From 5feeaf154a8b729873c729b4488f28536cf4ae24 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Sat, 21 Dec 2024 14:43:53 +0100
Subject: feat(ecs): add support for entities without components

---
 ecs/src/actions.rs   |  8 ++++++++
 ecs/src/archetype.rs | 22 ++++++++++------------
 ecs/src/component.rs | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+), 12 deletions(-)

(limited to 'ecs')

diff --git a/ecs/src/actions.rs b/ecs/src/actions.rs
index babd5a2..c062472 100644
--- a/ecs/src/actions.rs
+++ b/ecs/src/actions.rs
@@ -44,6 +44,10 @@ impl<'world> Actions<'world>
     {
         debug_assert_eq!(entity_uid.kind(), UidKind::Entity);
 
+        if Comps::metadata().len() == 0 {
+            return;
+        }
+
         self.action_queue.push(Action::AddComponents(
             entity_uid,
             components.into_vec(),
@@ -58,6 +62,10 @@ impl<'world> Actions<'world>
     {
         debug_assert_eq!(entity_uid.kind(), UidKind::Entity);
 
+        if Comps::metadata().len() == 0 {
+            return;
+        }
+
         self.action_queue.push(Action::RemoveComponents(
             entity_uid,
             Comps::metadata(),
diff --git a/ecs/src/archetype.rs b/ecs/src/archetype.rs
index d2ee36a..354d206 100644
--- a/ecs/src/archetype.rs
+++ b/ecs/src/archetype.rs
@@ -4,7 +4,6 @@ use crate::component::{
     IsOptional as ComponentIsOptional,
     Metadata as ComponentMetadata,
 };
-use crate::uid::Uid;
 
 /// Archetype ID.
 #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
@@ -19,10 +18,9 @@ impl Id
         components_metadata: &impl AsRef<[ComponentMetadata]>,
     ) -> Self
     {
-        debug_assert!(
-            components_metadata.as_ref().len() > 0,
-            "Cannot create a archetype ID from a empty component metadata list"
-        );
+        if components_metadata.as_ref().len() == 0 {
+            return Self { hash: 0 };
+        }
 
         debug_assert!(
             components_metadata
@@ -31,7 +29,7 @@ impl Id
             "Cannot create archetype ID from a unsorted component metadata list"
         );
 
-        Self::new(
+        let component_ids =
             components_metadata
                 .as_ref()
                 .iter()
@@ -41,13 +39,8 @@ impl Id
                     }
 
                     Some(component_metadata.id)
-                }),
-        )
-    }
+                });
 
-    /// Returns the ID of a archetype with the given components.
-    fn new(component_ids: impl IntoIterator<Item = Uid>) -> Self
-    {
         let mut hasher = DefaultHasher::new();
 
         for component_id in component_ids {
@@ -56,6 +49,11 @@ impl Id
 
         let hash = hasher.finish();
 
+        assert_ne!(
+            hash, 0,
+            "Archetype ID 0 is reserved for a archetype with zero components"
+        );
+
         Self { hash }
     }
 }
diff --git a/ecs/src/component.rs b/ecs/src/component.rs
index 85c556f..1fde5b4 100644
--- a/ecs/src/component.rs
+++ b/ecs/src/component.rs
@@ -351,3 +351,51 @@ macro_rules! inner {
 seq!(C in 0..=64 {
     inner!(C);
 });
+
+impl Sequence for ()
+{
+    type MutRefs<'component> = ();
+    type Refs<'component> = ();
+
+    fn into_vec(self) -> Vec<Box<dyn Component>>
+    {
+        Vec::new()
+    }
+
+    fn metadata() -> Vec<Metadata>
+    {
+        Vec::new()
+    }
+
+    fn added_event_ids() -> Vec<Uid>
+    {
+        Vec::new()
+    }
+
+    fn removed_event_ids() -> Vec<Uid>
+    {
+        Vec::new()
+    }
+
+    #[inline]
+    fn from_components_mut<'component>(
+        _components: &'component [EntityComponent],
+        _component_index_lookup: impl Fn(Uid) -> Option<usize>,
+        _world: &'component World,
+        _lock_component: fn(&EntityComponent) -> WriteGuard<'_, Box<dyn Component>>,
+    ) -> Self::MutRefs<'component>
+    {
+        ()
+    }
+
+    #[inline]
+    fn from_components<'component>(
+        _components: &'component [EntityComponent],
+        _component_index_lookup: impl Fn(Uid) -> Option<usize>,
+        _world: &'component World,
+        _lock_component: fn(&EntityComponent) -> ReadGuard<'_, Box<dyn Component>>,
+    ) -> Self::Refs<'component>
+    {
+        ()
+    }
+}
-- 
cgit v1.2.3-18-g5258