andex

andex (Array iNDEX) is a single-file, zero-dependency, rust crate that helps us create a strongly-typed, zero-cost, safe array index and the corresponding array type.

This is specially useful in scenarios where we have different arrays inside a struct and we want reference members without holding "hard" references. May also be useful when programming an Entity Component System.

Basic usage

use andex::*;
use andex::impl_andex_for;

// Create the type alias:
type MyIdx = Andex<12>;

// Create the array wrapper:
#[derive(Default)]
pub struct MyU32([u32; MyIdx::SIZE]);

// Use `impl_andex_for` to make it indexable:
impl_andex_for!(MyU32, u32, MyIdx);

fn example() {
    // Iterate:
    for i in MyIdx::iter() {
        println!("{:?}", i);
    }
    // Generate first index at compile time:
    const first = MyIdx::new::<0>();
    // Index the collection:
    let myu32 = MyU32::default();
    const first = MyIdx::new::<0>();
    println!("{:?}", myu32[first]);
}

Creating index instances

When an andex is created, it knows at compile time the size of the array it indexes, and all instances are assumed to be within bounds.

For this reason, it's useful to limit the way andex's are created. The ways we can get an instance is:

Via new, passing the value as a generic const argument:
```
const first = Idx::new::<0>::()
```
This checks that the value is valid at compile time, as long as you use it to create const variables.
Via try_from, which returns Result<Andex,Error> that has to be checked or explicitly ignored:
```
if let Ok(first) = Idx::try_from(0) {
    ...
}
```
By iterating:
```
for idx in Idx::iter() {
    ...
}
```

The assumption that the instances can only hold valid values allows us to use get_unsafe and get_unsafe_mut in the indexer implementation, which provides a bit of optimization by preventing the bound check when indexing.

Creating the indexable array wrapper

To use the index, we first create the array wrapper, and then use the impl_andex_for to make it indexable by the andex:

pub struct ArrayWrapper([u32; 12])

impl_andex_for!(ArrayWrapper, u32, Idx);

This macro creates the appropriate Index and IndexMut implementations. These implementations use get_unchecked and get_unchecked_mut under the wraps, as the array bounds are checked when the andex instance is created and we don't have to check them again.

Note: the user is responsible for making the limit of the andex and the wrapper equal.

Full example

use andex::*;
use std::convert::TryFrom;

/// A player with score
#[derive(Default)]
pub struct Player {
    pub score: i32,
}

/// All players in the game
#[derive(Default)]
pub struct Players([Player; 4]);

/// The player identifier
type PlayerId = Andex<4>;

// Make Players[PlayerId] work
impl_andex_for!(Players, Player, PlayerId);

/// The game state
#[derive(Default)]
pub struct Game {
    pub players: Players,
}

impl Game {
    pub fn play(&mut self) {
        // Increment all scores
        for playerid in PlayerId::iter() {
            self.players[playerid].score += 1;
        }
        // Increment the first player's score:
        self.players[PlayerId::new::<0>()].score += 1;
        // ^ note that we had to use a const generic parameter so that
        // the index bound is checked at compile time.
        // If we want to create an index at run time, we have to use
        // TryInto/TryFrom, which returns Result:
        if let Ok(playerid) = PlayerId::try_from(4) {
            self.players[playerid].score = 3;
        }
        // ^ This "if" is never true because 4 >= 4, which is out-of-bounds.
    }
}

Alternatives