point of the game, the current game state is a leaf of the tree.
You'd represent this kind of choice as a child node. When the user has made their choice, the code can return to the parent node with the choice being made so it can continue with the next "step" of the game.
This is the correct response. Hearthstone is structured like this internally.
If you are curious about it, I wrote a cc0 spec which stores hearthstone game state in xml. It’s based on how hearthstone stores game state on the server and client, and it was the first time a replay format was created for hearthstone:
https://hearthsim.info/hsreplay/
Incidentally the UI we wrote for hearthstone replays is a react app. It’s funny because looking back it was the first time I used react and typescript, and both were not at all adopted by the js community yet at the time.
That's the exact approach I'm considering for the new engine I mentioned!
Although that strategy enables you to store and recover the state of a game, it doesn't give you the ability to inspect a snapshot of that state. How can you print the card which has just been played, if that data only exists as an argument in the call stack of a suspended async function? In the same way that you can't inspect the local variables captured by a closure, mainstream languages also provide no way to inspect a suspended stack frame.
This problem interferes with debugging, consistency checks (e.g. hashing the game state to check that two clients are in sync), and unit testing.
