Remove redundant table and sparse set component IDs from Archetype (#…

…4927)

# Objective
Archetype is a deceptively large type in memory. It stores metadata about which components are in which storage in multiple locations, which is only used when creating new Archetypes while moving entities.

## Solution
Remove the redundant `Box<[ComponentId]>`s and iterate over the sparse set of component metadata instead. Reduces Archetype's size by 4 usizes (32 bytes on 64-bit systems), as well as the additional allocations for holding these slices.

It'd seem like there's a downside that the origin archetype has it's component metadata iterated over twice when creating a new archetype, but this change also removes the extra `Vec<ArchetypeComponentId>` allocations when creating a new archetype which may amortize out to a net gain here. This change likely negatively impacts creating new archetypes with a large number of components, but that's a cost mitigated by the fact that these archetypal relationships are cached in Edges and is incurred only once for each edge created.

## Additional Context
There are several other in-flight PRs that shrink Archetype:

 - #4800 merges the entities and rows Vecs together (shaves off 24 bytes per archetype) 
 - #4809 removes unique_components and moves it to it's own dedicated storage (shaves off 72 bytes per archetype)

---

## Changelog
Changed: `Archetype::table_components` and `Archetype::sparse_set_components` return iterators instead of slices. `Archetype::new` requires iterators instead of parallel slices/vecs.

## Migration Guide
Do I still need to do this? I really hope people were not relying on the public facing APIs changed here.

crates/bevy_ecs/src/archetype.rs

@@ -182,40 +182,32 @@ pub struct Archetype {
     table_id: TableId,
     edges: Edges,
     entities: Vec<ArchetypeEntity>,
-    table_components: Box<[ComponentId]>,
-    sparse_set_components: Box<[ComponentId]>,
     components: SparseSet<ComponentId, ArchetypeComponentInfo>,
 }
 
 impl Archetype {
     pub fn new(
         id: ArchetypeId,
         table_id: TableId,
-        table_components: Box<[ComponentId]>,
-        sparse_set_components: Box<[ComponentId]>,
-        table_archetype_components: Vec<ArchetypeComponentId>,
-        sparse_set_archetype_components: Vec<ArchetypeComponentId>,
+        table_components: impl Iterator<Item = (ComponentId, ArchetypeComponentId)>,
+        sparse_set_components: impl Iterator<Item = (ComponentId, ArchetypeComponentId)>,
     ) -> Self {
-        let mut components =
-            SparseSet::with_capacity(table_components.len() + sparse_set_components.len());
-        for (component_id, archetype_component_id) in
-            table_components.iter().zip(table_archetype_components)
-        {
+        let (min_table, _) = table_components.size_hint();
+        let (min_sparse, _) = sparse_set_components.size_hint();
+        let mut components = SparseSet::with_capacity(min_table + min_sparse);
+        for (component_id, archetype_component_id) in table_components {
             components.insert(
-                *component_id,
+                component_id,
                 ArchetypeComponentInfo {
                     storage_type: StorageType::Table,
                     archetype_component_id,
                 },
             );
         }
 
-        for (component_id, archetype_component_id) in sparse_set_components
-            .iter()
-            .zip(sparse_set_archetype_components)
-        {
+        for (component_id, archetype_component_id) in sparse_set_components {
             components.insert(
-                *component_id,
+                component_id,
                 ArchetypeComponentInfo {
                     storage_type: StorageType::SparseSet,
                     archetype_component_id,
@@ -225,10 +217,8 @@ impl Archetype {
         Self {
             id,
             table_id,
-            entities: Vec::new(),
             components,
-            table_components,
-            sparse_set_components,
+            entities: Default::default(),
             edges: Default::default(),
         }
     }
@@ -249,13 +239,19 @@ impl Archetype {
     }
 
     #[inline]
-    pub fn table_components(&self) -> &[ComponentId] {
-        &self.table_components
+    pub fn table_components(&self) -> impl Iterator<Item = ComponentId> + '_ {
+        self.components
+            .iter()
+            .filter(|(_, component)| component.storage_type == StorageType::Table)
+            .map(|(id, _)| *id)
     }
 
     #[inline]
-    pub fn sparse_set_components(&self) -> &[ComponentId] {
-        &self.sparse_set_components
+    pub fn sparse_set_components(&self) -> impl Iterator<Item = ComponentId> + '_ {
+        self.components
+            .iter()
+            .filter(|(_, component)| component.storage_type == StorageType::SparseSet)
+            .map(|(id, _)| *id)
     }
 
     #[inline]
@@ -484,38 +480,33 @@ impl Archetypes {
         table_components: Vec<ComponentId>,
         sparse_set_components: Vec<ComponentId>,
     ) -> ArchetypeId {
-        let table_components = table_components.into_boxed_slice();
-        let sparse_set_components = sparse_set_components.into_boxed_slice();
         let archetype_identity = ArchetypeIdentity {
-            sparse_set_components: sparse_set_components.clone(),
-            table_components: table_components.clone(),
+            sparse_set_components: sparse_set_components.clone().into_boxed_slice(),
+            table_components: table_components.clone().into_boxed_slice(),
         };
 
         let archetypes = &mut self.archetypes;
         let archetype_component_count = &mut self.archetype_component_count;
-        let mut next_archetype_component_id = move || {
-            let id = ArchetypeComponentId(*archetype_component_count);
-            *archetype_component_count += 1;
-            id
-        };
         *self
             .archetype_ids
             .entry(archetype_identity)
             .or_insert_with(move || {
                 let id = ArchetypeId(archetypes.len());
-                let table_archetype_components = (0..table_components.len())
-                    .map(|_| next_archetype_component_id())
-                    .collect();
-                let sparse_set_archetype_components = (0..sparse_set_components.len())
-                    .map(|_| next_archetype_component_id())
-                    .collect();
+                let table_start = *archetype_component_count;
+                *archetype_component_count += table_components.len();
+                let table_archetype_components =
+                    (table_start..*archetype_component_count).map(ArchetypeComponentId);
+                let sparse_start = *archetype_component_count;
+                *archetype_component_count += sparse_set_components.len();
+                let sparse_set_archetype_components =
+                    (sparse_start..*archetype_component_count).map(ArchetypeComponentId);
                 archetypes.push(Archetype::new(
                     id,
                     table_id,
-                    table_components,
-                    sparse_set_components,
-                    table_archetype_components,
-                    sparse_set_archetype_components,
+                    table_components.into_iter().zip(table_archetype_components),
+                    sparse_set_components
+                        .into_iter()
+                        .zip(sparse_set_archetype_components),
                 ));
                 id
             })

crates/bevy_ecs/src/bundle.rs

@@ -461,7 +461,7 @@ impl BundleInfo {
                 table_components = if new_table_components.is_empty() {
                     // if there are no new table components, we can keep using this table
                     table_id = current_archetype.table_id();
-                    current_archetype.table_components().to_vec()
+                    current_archetype.table_components().collect()
                 } else {
                     new_table_components.extend(current_archetype.table_components());
                     // sort to ignore order while hashing
@@ -477,7 +477,7 @@ impl BundleInfo {
                 };
 
                 sparse_set_components = if new_sparse_set_components.is_empty() {
-                    current_archetype.sparse_set_components().to_vec()
+                    current_archetype.sparse_set_components().collect()
                 } else {
                     new_sparse_set_components.extend(current_archetype.sparse_set_components());
                     // sort to ignore order while hashing

crates/bevy_ecs/src/world/entity_ref.rs

@@ -505,7 +505,7 @@ impl<'w> EntityMut<'w> {
             table_row = remove_result.table_row;
 
             for component_id in archetype.sparse_set_components() {
-                let sparse_set = world.storages.sparse_sets.get_mut(*component_id).unwrap();
+                let sparse_set = world.storages.sparse_sets.get_mut(component_id).unwrap();
                 sparse_set.remove(self.entity);
             }
             // SAFETY: table rows stored in archetypes always exist
@@ -843,8 +843,8 @@ unsafe fn remove_bundle_from_archetype(
             // components are already sorted
             removed_table_components.sort();
             removed_sparse_set_components.sort();
-            next_table_components = current_archetype.table_components().to_vec();
-            next_sparse_set_components = current_archetype.sparse_set_components().to_vec();
+            next_table_components = current_archetype.table_components().collect();
+            next_sparse_set_components = current_archetype.sparse_set_components().collect();
             sorted_remove(&mut next_table_components, &removed_table_components);
             sorted_remove(
                 &mut next_sparse_set_components,