4 files changed, 224 insertions, 0 deletions

diff --git a/ecs/examples/with_single.rs b/ecs/examples/with_single.rs
new file mode 100644
index 0000000..25b73d1
--- /dev/null
+++ b/ecs/examples/with_single.rs
@@ -0,0 +1,71 @@
+use ecs::component::single::Single;
+use ecs::event::{Event, Id as EventId};
+use ecs::{Component, Query, World};
+
+#[derive(Component)]
+struct Ammo
+{
+    ammo_left: u32,
+}
+
+#[derive(Component, Default)]
+struct AmmoCounter
+{
+    counter: u32,
+}
+
+fn count_ammo(query: Query<(Ammo,)>, mut ammo_counter: Single<AmmoCounter>)
+{
+    for (ammo,) in &query {
+        println!("Found {} ammo", ammo.ammo_left);
+
+        ammo_counter.counter += ammo.ammo_left;
+    }
+}
+
+fn print_total_ammo_count(ammo_counter: Single<AmmoCounter>)
+{
+    println!("Total ammo count: {}", ammo_counter.counter);
+
+    assert_eq!(ammo_counter.counter, 19);
+}
+
+#[derive(Debug)]
+struct EventA;
+
+impl Event for EventA
+{
+    fn id(&self) -> EventId
+    {
+        EventId::of::<Self>()
+    }
+}
+
+#[derive(Debug)]
+struct EventB;
+
+impl Event for EventB
+{
+    fn id(&self) -> EventId
+    {
+        EventId::of::<Self>()
+    }
+}
+
+fn main()
+{
+    let mut world = World::new();
+
+    world.register_system(&EventA, count_ammo);
+    world.register_system(&EventB, print_total_ammo_count);
+
+    world.create_entity((Ammo { ammo_left: 4 },));
+    world.create_entity((Ammo { ammo_left: 7 },));
+    world.create_entity((Ammo { ammo_left: 8 },));
+
+    world.add_single_component(AmmoCounter::default()).unwrap();
+
+    world.emit(&EventA);
+
+    world.emit(&EventB);
+}
diff --git a/ecs/src/component.rs b/ecs/src/component.rs
index 7a61f39..0e5f020 100644
--- a/ecs/src/component.rs
+++ b/ecs/src/component.rs
@@ -8,6 +8,7 @@ use crate::system::{ComponentRefMut, Input as SystemInput};
 use crate::type_name::TypeName;
 
 pub mod local;
+pub mod single;
 
 pub trait Component: SystemInput + Any + TypeName
 {
diff --git a/ecs/src/component/single.rs b/ecs/src/component/single.rs
new file mode 100644
index 0000000..a63dbe3
--- /dev/null
+++ b/ecs/src/component/single.rs
@@ -0,0 +1,98 @@
+use std::any::{type_name, Any, TypeId};
+use std::ops::{Deref, DerefMut};
+
+use crate::component::Component;
+use crate::system::{ComponentRefMut, NoInitParamFlag, Param as SystemParam, System};
+use crate::tuple::FilterExclude as TupleFilterExclude;
+use crate::WorldData;
+
+/// Holds a component which has a single instance and is shared globally.
+#[derive(Debug)]
+pub struct Single<'world, SingleComponent: Component>
+{
+    single_component: ComponentRefMut<'world, SingleComponent>,
+}
+
+unsafe impl<'world, SingleComponent> SystemParam<'world>
+    for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    type Flags = NoInitParamFlag;
+    type Input = TupleFilterExclude;
+
+    fn initialize<SystemImpl>(
+        _system: &mut impl System<'world, SystemImpl>,
+        _input: Self::Input,
+    )
+    {
+    }
+
+    fn new<SystemImpl>(
+        _system: &'world impl System<'world, SystemImpl>,
+        world_data: &'world WorldData,
+    ) -> Self
+    {
+        let single_component = world_data
+            .single_component_storage
+            .get::<SingleComponent>()
+            .expect("Single component was not found in world")
+            .write_nonblock()
+            .unwrap_or_else(|_| {
+                panic!(
+                    "Failed to aquire read-write lock to single component {}",
+                    type_name::<SingleComponent>()
+                )
+            });
+
+        Self {
+            single_component: ComponentRefMut::new(single_component),
+        }
+    }
+
+    fn is_compatible<Other: SystemParam<'world>>() -> bool
+    {
+        let other_comparable = Other::get_comparable();
+
+        let Some(comparable) = other_comparable.downcast_ref::<Comparable>() else {
+            // The other system param is not Single
+            return true;
+        };
+
+        TypeId::of::<SingleComponent>() != comparable.single_component_type_id
+    }
+
+    fn get_comparable() -> Box<dyn Any>
+    {
+        Box::new(Comparable {
+            single_component_type_id: TypeId::of::<SingleComponent>(),
+        })
+    }
+}
+
+impl<'world, SingleComponent> Deref for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    type Target = SingleComponent;
+
+    fn deref(&self) -> &Self::Target
+    {
+        &self.single_component
+    }
+}
+
+impl<'world, SingleComponent> DerefMut for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target
+    {
+        &mut self.single_component
+    }
+}
+
+struct Comparable
+{
+    single_component_type_id: TypeId,
+}
diff --git a/ecs/src/lib.rs b/ecs/src/lib.rs
index 33d981b..ce1a7d1 100644
--- a/ecs/src/lib.rs
+++ b/ecs/src/lib.rs
@@ -89,6 +89,20 @@ impl World
             });
     }
 
+    /// Adds a single component. This component will be globally shared.
+    ///
+    /// # Errors
+    /// Returns `Err` if this component has already been added as a single component.
+    pub fn add_single_component<SingleComponent>(
+        &mut self,
+        single_component: SingleComponent,
+    ) -> Result<(), SingleComponentAlreadyExistsError>
+    where
+        SingleComponent: Component,
+    {
+        self.data.single_component_storage.insert(single_component)
+    }
+
     pub fn register_system<'this, SystemImpl>(
         &'this mut self,
         event: &impl Event,
@@ -206,6 +220,7 @@ pub struct WorldData
 {
     events: HashMap<EventId, Vec<usize>>,
     component_storage: Arc<Lock<ComponentStorage>>,
+    single_component_storage: SingleComponentStorage,
     action_queue: Arc<Lock<ActionQueue>>,
 }
 
@@ -324,3 +339,42 @@ impl Drop for ComponentStorage
         }
     }
 }
+
+#[derive(Debug, thiserror::Error)]
+#[error("Single component {0} already exists")]
+pub struct SingleComponentAlreadyExistsError(pub &'static str);
+
+#[derive(Debug, Default)]
+struct SingleComponentStorage
+{
+    storage: HashMap<TypeId, Lock<Box<dyn Component>>>,
+}
+
+impl SingleComponentStorage
+{
+    fn get<SingleComponent: Component>(&self) -> Option<&Lock<Box<dyn Component>>>
+    {
+        self.storage.get(&TypeId::of::<SingleComponent>())
+    }
+
+    fn insert<SingleComponent: Component>(
+        &mut self,
+        single_component: SingleComponent,
+    ) -> Result<(), SingleComponentAlreadyExistsError>
+    {
+        let single_component_type_id = TypeId::of::<SingleComponent>();
+
+        if self.storage.contains_key(&single_component_type_id) {
+            return Err(SingleComponentAlreadyExistsError(type_name::<
+                SingleComponent,
+            >()));
+        }
+
+        self.storage.insert(
+            single_component_type_id,
+            Lock::new(Box::new(single_component)),
+        );
+
+        Ok(())
+    }
+}