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: 2018-05-04 02:52 pm (UTC)Yes! And not just adventure games, either. I always used to want that feature in puzzle games of the XOR family (which also include the 'Enigma' game on my web page, and the spiritual sequel 'Chroma' by its original designer). Those too have the property that you get 700 moves into the game and then realise that if only you'd done something slightly differently in room 1 and then done the rest of your current solution unchanged, you could make progress.
I never got round to it, but I always quite wanted to try to build a rebasing UI for Enigma. I felt it might well be tractable in UI terms because it wouldn't make sense to rebase a move sequence from parent state A to state B unless the player character's location was the same between A and B, which would mean that when you selected a chunk of moves and tried to drag them, the UI would be able to narrow down to quite a small number of positions that it would even make sense to rebase them to, and mark them / auto-snap to them / generally make it easy to land on the right one.
(I think one reason I never got round to it is because those games depend critically on human-designed levels, and there's a finite supply of those, so once you've solved them all yourself, your motivation to design and implement an elaborate UI to make your own life easier suddenly vanishes...)
no subject
Date: 2018-05-04 03:40 pm (UTC)