termite 0.1.0-alpha

A simple modern Entity Component System
Documentation 
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use std::borrow::Cow;

use crate::{
    Access, ComponentId, SystemParam, SystemParamFetch, SystemParamItem, SystemParamState, World,
    WorldId, MAX_CHANGE_AGE,
};

pub type BoxedSystem<In, Out> = Box<dyn System<In = In, Out = Out>>;

pub struct SystemMeta {
    pub name: Cow<'static, str>,
    pub access: Access<ComponentId>,
    pub last_change_tick: u32,
}

impl SystemMeta {
    #[inline]
    pub fn new<T>() -> Self {
        Self {
            name: std::any::type_name::<T>().into(),
            access: Access::default(),
            last_change_tick: 0,
        }
    }

    #[inline]
    pub fn name(&self) -> &str {
        &self.name
    }
}

pub struct SystemState<Param: SystemParam + 'static> {
    meta: SystemMeta,
    param_state: <Param as SystemParam>::Fetch,
    world_id: WorldId,
}

impl<Param: SystemParam + 'static> SystemState<Param> {
    #[inline]
    pub fn new(world: &mut World) -> Self {
        let mut meta = SystemMeta::new::<Param>();
        meta.last_change_tick = world.change_tick().wrapping_sub(MAX_CHANGE_AGE);
        let param_state = <Param::Fetch as SystemParamState>::init(world, &mut meta);
        let world_id = world.id();

        Self {
            meta,
            param_state,
            world_id,
        }
    }

    #[inline]
    pub fn meta(&self) -> &SystemMeta {
        &self.meta
    }

    #[inline]
    pub fn world_id(&self) -> WorldId {
        self.world_id
    }

    #[inline]
    pub fn get<'w, 's>(&'s mut self, world: &'w World) -> SystemParamItem<'w, 's, Param> {
        self.validate_world(world);

        unsafe { self.get_unchecked_manual(world) }
    }

    #[inline]
    pub fn apply(&mut self, world: &mut World) {
        self.param_state.apply(world);
    }

    #[inline]
    pub fn matches_world(&self, world: &World) -> bool {
        self.world_id == world.id()
    }

    #[inline]
    fn validate_world(&self, world: &World) {
        if self.matches_world(world) {
            panic!("System state was created for a different world");
        }
    }

    #[inline]
    pub unsafe fn get_unchecked_manual<'w, 's>(
        &'s mut self,
        world: &'w World,
    ) -> <Param::Fetch as SystemParamFetch<'w, 's>>::Item {
        let change_tick = world.increment_change_tick();
        let param = unsafe {
            <Param::Fetch as SystemParamFetch>::get_param(
                &mut self.param_state,
                &self.meta,
                world,
                change_tick,
            )
        };
        self.meta.last_change_tick = change_tick;
        param
    }
}

pub trait System: Send + Sync + 'static {
    type In;
    type Out;

    fn meta(&self) -> &SystemMeta;

    /// # Safety
    /// - `SystemMeta::access` must not be modified
    unsafe fn meta_mut(&mut self) -> &mut SystemMeta;

    fn init(&mut self, _world: &mut World) {}

    /// # Safety
    /// - `world` must be the same world that was used to [`System::init`] this system.
    /// - This system must not run at the same time as another system that has conflicting [`Access`].
    ///   Use [`System::meta`] to guarantee this.
    unsafe fn run(&mut self, input: Self::In, world: &World) -> Self::Out;

    fn apply(&mut self, _world: &mut World) {}

    fn check_change_tick(&mut self, change_tick: u32) {
        // SAFETY: `SystemMeta::access` is not modified
        let meta = unsafe { self.meta_mut() };

        let age = change_tick.wrapping_sub(meta.last_change_tick);

        if age > MAX_CHANGE_AGE {
            meta.last_change_tick = change_tick.wrapping_sub(MAX_CHANGE_AGE);
        }
    }

    fn set_last_change_tick(&mut self, last_change_tick: u32) {
        // SAFETY: `SystemMeta::access` is not modified
        let meta = unsafe { self.meta_mut() };

        meta.last_change_tick = last_change_tick;
    }
}