bevy/crates/bevy_ecs/src/system.rs

use crate::{Resources, World};
use std::borrow::Cow;
use fixedbitset::FixedBitSet;
use hecs::{Query, Access};

#[derive(Copy, Clone)]
pub enum ThreadLocalExecution {
    Immediate,
    NextFlush,
}

#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct SystemId(pub u32);

impl SystemId {
    pub fn new() -> Self {
        SystemId(rand::random::<u32>())
    }
}

pub trait System: Send + Sync {
    fn name(&self) -> Cow<'static, str>;
    fn id(&self) -> SystemId;
    fn update_archetype_access(&mut self, world: &World);
    fn get_archetype_access(&self) -> &ArchetypeAccess;
    fn thread_local_execution(&self) -> ThreadLocalExecution;
    fn run(&mut self, world: &World, resources: &Resources);
    fn run_thread_local(&mut self, world: &mut World, resources: &mut Resources);
    fn initialize(&mut self, _resources: &mut Resources) {}
}

// credit to Ratysz from the Yaks codebase
#[derive(Default)]
pub struct ArchetypeAccess {
    pub immutable: FixedBitSet,
    pub mutable: FixedBitSet,
}

impl ArchetypeAccess {
    pub fn is_compatible(&self, other: &ArchetypeAccess) -> bool {
        self.mutable.is_disjoint(&other.mutable)
            && self.mutable.is_disjoint(&other.immutable)
            && self.immutable.is_disjoint(&other.mutable)
    }

    pub fn union(&mut self, other: &ArchetypeAccess) {
        self.mutable.union_with(&other.mutable);
        self.immutable.union_with(&other.immutable);
    }

    pub fn set_access_for_query<Q>(&mut self, world: &World)
    where
        Q: Query,
    {
        self.immutable.clear();
        self.mutable.clear();
        let iterator = world.archetypes();
        let bits = iterator.len();
        self.immutable.grow(bits);
        self.mutable.grow(bits);
        iterator
            .enumerate()
            .filter_map(|(index, archetype)| archetype.access::<Q>().map(|access| (index, access)))
            .for_each(|(archetype, access)| match access {
                Access::Read => self.immutable.set(archetype, true),
                Access::Write => self.mutable.set(archetype, true),
                Access::Iterate => (),
            });
    }
}