Endianness based Entity representation

crates/bevy_ecs/src/entity/mod.rs

@@ -30,8 +30,11 @@ pub use map_entities::*;
 
 use crate::{archetype::ArchetypeId, storage::SparseSetIndex};
 use std::{
+    cmp::{Ordering as CmpOrdering, PartialEq},
     convert::TryFrom,
-    fmt, mem,
+    fmt,
+    hash::{Hash, Hasher},
+    mem,
     sync::atomic::{AtomicI64, Ordering},
 };
 
@@ -44,9 +47,15 @@ use std::{
 /// `Entity` can be a part of a query, e.g. `Query<(Entity, &MyComponent)>`.
 /// Components of a specific entity can be accessed using
 /// [`Query::get`](crate::system::Query::get) and related methods.
-#[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy)]
+#[repr(C, align(8))]
 pub struct Entity {
+    // Do not reorder the fields here. The ordering is explicitly used by repr(C)
+    // to make this struct equivalent to a u64.
+    #[cfg(target_endian = "little")]
+    pub(crate) id: u32,
     pub(crate) generation: u32,
+    #[cfg(target_endian = "big")]
     pub(crate) id: u32,
 }
 
@@ -114,6 +123,7 @@ impl Entity {
     /// for serialization between runs.
     ///
     /// No particular structure is guaranteed for the returned bits.
+    #[inline(always)]
     pub fn to_bits(self) -> u64 {
         u64::from(self.generation) << 32 | u64::from(self.id)
     }
@@ -147,6 +157,38 @@ impl Entity {
     }
 }
 
+// Required for ordering correctness. Cannot be done with a derive macro.
+impl PartialOrd for Entity {
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<CmpOrdering> {
+        Some(self.to_bits().cmp(&other.to_bits()))
+    }
+}
+
+// Required for ordering correctness. Cannot be done with a derive macro.
+impl Ord for Entity {
+    #[inline]
+    fn cmp(&self, other: &Self) -> CmpOrdering {
+        self.to_bits().cmp(&other.to_bits())
+    }
+}
+
+impl Hash for Entity {
+    #[inline]
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        self.to_bits().hash(hasher)
+    }
+}
+
+impl PartialEq for Entity {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.to_bits() == other.to_bits()
+    }
+}
+
+impl Eq for Entity {}
+
 impl fmt::Debug for Entity {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}v{}", self.id, self.generation)