entity-trait-system 1.0.1

An alternative to ECS
Documentation 
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# entity-trait-system

[![Crates.io](https://img.shields.io/crates/v/entity-trait-system.svg)](https://crates.io/crates/entity-trait-system)
[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)

An alternative to [ECS](https://en.wikipedia.org/wiki/Entity_component_system).
Here is a [video](https://youtu.be/AezHJdwDfW0) summary.

## Setup

Requires the nightly compiler and
[specialization](https://std-dev-guide.rust-lang.org/policy/specialization.html)
feature - it's only used in a sound way.

```bash
rustup override set nightly # enable nightly compiler
cargo add entity-trait-system # get lib
```

```rs
// explicitly opt in to language feature
#![allow(incomplete_features)]
#![feature(specialization)]

// declare world, entities, and traits which the entities could have
entity_trait_system::world!(MyWorld, Enemy, Player; TestTrait, SecondTestTrait);

let mut world = MyWorld::default();
// directly access arena member
let player_id = world.player.insert(Player { id: 1 });
// compile time type accessor of arena member (similar)
world.arena_mut::<Enemy>().insert(Enemy { hp: 10 });

// visit all arenas with types that implement trait (likely static dispatch)
#[cfg(feature = "rayon")]
world.par_visit_test_trait(|e| e.do_something());
#[cfg(not(feature = "rayon"))]
world.visit_test_trait(|e| e.do_something());

// runtime type API - access type-erased (Any) arena
let arena_id = MyWorld::arena_id::<Player>();
let player_arena = world.any_arena_mut(arena_id);
// unwrap: I know that this is a player and that the reference is valid
let player = player_arena
    .get_mut(player_id).unwrap()
    .downcast_mut::<Player>().unwrap();
player.do_something_else();
```

## API Overview

### Per-Trait Methods (generated for each trait)
- `visit_<trait>` - Immutable iteration over implementing entities
- `visit_mut_<trait>` - Mutable iteration over implementing entities
- `visit_key_<trait>` - Immutable iteration with `(Key, &Value)` tuples
- `visit_key_mut_<trait>` - Mutable iteration with `(Key, &mut Value)` tuples
- `retain_<trait>` - Keep entities matching predicate
- `retain_with_default_<trait>` - Keep with control over non-implementing types
- `diff_<trait>` - Gather diff vector from immutable view, to apply later
- `diff_mut_<trait>` - Same as previous, but from mutable view
- `diff_apply_<trait>` - Apply diff vector
- `clear_<trait>` - Clear all arenas implementing trait
- `len_<trait>` - Count entities implementing trait
- `any_arenas_<trait>` - Get array of type-erased arenas implementing trait
- `any_arenas_mut_<trait>` - Mutable version of above

### World Methods
- `arena<T>()` / `arena_mut<T>()` - Compile-time typed arena access
- `any_arena()` / `any_arena_mut()` - Runtime type-erased access
- `arena_id<T>()` - Get stable arena identifier - serializes to type name.
- `clear()` - Clear all arenas
- `len()` - Total entity count across all arenas

## Rayon Support
Parallel operations exposed via `par_*` variants.

## Serde Support
Both map (serde_json) and seq (bincode) style ser/deserialization.

## Performance Note

This can be found in the implementation of `visit_*`:

```rust
fn v_if_applicable<F>(arena: &DenseSlotMap<DefaultKey, T>, mut handler: F)
where F: FnMut(&dyn $trait_name) {
    arena.values_as_slice().iter()
        .for_each(|entity| {
            handler(entity) // implicit type erase T -> &dyn $trait_name
        });
}
```

The handler is typically inlined and devirtualized to erase dynamic dispatch,
since the type is known at compile time and is type erased just before use.
This means that static dispatch is reliant on compiler optimization; likely but not guaranteed.

License: MIT