bevymodindex

A Rust crate that allows efficient querying for components by their values in the game engine [Bevy].

Use Cases

It is quite common to want to write code in a system that only operates on components that have a certain value, e.g.: rust fn move_living_players(mut players: Query<&mut Transform, &Player>) { for (mut transform, player) in &players { if player.is_alive() { move_player(transform); } } }

With an index, we can change the code to: rust fn move_living_players( mut transforms: Query<&mut Transform>, player_alive_idx: Index<PlayerAlive> ) { for entity in &player_alive_idx.get(true) { transforms.get(entity).unwrap().move_player(transform); } }

There are a few cases where a change like this may be beneficial: - If is_alive is expensive to calculate, indexes can we can save work by caching the results and only recomputing when the data actually changes. - If the component data that the result is calculated from doesn't change often, we can use the cached values across frames. - If components tend to change only in the beginning of a frame, and the results are needed multiple times later on, we can use the cached values across different systems, (or even the same system if it had been calculated multiple times). - If we don't care too much about performance, indexes can provide a nicer API to work with.

Indexes add a non-zero amount of overhead, though, so introducing them can make your systems slower. Make sure to profile your systems before and after introducing indexes if you care about performance.

Getting Started

First, import the prelude. rust use bevy_mod_index::prelude::*;

Next, implement the IndexInfo trait. If your component only needs one index, you can implement this trait directly on the component. If you need more than one, you can use a simple unit struct for each index beyond the first. You can also use unit structs to give more descriptive names, even if you only need one index.

You must specify: - the type of component to be indexed, - the type of value that you want to be able to use for lookups, - a function for calculating that value for a component, and - how to store the relationship between an entity and the value calculated from its appropriate component. ```rust struct NearOrigin {} impl IndexInfo for NearOrigin { type Component = Transform; type Value = bool; type Storage = HashmapStorage;

fn value(t: &Transform) -> bool { t.translation.length() < 5.0 } } ```

Finally, include the Index system param in your systems and use it to query for entities! ```rust fn countplayersandenemiesnearspawn( players: Query<(), With<(&Player, &Transform)>>, enemies: Query<(), With<(&Enemy, &Transform)>>, index: Index, ) { let (mut playercount, mut enemy_count) = (0, 0);

let entitiesnearspawn: HashSet = index.lookup(true); for entity in entitiesnearspawn.intoiter() { if let Ok(()) = players.get(entity) { playercount += 1; } if let Ok(()) = enemies.get(entity) { enemy_count += 1; } }

println!("There are {} players and {} enemies near spawn!", playercount, enemycount) } ```

Implementations

HashmapStorage uses a custom SystemParam that updates the index whenever it is used. The update is done by using a query to loop over all components, and only reading the actual data/re-computing the index value when a component is changed since the last update. If the index is not used, it will not update, even if its system runs, which can be useful if you only need up-to-date data in certain circumstances (e.g. when the mouse is clicked) to save re-computing values for rapidly changing data.

NoStorage, as the name implies, does not store any index data. Instead, it loops over all data each time it is queried, computing the value function for each component, exactly like the first move_living_players example above. This option allows you to use the index API without incurring as much overhead as HashmapStorage (though still more than directly looping over all components yourself)

Compatability

| Bevy Version | bevy_mod_index Version | |--------------|--------------------------| | 0.11 | 0.2.0 | | 0.10 | 0.1.0 |

API Stability

Consider the API to be extremely unstable as I experiment with what names and patterns feel most natural and expressive, and also work on supporting new features.

Performance

I have not put a lot of effort into optimizing the performance indexes yet. However, I have done some initial tests under to get a sense of approximately how much overhead they add.

With 1 million entities, while none of the components change frame-to-frame, using the component itself as the index value, operation on ~300 entities takes: - 2-4x as long as a naive iteration when using NoStorage. - 3-5x as long as a naive iteration when using HashmapStorage.

With the same setup, except that 5% of the entities are updated every frame, performance for HashmapStorage drops to 30-40x as long as naive iteration.

I am currently in the process of adding more concrete benchmarks, and I do have some plans for changes that will affect performance.

Get in contact

If you have suggestions for improvements to the API, or ideas about improving performance, I'd love to hear them. File an issue, or even better, reach out in the bevy_mod_index #crate-help thread on Bevy's [discord].

Troubleshooting

Future work