Crate tiny_ecs

Tiny ECS

The intention of this crate is that a basic ECS is provided, where you will be required to exercise a little additional control. This is somewhat due to some limitations, and also due to trying to maintain as little overhead as possible - this means no unneccesary copies/clones. In all cases you will need to use downcast_ref or downcast_mut to get the correct types to work with.

Both downcast_ref or downcast_mut return a reference to the type, in this case it will be PartMap<T> where T is your stored type. The downcast is required because the PartMap is stored as an Any trait object to enable storage of multiple PartMap<T>.

The basis of this ECS is the use of bitmasks. Each entity ID is in practice an internal index number in to an array which contains bitmasks. The bitmasks themselves keep track of what components the entity has. For the most part, bitmasks are handled for you, and some helper methods are available to hide their use, but there are also methods to get the bitmask for any ID if you are inclined to do some manual management.

Examples

Init with a capacity

This is good to do if you know the size required as it will prevent many reallocs/moves as data is added. This affects both the entity and partmaps allocs (they will be equal in size).

use tiny_ecs::Entities;

let mut entities = Entities::new(Some(1000), Some(24));

Demonstrating use

use tiny_ecs::Entities;

// These are the "components" we will use
struct Vector1 {
    x: i32,
}
struct Vector2 {
    x: i32,
    y: i32,
}
struct Vector3 {
    x: i32,
    y: i32,
    z: i32,
}

// Initialize the Entity collection
let mut entities = Entities::new(Some(3), Some(3));

// To create an entity you only need to add the first part using
// a free slot
let entity_1 = entities.get_free_slot().unwrap();
entities.add_part(entity_1, Vector1 { x: 42 });
// And you can add more parts to it
// The entity is only considered newly created if no parts existed before
entities.add_part(entity_1,
                  Vector3 { x: 3,
                            y: 10,
                            z: -12 });

// To add another entity you need another free slot
let entity_2 = entities.get_free_slot().unwrap();
entities.add_part(entity_2, Vector2 { x: 66, y: 6 });
entities.add_part(entity_2, Vector1 { x: 6 });

Access an entities part of type <T>

// To get access to a part belonging to an entity you need
// first to get the partmap created for the part type
// You need to 'anchor' this with a let or the ref is
// dropped before you can use it
let mut partmap = entities.get_map_mut::<Vector3>()
                  .expect("No part found for entity");
// To access the partmap proper you need to downcast
let mut partmap = partmap.downcast_mut::<PartMap<Vector3>>().unwrap();
// You can then use the part by getting a reference
let mut part = partmap.get_part_mut(entity_1).unwrap();
assert_eq!(part.z, -12);

Check if Entity contains a part type + remove part

// You can check if an entity contains a part with the type signature
if entities.entity_contains::<Vector1>(entity_1) {
    assert!(entities.rm_part::<Vector1>(entity_1));
}
assert_eq!(entities.entity_contains::<Vector1>(entity_1), false);

A system that uses an iter_mut()

use std::cell::RefMut;

// Make a system of some form that takes a `PartMap<T>` arg
fn some_system(mut partmap: &mut PartMap<Vector1>) {
    // You can then iterate over the parts directly
    for (k, v) in partmap.iter_mut() {
        v.x += 1;
        assert!(v.x > *k as i32);
    }
}
some_system(&mut partmap);

Get parts for an entity ID list

use std::cell::{Ref, RefMut};

// A system that fetches the parts for only the entities you are require
fn second_system(active: &[u32], mut v1_map: &mut PartMap<Vector1>) {
    for id in active {
        if let Ok(part) = v1_map.get_part_mut(*id) {
            part.x = 42;
        }
    }
}
second_system(&[0, 1, 2], &mut partmap);

A more complex system using PartMaps directly

// Or a system handles the `Entities` container directly
fn other_system(active_ents: &[u32], entities: &mut Entities) {
    // You can mutably borrow multiple part types at once
    let mut v1_partmap = entities.get_map_mut::<Vector1>()
                .expect("No part found for entity");
    let mut v1_partmap = v1_partmap
                .downcast_mut::<PartMap<Vector1>>().unwrap();

    let mut v2_partmap = entities.get_map_mut::<Vector2>()
                .expect("No part found for entity");
    let mut v2_partmap = v2_partmap
                .downcast_mut::<PartMap<Vector2>>().unwrap();

    // But not have a mutable borrow and immutable borrow to the same
    // Fails at runtime!
    // let v2_partmap = entities.get_map_ref::<Vector2>().unwrap();
    // But you can have multiple immutable references to the same part
    for id in active_ents {
        if entities.entity_contains::<Vector1>(*id) &&
           entities.entity_contains::<Vector2>(*id) {
            let v1_part = v1_partmap.get_part_mut(*id).unwrap();
            let v2_part = v2_partmap.get_part_mut(*id).unwrap();
            v1_part.x = 42;
            assert_ne!(v1_part.x, 43);
            assert_eq!(v1_part.x, 42);
        }
    }
}
other_system(&[0, 1, 2], &mut entities);

Structs

Entities	This is the root of the ECS implementation
PartMap	PartMap is a container type for the parts used by entities

Constants

EMPTY

Bitmask used to fill the initial mask list, and replace deleted entities