hecs

hecs provides a high-performance, minimalist entity-component-system (ECS) world. It is a library, not a framework. In place of an explicit "System" abstraction, a World's entities are easily queried from regular code. Organize your application however you like!

Example

let mut world = World::new();
// Nearly any type can be used as a component with zero boilerplate
let a = world.spawn((123, true, "abc"));
let b = world.spawn((42, false));
// Systems can be simple for loops
for (id, (number, &flag)) in world.query_mut::<(&mut i32, &bool)>() {
  if flag { *number *= 2; }
}
// Random access is simple and safe
assert_eq!(*world.get::<&i32>(a).unwrap(), 246);
assert_eq!(*world.get::<&i32>(b).unwrap(), 42);

Why ECS?

Entity-component-system architecture makes it easy to compose loosely-coupled state and behavior. An ECS world consists of:

• any number of entities, which represent distinct objects
• a collection of component data associated with each entity, where each entity has at most one component of any type, and two entities may have different components

That world is then manipulated by systems, each of which accesses all entities having a particular set of component types. Systems implement self-contained behavior like physics (e.g. by accessing "position", "velocity", and "collision" components) or rendering (e.g. by accessing "position" and "sprite" components).

New components and systems can be added to a complex application without interfering with existing logic, making the ECS paradigm well suited to applications where many layers of overlapping behavior will be defined on the same set of objects, particularly if new behaviors will be added in the future. This flexibility sets it apart from traditional approaches based on heterogeneous collections of explicitly defined object types, where implementing new combinations of behaviors (e.g. a vehicle which is also a questgiver) can require far-reaching changes.

Performance

In addition to having excellent composability, the ECS paradigm can also provide exceptional speed and cache locality. hecs internally tracks groups of entities which all have the same components. Each group has a dense, contiguous array for each type of component. When a system accesses all entities with a certain set of components, a fast linear traversal can be made through each group having a superset of those components. This is effectively a columnar database, and has the same benefits: the CPU can accurately predict memory accesses, bypassing unneeded data, maximizing cache use and minimizing latency.

Why Not ECS?

hecs strives to be lightweight and unobtrusive so it can be useful in a wide range of applications. Even so, it's not appropriate for every game. If your game will have few types of entities, consider a simpler architecture such as storing each type of entity in a separate plain Vec. Similarly, ECS may be overkill for games that don't call for batch processing of entities.

Even for games that benefit, an ECS world is not a be-all end-all data structure. Most games will store significant amounts of state in other structures. For example, many games maintain a spatial index structure (e.g. a tile map or bounding volume hierarchy) used to find entities and obstacles near a certain location for efficient collision detection without searching the entire world.

If you need to search for specific entities using criteria other than the types of their components, consider maintaining a specialized index beside your world, storing Entity handles and whatever other data is necessary. Insert into the index when spawning relevant entities, and include a component with that allows efficiently removing them from the index when despawning.

Other Libraries

hecs owes a great deal to the free exchange of ideas in Rust's ECS library ecosystem. Particularly influential examples include:

• bevy, which continually pushes the envelope for performance and ergonomics in the context of a batteries-included framework
• specs, which was key in popularizing ECS in Rust
• legion, which demonstrated archetypal memory layout and trait-less components

If hecs doesn't suit you, one of those might do the trick!

License

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Disclaimer

This is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.