1 files changed, 134 insertions, 0 deletions
diff --git a/ecs/src/sole.rs b/ecs/src/sole.rs
new file mode 100644
index 0000000..e5166e9
--- /dev/null
+++ b/ecs/src/sole.rs
@@ -0,0 +1,134 @@
+use std::any::{type_name, Any, TypeId};
+use std::fmt::Debug;
+use std::marker::PhantomData;
+use std::ops::{Deref, DerefMut};
+
+use crate::lock::WriteGuard;
+use crate::system::{NoInitParamFlag, Param as SystemParam, System};
+use crate::tuple::FilterExclude as TupleFilterExclude;
+use crate::type_name::TypeName;
+use crate::WorldData;
+
+/// A type which has a single instance and is shared globally.
+pub trait Sole: Any + TypeName
+{
+    fn as_any_mut(&mut self) -> &mut dyn Any;
+
+    fn as_any(&self) -> &dyn Any;
+}
+
+impl dyn Sole
+{
+    pub fn downcast_mut<Real: 'static>(&mut self) -> Option<&mut Real>
+    {
+        self.as_any_mut().downcast_mut()
+    }
+
+    pub fn downcast_ref<Real: 'static>(&self) -> Option<&Real>
+    {
+        self.as_any().downcast_ref()
+    }
+}
+
+impl Debug for dyn Sole
+{
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result
+    {
+        formatter.debug_struct("Sole").finish_non_exhaustive()
+    }
+}
+
+impl TypeName for Box<dyn Sole>
+{
+    fn type_name(&self) -> &'static str
+    {
+        self.as_ref().type_name()
+    }
+}
+
+/// Holds a reference to a globally shared singleton value.
+#[derive(Debug)]
+pub struct Single<'world, SoleT: Sole>
+{
+    sole: WriteGuard<'world, Box<dyn Sole>>,
+    _ph: PhantomData<SoleT>,
+}
+
+unsafe impl<'world, SoleT> SystemParam<'world> for Single<'world, SoleT>
+where
+    SoleT: Sole,
+{
+    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 sole = world_data
+            .sole_storage
+            .get::<SoleT>()
+            .expect("Sole was not found in world")
+            .write_nonblock()
+            .unwrap_or_else(|_| {
+                panic!(
+                    "Failed to aquire read-write lock to single component {}",
+                    type_name::<SoleT>()
+                )
+            });
+
+        Self { sole, _ph: PhantomData }
+    }
+
+    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::<SoleT>() != comparable.sole_type_id
+    }
+
+    fn get_comparable() -> Box<dyn Any>
+    {
+        Box::new(Comparable { sole_type_id: TypeId::of::<SoleT>() })
+    }
+}
+
+impl<'world, SoleT> Deref for Single<'world, SoleT>
+where
+    SoleT: Sole,
+{
+    type Target = SoleT;
+
+    fn deref(&self) -> &Self::Target
+    {
+        self.sole.downcast_ref().unwrap()
+    }
+}
+
+impl<'world, SoleT> DerefMut for Single<'world, SoleT>
+where
+    SoleT: Sole,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target
+    {
+        self.sole.downcast_mut().unwrap()
+    }
+}
+
+struct Comparable
+{
+    sole_type_id: TypeId,
+}