From 3c8c135b9f41acf6a170822c0cba21a402f881d6 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Thu, 4 Apr 2024 21:07:11 +0200
Subject: feat(ecs): allow components to be specified to be dropped last

---
 ecs/src/component.rs |  2 ++
 ecs/src/lib.rs       | 71 ++++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 66 insertions(+), 7 deletions(-)

diff --git a/ecs/src/component.rs b/ecs/src/component.rs
index 120ba30..c1f8168 100644
--- a/ecs/src/component.rs
+++ b/ecs/src/component.rs
@@ -16,6 +16,8 @@ use crate::WorldData;
 
 pub trait Component: SystemInput + Any + TypeName
 {
+    fn drop_last(&self) -> bool;
+
     #[doc(hidden)]
     fn as_any_mut(&mut self) -> &mut dyn Any;
 
diff --git a/ecs/src/lib.rs b/ecs/src/lib.rs
index 7a56f37..4d6e4e5 100644
--- a/ecs/src/lib.rs
+++ b/ecs/src/lib.rs
@@ -4,6 +4,7 @@ use std::any::{type_name, Any, TypeId};
 use std::collections::{HashMap, HashSet};
 use std::fmt::Debug;
 use std::marker::PhantomData;
+use std::mem::ManuallyDrop;
 use std::ops::RangeBounds;
 use std::slice::Iter as SliceIter;
 use std::sync::{Arc, Weak};
@@ -35,7 +36,7 @@ pub use ecs_macros::Component;
 #[derive(Debug, Default)]
 struct Entity
 {
-    components: Vec<EntityComponent>,
+    components: Vec<ManuallyDrop<EntityComponent>>,
 }
 
 #[derive(Debug)]
@@ -43,6 +44,7 @@ struct EntityComponent
 {
     id: TypeId,
     component: Lock<Box<dyn Component>>,
+    drop_last: bool,
 }
 
 #[derive(Debug, Default)]
@@ -73,9 +75,14 @@ impl World
                 components: components
                     .into_vec()
                     .into_iter()
-                    .map(|component| EntityComponent {
-                        id: (*component).type_id(),
-                        component: Lock::new(component),
+                    .map(|component| {
+                        let drop_last = component.drop_last();
+
+                        ManuallyDrop::new(EntityComponent {
+                            id: (*component).type_id(),
+                            component: Lock::new(component),
+                            drop_last,
+                        })
                     })
                     .collect(),
             });
@@ -144,9 +151,14 @@ impl World
                         .push(Entity {
                             components: components
                                 .into_iter()
-                                .map(|component| EntityComponent {
-                                    id: (*component).type_id(),
-                                    component: Lock::new(component),
+                                .map(|component| {
+                                    let drop_last = component.drop_last();
+
+                                    ManuallyDrop::new(EntityComponent {
+                                        id: (*component).type_id(),
+                                        component: Lock::new(component),
+                                        drop_last,
+                                    })
                                 })
                                 .collect(),
                         });
@@ -443,3 +455,48 @@ impl TypeName for ComponentStorage
         type_name::<Self>()
     }
 }
+
+impl Drop for ComponentStorage
+{
+    fn drop(&mut self)
+    {
+        let mut components_to_drop_last = Vec::new();
+
+        for entity in &mut self.entities {
+            for component in &mut entity.components {
+                if component.drop_last {
+                    #[cfg(feature = "debug")]
+                    tracing::debug!(
+                        "Component {} pushed to dropping last queue",
+                        component.component.read_nonblock().unwrap().type_name()
+                    );
+
+                    components_to_drop_last.push(component);
+                    continue;
+                }
+
+                #[cfg(feature = "debug")]
+                tracing::debug!(
+                    "Dropping component {}",
+                    component.component.read_nonblock().unwrap().type_name()
+                );
+
+                unsafe {
+                    ManuallyDrop::drop(component);
+                }
+            }
+        }
+
+        for component in &mut components_to_drop_last {
+            #[cfg(feature = "debug")]
+            tracing::debug!(
+                "Dropping component {} last",
+                component.component.read_nonblock().unwrap().type_name()
+            );
+
+            unsafe {
+                ManuallyDrop::drop(component);
+            }
+        }
+    }
+}
-- 
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