Implementing 2D vector syntax for accessing a 1D vector? - vector

Question

I'm making a toy roguelike and have a Level structure for storing the game map, for which the most naive implementation is a 2D vector.
I'm following this tutorial which uses a Vector of Vectors, but states that for performance gains it's also possible to use a single Vector of size MAP_HEIGHT * MAP_WIDTH, and to access a tile at (x, y) one can simply access map[y * MAP_WIDTH + x].
I'm trying to implement this faster method but using getters and setters is clunky, and public fields aren't that great either. I'd much prefer it to feel like a 2D vector.
In order to do that I need to implement the Index trait for my class, but I'm not sure how to get the result I want. Maybe by nesting the impls? I really no idea.
Here is my code with a terrible attempt at implementing Index for my structure, which obviously won't work for my purposes because it's one dimensional:
const MAP_WIDTH: i32 = 80;
const MAP_HEIGHT: i32 = 45;
pub struct Level {
map: Vec<Tile>,
}
impl Level {
pub fn new() -> Self {
Level { map: vec![Tile::empty(); (MAP_HEIGHT * MAP_WIDTH) as usize] }
}
}
impl std::ops::Index<i32> for Level {
type Output = Tile;
fn index(&self, x: i32) -> &Self::Output {
self[MAP_WIDTH + x]; // We have x and y values; how do we make this work?
}
}

Answer 1

Make your struct indexible over objects of type (i32, i32).
type Pos = (i32, i32);
impl std::ops::Index<Pos> for Level {
type Output = Tile;
fn index(&self, (x, y): Pos) -> &Self::Output {
&self.map[(y * MAP_WIDTH + x) as usize]
}
}
Which you can then access with, for example:
let tile = level[(3, 4)];
Since you are using i32, you need to make sure that the values are within range, and can be coerced to usize, which is what Vecs are indexed over. Probably you should just stick with u32 or usize values from the start. Otherwise, you'll need to keep track of the minimum x and y values, and subtract them, to keep the position in range. It's definitely simpler to deal with positive coordinates and make the assumption that the corner of your map is (0, 0).

Answer 2

It is possible, though not obvious.
First of all, I suggest having the MAP_WIDTH and MAP_HEIGHT in usize, as they are positive integers:
const MAP_WIDTH: usize = 80;
const MAP_HEIGHT: usize = 45;
Then you need to implement Index (and possibly IndexMut) to return a slice; in this case I'm assuming that you want the first coordinate to be the row:
impl std::ops::Index<usize> for Level {
type Output = [Tile];
fn index(&self, row: usize) -> &[Tile] {
let start = MAP_WIDTH * row;
&self.map[start .. start + MAP_WIDTH]
}
}
impl std::ops::IndexMut<usize> for Level {
fn index_mut(&mut self, row: usize) -> &mut [Tile] {
let start = MAP_WIDTH * row;
&mut self.map[start .. start + MAP_WIDTH]
}
}
Then, when you index a Level, it first returns a slice with the applicable row; then you can index that slice with the column number.
Below is an example implementation with a substitute Tile:
const MAP_WIDTH: usize = 80;
const MAP_HEIGHT: usize = 45;
#[derive(Clone, Debug)]
pub struct Tile {
x: u32,
y: u32
}
pub struct Level {
map: Vec<Tile>,
}
impl Level {
pub fn new() -> Self {
Level { map: vec![Tile { x: 0, y: 0 }; (MAP_HEIGHT * MAP_WIDTH) as usize] }
}
}
impl std::ops::Index<usize> for Level {
type Output = [Tile];
fn index(&self, row: usize) -> &[Tile] {
let start = MAP_WIDTH * row;
&self.map[start .. start + MAP_WIDTH]
}
}
impl std::ops::IndexMut<usize> for Level {
fn index_mut(&mut self, row: usize) -> &mut [Tile] {
let start = MAP_WIDTH * row;
&mut self.map[start .. start + MAP_WIDTH]
}
}
fn main() {
let mut lvl = Level::new();
lvl[5][2] = Tile { x: 5, y: 2 };
println!("{:?}", lvl[5][2]); // Tile { x: 5, y: 2 }
}

Answer 3

You cannot do this without exposing internal details about your implementation. Index is defined as:
pub trait Index<Idx>
where
Idx: ?Sized,
{
type Output: ?Sized;
fn index(&self, index: Idx) -> &Self::Output;
}
In order to support game[x][y], the return value of game[x] would need to:
Be a reference to something. (&Self::Output)
Implement Index itself.
There's no value to return a reference to other than self, and self would already implement Index for a usize so you can't reuse it.
Instead, you can implement indexing for a tuple:
impl std::ops::Index<(usize, usize)> for Level {
type Output = Tile;
fn index(&self, (x, y): (usize, usize)) -> &Self::Output {
&self.map[MAP_WIDTH as usize * y + x]
}
}
This can be used as level[(43, 12)].
If you implement Index to return a slice, you should be aware that you are forever requiring that your internal data structure be something that is based on slices. For example, you cannot use a "sparse" structure like a HashMap because it cannot return a &[Tile]. The ability to return a &[Tile] is now a part of the public API of the struct. It's certainly a possibility that the representation will change, especially since it's already changed once.