Data structures and stuff
May. 4th, 2018 12:00 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
jackThe premise
I have a tile-based computer adventure game. The current state is represented in memory as a 2d array of tiles representing the play area, each with one (or possibly more) objects on that tile. There is also an "undo" history, tracking recent actions, and the difference (objects moved, removed or added) so the previous or following state can be recreated.
In addition, each object remembers the most recent undo step which affected it, so you can click "undo" on an object and return it to the previous state (also undoing any following actions), not just undo one at a time until you rewind to the appropriate point.
I need to check the code, but as I remember, this is represented by each object having a pointer (well, the python equivalent) to one of the elements of the undo sequence. And when you redo or undo the pointers are updated to refer to the newly-correct object.
Now I'm unsure, but IIRC the undo steps refer to an object by coordinates in the play area, not a pointer to the object (in general, we assume the play area might store game objects as just ids or something, not as ab object in memory).
What happens when we want to save the game
We need to be able to save the game -- indeed, a modern game (especially one where actions aren't irreversible) should just save as it goes along, not require a separate load/save action to do so.
This means my instinctive layout above doesn't work. You can't save "a pointer". The best option is probably to use an index into the undo list which the undo list understands.
That can also cut out other possible bugs. If you have a pointer, it could be anywhere in memory. If you have an index into the undo list, you can choose to have the list check that the dereference is valid (and retain the option to turn those checks off if performance matters).
There's other possibilities but I think that's the best one. It is uncomfortably close to designing our own ORM -- we could alternatively have ALL objects represented by a unique id and index things by that instead (either via some global list of ids or only when we load from disk).
I run into this often when I'm game programming, the best way of 'referring' somehow to another game object -- by value or reference? by game coordinates or pointer to memory? But not in other types of programming. Does this experience ring a bell to anyone else?
But now I'm thinking...
This also reminds me of a problem I ran into when I started thinking about rust's memory models. If you have a class, that creates a bunch of other classes, and those classes want to have pointers to each other, there's no easy way of doing it iirc.
I think you need to rely on reference-counted pointers even though it feels like you shouldn't. That's not a problem in practice -- the "store an index" method above also has an indirection every time you need to access the object. But it feels like, you shouldn't need to. And a similar sort of "I want to refer to one of the classes which this big class is responsible for".
But I'm not sure if there's a way of combining these thoughts.
I have a tile-based computer adventure game. The current state is represented in memory as a 2d array of tiles representing the play area, each with one (or possibly more) objects on that tile. There is also an "undo" history, tracking recent actions, and the difference (objects moved, removed or added) so the previous or following state can be recreated.
In addition, each object remembers the most recent undo step which affected it, so you can click "undo" on an object and return it to the previous state (also undoing any following actions), not just undo one at a time until you rewind to the appropriate point.
I need to check the code, but as I remember, this is represented by each object having a pointer (well, the python equivalent) to one of the elements of the undo sequence. And when you redo or undo the pointers are updated to refer to the newly-correct object.
Now I'm unsure, but IIRC the undo steps refer to an object by coordinates in the play area, not a pointer to the object (in general, we assume the play area might store game objects as just ids or something, not as ab object in memory).
What happens when we want to save the game
We need to be able to save the game -- indeed, a modern game (especially one where actions aren't irreversible) should just save as it goes along, not require a separate load/save action to do so.
This means my instinctive layout above doesn't work. You can't save "a pointer". The best option is probably to use an index into the undo list which the undo list understands.
That can also cut out other possible bugs. If you have a pointer, it could be anywhere in memory. If you have an index into the undo list, you can choose to have the list check that the dereference is valid (and retain the option to turn those checks off if performance matters).
There's other possibilities but I think that's the best one. It is uncomfortably close to designing our own ORM -- we could alternatively have ALL objects represented by a unique id and index things by that instead (either via some global list of ids or only when we load from disk).
I run into this often when I'm game programming, the best way of 'referring' somehow to another game object -- by value or reference? by game coordinates or pointer to memory? But not in other types of programming. Does this experience ring a bell to anyone else?
But now I'm thinking...
This also reminds me of a problem I ran into when I started thinking about rust's memory models. If you have a class, that creates a bunch of other classes, and those classes want to have pointers to each other, there's no easy way of doing it iirc.
I think you need to rely on reference-counted pointers even though it feels like you shouldn't. That's not a problem in practice -- the "store an index" method above also has an indirection every time you need to access the object. But it feels like, you shouldn't need to. And a similar sort of "I want to refer to one of the classes which this big class is responsible for".
But I'm not sure if there's a way of combining these thoughts.
no subject
Date: 2018-05-13 02:08 pm (UTC)My idea was that always requiring that characters have an in-world way of reversing mistakes and finding objects has quite a lot of constraints on the plot -- it means that if the character ever moves from one place to another, or enters some dangerous situation, they either need to be required to have picked up all relevant objects and taken relevant actions before then, or have some way of leaving the current situation to do so now.
However, if you take the point of view that, near the end of the game, you can teleport back in space *and time* to do the thing you missed, and then teleport forward again to finish the climax, you have a lot more flexibility to add interesting interactions between objects and events. For clarity, I don't mean in-universe teleport, I just mean that the player can select a place (or equivalently, an earlier point in time) and do a thing, and then go back to the furthest point they reached.
To me that's the opposite of forcing them to replay it repeatedly, because they don't have to retake all the actions inbetween, they just click one button and go straight to the end of the game. But I'm aware, this is an idea I haven't really tested, so if people who have the same basic starting point as me are put off by it, I should reconsider if it's actually desirable.
What I like about it is that there's LOTS of puzzles that would be utterly unfair in a normal game, that work fine if the character has the option of exploring other areas to find a solution, even if those areas wouldn't logically be available to the character at that point in time. Random ideas like, you have a cell, you're breaking out, it strains credibility that if you do it wrong you can do it again without anyone getting suspicious, but that's what lots of games require, but this game wouldn't. And you have a bunch of actions you can take to do so, but the puzzle is getting them in the right order. Or it's often logical that you can *throw something away*. Most games prevent that (or you just lose if you throw away the necessary plot item). That means you can't have any puzzles that involve throwing something in the sea or whatever, unless the character immediately announces that doing so is relevant simply by letting you do so. This game you could easily do that, because you can undo that single action (either immediately, or rewind that one action individually much later on in the game).