3 files changed, 678 insertions, 0 deletions
diff --git a/engine-ecs/src/system/initializable.rs b/engine-ecs/src/system/initializable.rs
new file mode 100644
index 0000000..b6ec8e8
--- /dev/null
+++ b/engine-ecs/src/system/initializable.rs
@@ -0,0 +1,131 @@
+use std::marker::PhantomData;
+
+use seq_macro::seq;
+
+use crate::system::{Input, Param as SystemParam, System};
+use crate::tuple::{Reduce as TupleReduce, ReduceElement as TupleReduceElement, Tuple};
+
+/// A initializable system.
+pub trait Initializable<'world, Impl>: System<'world, Impl>
+{
+    type Inputs;
+
+    #[must_use]
+    fn initialize(self, inputs: Self::Inputs) -> Self;
+}
+
+pub trait Param<'world, SystemT>: SystemParam<'world>
+{
+    fn initialize(system: &mut SystemT, input: Self::Input);
+}
+
+pub struct ParamTupleFilter<'world, SystemT>
+{
+    _pd: PhantomData<(&'world (), SystemT)>,
+}
+
+impl<'world, SystemT, ParamT, Accumulator>
+    TupleReduceElement<Accumulator, ParamTupleFilter<'world, SystemT>> for ParamT
+where
+    ParamT: SystemParam<
+        'world,
+        Input: AppendInitializableParam<'world, Accumulator, ParamT, SystemT>,
+    >,
+    Accumulator: Tuple,
+{
+    type Return = <ParamT::Input as AppendInitializableParam<
+        'world,
+        Accumulator,
+        ParamT,
+        SystemT,
+    >>::Return;
+}
+
+pub trait AppendInitializableParam<'world, Accumulator, ParamT, SystemT>
+{
+    type Return;
+}
+
+impl<'world, InputT, ParamT, Accumulator, SystemT>
+    AppendInitializableParam<'world, Accumulator, ParamT, SystemT> for InputT
+where
+    InputT: Input,
+    Accumulator: Tuple,
+    ParamT: Param<'world, SystemT>,
+{
+    type Return = Accumulator::WithElementAtEnd<ParamT>;
+}
+
+impl<ParamT, Accumulator, SystemT>
+    AppendInitializableParam<'_, Accumulator, ParamT, SystemT> for ()
+where
+    Accumulator: Tuple,
+{
+    type Return = Accumulator;
+}
+
+pub trait ParamTuple<'world, SystemT>
+{
+    type Inputs;
+
+    fn initialize_all(system: &mut SystemT, inputs: Self::Inputs);
+}
+
+macro_rules! impl_initializable_param_tuple {
+    ($c: tt) => {
+        seq!(I in 0..$c {
+            impl<'world, SystemT, #(Param~I,)*> ParamTuple<'world, SystemT>
+                for (#(Param~I,)*)
+            where
+                #(Param~I: Param<'world, SystemT>,)*
+            {
+                type Inputs = (#(Param~I::Input,)*);
+
+                fn initialize_all(
+                    system: &mut SystemT,
+                    inputs: Self::Inputs,
+                ) {
+                    #(
+                        <Param~I as Param<'world, SystemT>>::initialize(
+                            system,
+                            inputs.I
+                        );
+                    )*
+                }
+            }
+        });
+    };
+}
+
+seq!(C in 1..16 {
+    impl_initializable_param_tuple!(C);
+});
+
+impl<SystemT> ParamTuple<'_, SystemT> for ()
+{
+    type Inputs = ();
+
+    fn initialize_all(_system: &mut SystemT, _inputs: Self::Inputs) {}
+}
+
+/// A tuple of system parameters that may or may not be initializable.
+pub trait MaybeInitializableParamTuple<'world, SystemT>
+{
+    /// A tuple of the inputs of the initializable system parameters in this tuple.
+    type Inputs;
+
+    fn init_initializable(system: &mut SystemT, inputs: Self::Inputs);
+}
+
+impl<'world, SystemT, Params> MaybeInitializableParamTuple<'world, SystemT> for Params
+where
+    Params:
+        TupleReduce<ParamTupleFilter<'world, SystemT>, Out: ParamTuple<'world, SystemT>>,
+{
+    type Inputs = <Params::Out as ParamTuple<'world, SystemT>>::Inputs;
+
+    fn init_initializable(system: &mut SystemT, inputs: Self::Inputs)
+    {
+        Params::Out::initialize_all(system, inputs);
+    }
+}
diff --git a/engine-ecs/src/system/observer.rs b/engine-ecs/src/system/observer.rs
new file mode 100644
index 0000000..1ad7496
--- /dev/null
+++ b/engine-ecs/src/system/observer.rs
@@ -0,0 +1,278 @@
+use std::any::type_name;
+use std::fmt::Debug;
+use std::marker::PhantomData;
+use std::mem::transmute;
+use std::slice::Iter as SliceIter;
+
+use seq_macro::seq;
+
+use crate::entity::Handle as EntityHandle;
+use crate::error::Error;
+use crate::event::Emitted as EmittedEvent;
+use crate::pair::Pair;
+use crate::system::{
+    Metadata,
+    NoCallbacks,
+    Param,
+    ReturnValue as SystemReturnValue,
+    System,
+    TypeErased as TypeErasedSystem,
+};
+use crate::uid::Uid;
+use crate::util::Array;
+use crate::{Component, World};
+
+pub trait Observed
+{
+    type Events: Array<Pair<Uid, Uid>>;
+
+    fn events() -> Self::Events;
+}
+
+impl<Relation, Target> Observed for Pair<Relation, Target>
+where
+    Relation: Component,
+    Target: Component,
+{
+    type Events = [Pair<Uid, Uid>; 1];
+
+    fn events() -> Self::Events
+    {
+        [Pair::builder()
+            .relation::<Relation>()
+            .target::<Target>()
+            .build()]
+    }
+}
+
+/// Observer system.
+pub trait Observer<'world, Impl>: System<'world, Impl>
+{
+    type ObservedEvents: Array<Pair<Uid, Uid>>;
+
+    fn observed_events() -> Self::ObservedEvents;
+
+    fn finish_observer(self) -> (WrapperComponent, Self::Callbacks);
+}
+
+pub struct Observe<'world, ObservedT: Observed>
+{
+    _pd: PhantomData<ObservedT>,
+    world: &'world World,
+    emitted_event: EmittedEvent<'world>,
+}
+
+impl<'world, ObservedT: Observed> Observe<'world, ObservedT>
+{
+    pub fn new(world: &'world World, emitted_event: EmittedEvent<'world>) -> Self
+    {
+        Self {
+            _pd: PhantomData,
+            world,
+            emitted_event,
+        }
+    }
+
+    #[must_use]
+    pub fn event(&self) -> Uid
+    {
+        self.emitted_event.event
+    }
+}
+
+impl<ObservedT: Observed> Observe<'_, ObservedT>
+{
+    #[must_use]
+    pub fn iter(&self) -> ObserveIter<'_, ObservedT>
+    {
+        ObserveIter {
+            world: self.world,
+            inner: self.emitted_event.match_ids.iter(),
+            _pd: PhantomData,
+        }
+    }
+}
+
+impl<'a, ObservedT: Observed> IntoIterator for &'a Observe<'_, ObservedT>
+{
+    type IntoIter = ObserveIter<'a, ObservedT>;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    fn into_iter(self) -> Self::IntoIter
+    {
+        self.iter()
+    }
+}
+
+pub struct ObserveIter<'observe, ObservedT: Observed>
+{
+    world: &'observe World,
+    inner: SliceIter<'observe, Uid>,
+    _pd: PhantomData<ObservedT>,
+}
+
+impl<'observe, ObservedT: Observed> Iterator for ObserveIter<'observe, ObservedT>
+{
+    type Item = EventMatch<'observe, ObservedT>;
+
+    fn next(&mut self) -> Option<Self::Item>
+    {
+        let match_id = *self.inner.next()?;
+
+        Some(EventMatch {
+            world: self.world,
+            id: match_id,
+            _pd: PhantomData,
+        })
+    }
+}
+
+/// A event match.
+#[derive(Debug)]
+pub struct EventMatch<'world, ObservedT: Observed>
+{
+    world: &'world World,
+    id: Uid,
+    _pd: PhantomData<ObservedT>,
+}
+
+impl<'world, ObservedT: Observed> EventMatch<'world, ObservedT>
+{
+    #[must_use]
+    pub fn entity_id(&self) -> Uid
+    {
+        self.id
+    }
+
+    /// Attempts to get the entity with the id of this match.
+    #[must_use]
+    pub fn try_get_entity(&self) -> Option<EntityHandle<'world>>
+    {
+        self.world.get_entity(self.id)
+    }
+}
+
+macro_rules! impl_observer {
+    ($c: tt) => {
+        seq!(I in 0..$c {
+            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> System<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Func
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: SystemReturnValue,
+                #(TParam~I: Param<'world, Input = ()>,)*
+            {
+                type Callbacks = NoCallbacks;
+
+                fn finish(self) -> (TypeErasedSystem, NoCallbacks)
+                {
+                    const {
+                        panic!("Observers cannot be used as regular systems");
+                    }
+                }
+            }
+
+            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> Observer<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Func
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: SystemReturnValue,
+                #(TParam~I: Param<'world, Input = ()>,)*
+            {
+                type ObservedEvents = ObservedT::Events;
+
+                fn observed_events() -> Self::ObservedEvents
+                {
+                    ObservedT::events()
+                }
+
+                fn finish_observer(self) -> (WrapperComponent, NoCallbacks)
+                {
+                    #[allow(unused)]
+
+                    let wrapper_comp = WrapperComponent::new(
+                        move |world, metadata, emitted_event| {
+                            // SAFETY: The caller of TypeErased::run ensures the lifetime
+                            // is correct
+                            let world = unsafe { &*std::ptr::from_ref(world) };
+
+                            // SAFETY: The caller of TypeErased::run ensures the lifetime
+                            // is correct
+                            let emitted_event = unsafe {
+                                transmute::<EmittedEvent<'_>, EmittedEvent<'_>>(
+                                    emitted_event
+                                )
+                            };
+
+                            self(Observe::new(world, emitted_event), #({
+                                TParam~I::new(world, &metadata)
+                            },)*).into_result()
+                        },
+                        type_name::<Func>()
+                    );
+
+                    (wrapper_comp, NoCallbacks)
+                }
+            }
+        });
+    };
+}
+
+seq!(C in 0..16 {
+    impl_observer!(C);
+});
+
+#[derive(Component)]
+pub struct WrapperComponent
+{
+    run: Box<RunFn>,
+    name: &'static str,
+}
+
+impl WrapperComponent
+{
+    pub fn new(
+        run: impl Fn(&World, Metadata, EmittedEvent<'_>) -> Result<(), Error> + 'static,
+        name: &'static str,
+    ) -> Self
+    {
+        Self { run: Box::new(run), name }
+    }
+
+    /// Runs the observer system.
+    ///
+    /// # Safety
+    /// `world` must live at least as long as the [`World`] the system belongs to.
+    pub unsafe fn run(
+        &self,
+        world: &World,
+        metadata: Metadata,
+        emitted_event: EmittedEvent<'_>,
+    ) -> Result<(), Error>
+    {
+        (self.run)(world, metadata, emitted_event)
+    }
+
+    pub fn name(&self) -> &'static str
+    {
+        self.name
+    }
+}
+
+impl Debug for WrapperComponent
+{
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result
+    {
+        formatter
+            .debug_struct("WrapperComponent")
+            .finish_non_exhaustive()
+    }
+}
+
+type RunFn = dyn Fn(&World, Metadata, EmittedEvent<'_>) -> Result<(), Error>;
diff --git a/engine-ecs/src/system/stateful.rs b/engine-ecs/src/system/stateful.rs
new file mode 100644
index 0000000..3e0076a
--- /dev/null
+++ b/engine-ecs/src/system/stateful.rs
@@ -0,0 +1,269 @@
+use std::any::type_name;
+use std::mem::transmute;
+
+use seq_macro::seq;
+
+use crate::World;
+use crate::component::Parts as ComponentParts;
+use crate::component::local::SystemWithLocalComponents;
+use crate::event::Emitted as EmittedEvent;
+use crate::system::initializable::{Initializable, MaybeInitializableParamTuple};
+use crate::system::observer::{
+    Observe,
+    Observed,
+    Observer,
+    WrapperComponent as ObserverWrapperComponent,
+};
+use crate::system::{
+    Into as IntoSystem,
+    Metadata,
+    Param,
+    ReturnValue,
+    System,
+    TypeErased,
+};
+
+/// A stateful system.
+pub struct Stateful<Func>
+{
+    func: Func,
+    local_components: Vec<ComponentParts>,
+}
+
+macro_rules! impl_system {
+    ($c: tt) => {
+        seq!(I in 0..$c {
+            impl<'world, Func, Ret, #(TParam~I,)*>
+                System<'world, fn(#(TParam~I,)*) -> Ret> for Stateful<Func>
+            where
+                Func: Fn(#(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world, Input: 'static>,)*
+            {
+                type Callbacks = Callbacks;
+
+                fn finish(self) -> (TypeErased, Self::Callbacks)
+                {
+                    let Self { func, local_components } = self;
+
+                    let callbacks = Callbacks { local_components };
+
+                    let type_erased = TypeErased {
+                        run: Box::new(move |world, metadata| {
+                            // SAFETY: The caller of TypeErased::run ensures the lifetime
+                            // is correct
+                            let world = unsafe { &*std::ptr::from_ref(world) };
+
+                            func(#({
+                                TParam~I::new(&world, &metadata)
+                            },)*);
+
+                            Ok(())
+                        }),
+                        name: type_name::<Func>()
+                    };
+
+
+                    (type_erased, callbacks)
+                }
+            }
+
+            impl<'world, Func, Ret, #(TParam~I,)*>
+                Initializable<'world, fn(#(TParam~I,)*) -> Ret> for Stateful<Func>
+            where
+                Func: Fn(#(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world, Input: 'static>,)*
+                (#(TParam~I,)*): MaybeInitializableParamTuple<'world, Self>
+            {
+                type Inputs = <
+                    (#(TParam~I,)*) as MaybeInitializableParamTuple<'world, Self>
+                >::Inputs;
+
+                fn initialize(mut self, inputs: Self::Inputs) -> Self
+                {
+                    init_initializable_params::<_, (#(TParam~I,)*)>(&mut self, inputs);
+
+                    self
+                }
+            }
+
+            impl<'world, Func, Ret, #(TParam~I,)*>
+                IntoSystem<'world, fn(#(TParam~I,)*) -> Ret> for Func
+            where
+                Func: Fn(#(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world>,)*
+            {
+                type System = Stateful<Func>;
+
+                fn into_system(self) -> Self::System
+                {
+                    Self::System {
+                        func: self,
+                        local_components: Vec::new(), // TODO: Use Vec::with_capacity
+                    }
+                }
+            }
+        });
+    };
+}
+
+seq!(C in 1..16 {
+    impl_system!(C);
+});
+
+impl<Func> SystemWithLocalComponents for Stateful<Func>
+{
+    fn add_local_component(&mut self, component_parts: ComponentParts)
+    {
+        self.local_components.push(component_parts);
+    }
+}
+
+#[derive(Debug)]
+pub struct Callbacks
+{
+    local_components: Vec<ComponentParts>,
+}
+
+impl crate::system::Callbacks for Callbacks
+{
+    fn on_created(&mut self, world: &mut World, metadata: Metadata)
+    {
+        for local_comp_parts in self.local_components.drain(..) {
+            world.add_component(metadata.ent_id, local_comp_parts);
+        }
+    }
+}
+
+fn init_initializable_params<'world, SystemT, Params>(
+    system: &mut SystemT,
+    inputs: Params::Inputs,
+) where
+    Params: MaybeInitializableParamTuple<'world, SystemT>,
+{
+    Params::init_initializable(system, inputs);
+}
+
+macro_rules! impl_observer {
+    ($c: tt) => {
+        seq!(I in 0..$c {
+            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> System<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Stateful<Func>
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world>,)*
+            {
+                type Callbacks = Callbacks;
+
+                fn finish(self) -> (TypeErased, Callbacks)
+                {
+                    unimplemented!();
+                }
+            }
+
+            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> Initializable<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Stateful<Func>
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world>,)*
+                (#(TParam~I,)*): MaybeInitializableParamTuple<'world, Self>
+            {
+                type Inputs = <
+                    (#(TParam~I,)*) as MaybeInitializableParamTuple<'world, Self>
+                >::Inputs;
+
+                fn initialize(mut self, inputs: Self::Inputs) -> Self
+                {
+                    init_initializable_params::<_, (#(TParam~I,)*)>(&mut self, inputs);
+
+                    self
+                }
+            }
+
+            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> Observer<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Stateful<Func>
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world>,)*
+            {
+                type ObservedEvents = ObservedT::Events;
+
+                fn observed_events() -> Self::ObservedEvents
+                {
+                    ObservedT::events()
+                }
+
+                fn finish_observer(self) -> (ObserverWrapperComponent, Callbacks)
+                {
+                    #![allow(unused)]
+
+                    let Self { func, local_components } = self;
+
+                    let callbacks = Callbacks { local_components };
+
+                    let wrapper_comp = ObserverWrapperComponent::new(
+                        move |world, metadata, emitted_event| {
+                            // SAFETY: The caller of TypeErased::run ensures the lifetime
+                            // is correct
+                            let world = unsafe { &*std::ptr::from_ref(world) };
+
+                            // SAFETY: The caller of TypeErased::run ensures the lifetime
+                            // is correct
+                            let emitted_event = unsafe {
+                                transmute::<EmittedEvent<'_>, EmittedEvent<'_>>(
+                                    emitted_event
+                                )
+                            };
+
+                            func(Observe::new(world, emitted_event), #({
+                                TParam~I::new(world, &metadata)
+                            },)*).into_result()
+                        },
+                        type_name::<Func>()
+                    );
+
+                    (wrapper_comp, callbacks)
+                }
+            }
+
+            impl<'world, Func, Ret, ObservedT, #(TParam~I,)*> IntoSystem<
+                'world,
+                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
+            > for Func
+            where
+                ObservedT: Observed,
+                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + 'static,
+                Ret: ReturnValue,
+                #(TParam~I: Param<'world>,)*
+            {
+                type System = Stateful<Func>;
+
+                fn into_system(self) -> Stateful<Func>
+                {
+                    Stateful {
+                        func: self,
+                        local_components: Vec::new(), // TODO: Use Vec::with_capacity
+                    }
+                }
+            }
+        });
+    };
+}
+
+seq!(C in 0..16 {
+    impl_observer!(C);
+});