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use alloc::vec::Vec;
use core::cmp;
use core::convert::TryFrom;
use core::iter::ExactSizeIterator;
use core::num::{NonZeroU32, NonZeroU64};
use core::ops::Range;
use core::sync::atomic::{AtomicIsize, Ordering};
use core::{fmt, mem};
#[cfg(feature = "std")]
use std::error::Error;
/// Lightweight unique ID, or handle, of an entity
///
/// Obtained from `World::spawn`. Can be stored to refer to an entity in the future.
///
/// Enable the `serde` feature on the crate to make this `Serialize`able. Some applications may be
/// able to save space by only serializing the output of `Entity::id`.
#[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
pub struct Entity {
pub(crate) generation: NonZeroU32,
pub(crate) id: u32,
}
impl Entity {
/// An [`Entity`] that does not necessarily correspond to data in any `World`
///
/// Useful as a dummy value. It is possible (albeit unlikely) for a `World` to contain this
/// entity.
pub const DANGLING: Entity = Entity {
generation: match NonZeroU32::new(u32::MAX) {
Some(x) => x,
None => unreachable!(),
},
id: u32::MAX,
};
/// Convert to a form convenient for passing outside of rust
///
/// No particular structure is guaranteed for the returned bits.
///
/// Useful for storing entity IDs externally, or in conjunction with `Entity::from_bits` and
/// `World::spawn_at` for easy serialization. Alternatively, consider `id` for more compact
/// representation.
pub fn to_bits(self) -> NonZeroU64 {
unsafe {
NonZeroU64::new_unchecked(u64::from(self.generation.get()) << 32 | u64::from(self.id))
}
}
/// Reconstruct an `Entity` previously destructured with `to_bits` if the bitpattern is valid,
/// else `None`
///
/// Useful for storing entity IDs externally, or in conjunction with `Entity::to_bits` and
/// `World::spawn_at` for easy serialization.
pub fn from_bits(bits: u64) -> Option<Self> {
Some(Self {
generation: NonZeroU32::new((bits >> 32) as u32)?,
id: bits as u32,
})
}
/// Extract a transiently unique identifier
///
/// No two simultaneously-live entities share the same ID, but dead entities' IDs may collide
/// with both live and dead entities. Useful for compactly representing entities within a
/// specific snapshot of the world, such as when serializing.
///
/// See also `World::find_entity_from_id`.
pub fn id(self) -> u32 {
self.id
}
}
impl fmt::Debug for Entity {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{}v{}", self.id, self.generation)
}
}
#[cfg(feature = "serde")]
impl serde::Serialize for Entity {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where
S: serde::Serializer,
{
self.to_bits().serialize(serializer)
}
}
#[cfg(feature = "serde")]
impl<'de> serde::Deserialize<'de> for Entity {
fn deserialize<D>(deserializer: D) -> Result<Entity, D::Error>
where
D: serde::Deserializer<'de>,
{
let bits = u64::deserialize(deserializer)?;
match Entity::from_bits(bits) {
Some(ent) => Ok(ent),
None => Err(serde::de::Error::invalid_value(
serde::de::Unexpected::Unsigned(bits),
&"`a valid `Entity` bitpattern",
)),
}
}
}
/// An iterator returning a sequence of Entity values from `Entities::reserve_entities`.
pub struct ReserveEntitiesIterator<'a> {
// Metas, so we can recover the current generation for anything in the freelist.
meta: &'a [EntityMeta],
// Reserved IDs formerly in the freelist to hand out.
id_iter: core::slice::Iter<'a, u32>,
// New Entity IDs to hand out, outside the range of meta.len().
id_range: core::ops::Range<u32>,
}
impl<'a> Iterator for ReserveEntitiesIterator<'a> {
type Item = Entity;
fn next(&mut self) -> Option<Self::Item> {
self.id_iter
.next()
.map(|&id| Entity {
generation: self.meta[id as usize].generation,
id,
})
.or_else(|| {
self.id_range.next().map(|id| Entity {
generation: NonZeroU32::new(1).unwrap(),
id,
})
})
}
fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.id_iter.len() + self.id_range.len();
(len, Some(len))
}
}
impl<'a> ExactSizeIterator for ReserveEntitiesIterator<'a> {}
#[derive(Default)]
pub(crate) struct Entities {
pub meta: Vec<EntityMeta>,
// The `pending` and `free_cursor` fields describe three sets of Entity IDs
// that have been freed or are in the process of being allocated:
//
// - The `freelist` IDs, previously freed by `free()`. These IDs are available to any
// of `alloc()`, `reserve_entity()` or `reserve_entities()`. Allocation will
// always prefer these over brand new IDs.
//
// - The `reserved` list of IDs that were once in the freelist, but got
// reserved by `reserve_entities` or `reserve_entity()`. They are now waiting
// for `flush()` to make them fully allocated.
//
// - The count of new IDs that do not yet exist in `self.meta()`, but which
// we have handed out and reserved. `flush()` will allocate room for them in `self.meta()`.
//
// The contents of `pending` look like this:
//
// ```
// ----------------------------
// | freelist | reserved |
// ----------------------------
// ^ ^
// free_cursor pending.len()
// ```
//
// As IDs are allocated, `free_cursor` is atomically decremented, moving
// items from the freelist into the reserved list by sliding over the boundary.
//
// Once the freelist runs out, `free_cursor` starts going negative.
// The more negative it is, the more IDs have been reserved starting exactly at
// the end of `meta.len()`.
//
// This formulation allows us to reserve any number of IDs first from the freelist
// and then from the new IDs, using only a single atomic subtract.
//
// Once `flush()` is done, `free_cursor` will equal `pending.len()`.
pending: Vec<u32>,
free_cursor: AtomicIsize,
len: u32,
}
impl Entities {
/// Reserve entity IDs concurrently
///
/// Storage for entity generation and location is lazily allocated by calling `flush`.
pub fn reserve_entities(&self, count: u32) -> ReserveEntitiesIterator {
// Use one atomic subtract to grab a range of new IDs. The range might be
// entirely nonnegative, meaning all IDs come from the freelist, or entirely
// negative, meaning they are all new IDs to allocate, or a mix of both.
let range_end = self
.free_cursor
.fetch_sub(count as isize, Ordering::Relaxed);
let range_start = range_end - count as isize;
let freelist_range = range_start.max(0) as usize..range_end.max(0) as usize;
let (new_id_start, new_id_end) = if range_start >= 0 {
// We satisfied all requests from the freelist.
(0, 0)
} else {
// We need to allocate some new Entity IDs outside of the range of self.meta.
//
// `range_start` covers some negative territory, e.g. `-3..6`.
// Since the nonnegative values `0..6` are handled by the freelist, that
// means we need to handle the negative range here.
//
// In this example, we truncate the end to 0, leaving us with `-3..0`.
// Then we negate these values to indicate how far beyond the end of `meta.end()`
// to go, yielding `meta.len()+0 .. meta.len()+3`.
let base = self.meta.len() as isize;
let new_id_end = u32::try_from(base - range_start).expect("too many entities");
// `new_id_end` is in range, so no need to check `start`.
let new_id_start = (base - range_end.min(0)) as u32;
(new_id_start, new_id_end)
};
ReserveEntitiesIterator {
meta: &self.meta[..],
id_iter: self.pending[freelist_range].iter(),
id_range: new_id_start..new_id_end,
}
}
/// Reserve one entity ID concurrently
///
/// Equivalent to `self.reserve_entities(1).next().unwrap()`, but more efficient.
pub fn reserve_entity(&self) -> Entity {
let n = self.free_cursor.fetch_sub(1, Ordering::Relaxed);
if n > 0 {
// Allocate from the freelist.
let id = self.pending[(n - 1) as usize];
Entity {
generation: self.meta[id as usize].generation,
id,
}
} else {
// Grab a new ID, outside the range of `meta.len()`. `flush()` must
// eventually be called to make it valid.
//
// As `self.free_cursor` goes more and more negative, we return IDs farther
// and farther beyond `meta.len()`.
Entity {
generation: NonZeroU32::new(1).unwrap(),
id: u32::try_from(self.meta.len() as isize - n).expect("too many entities"),
}
}
}
/// Check that we do not have pending work requiring `flush()` to be called.
fn verify_flushed(&mut self) {
debug_assert!(
!self.needs_flush(),
"flush() needs to be called before this operation is legal"
);
}
/// Allocate an entity ID directly
///
/// Location should be written immediately.
pub fn alloc(&mut self) -> Entity {
self.verify_flushed();
self.len += 1;
if let Some(id) = self.pending.pop() {
let new_free_cursor = self.pending.len() as isize;
*self.free_cursor.get_mut() = new_free_cursor;
Entity {
generation: self.meta[id as usize].generation,
id,
}
} else {
let id = u32::try_from(self.meta.len()).expect("too many entities");
self.meta.push(EntityMeta::EMPTY);
Entity {
generation: NonZeroU32::new(1).unwrap(),
id,
}
}
}
/// Allocate and set locations for many entity IDs laid out contiguously in an archetype
///
/// `self.finish_alloc_many()` must be called after!
pub fn alloc_many(&mut self, n: u32, archetype: u32, mut first_index: u32) -> AllocManyState {
self.verify_flushed();
let fresh = (n as usize).saturating_sub(self.pending.len()) as u32;
assert!(
(self.meta.len() + fresh as usize) < u32::MAX as usize,
"too many entities"
);
let pending_end = self.pending.len().saturating_sub(n as usize);
for &id in &self.pending[pending_end..] {
self.meta[id as usize].location = Location {
archetype,
index: first_index,
};
first_index += 1;
}
let fresh_start = self.meta.len() as u32;
self.meta.extend(
(first_index..(first_index + fresh)).map(|index| EntityMeta {
generation: NonZeroU32::new(1).unwrap(),
location: Location { archetype, index },
}),
);
self.len += n;
AllocManyState {
fresh: fresh_start..(fresh_start + fresh),
pending_end,
}
}
/// Remove entities used by `alloc_many` from the freelist
///
/// This is an awkward separate function to avoid borrowck issues in `SpawnColumnBatchIter`.
pub fn finish_alloc_many(&mut self, pending_end: usize) {
self.pending.truncate(pending_end);
}
/// Allocate a specific entity ID, overwriting its generation
///
/// Returns the location of the entity currently using the given ID, if any. Location should be written immediately.
pub fn alloc_at(&mut self, entity: Entity) -> Option<Location> {
self.verify_flushed();
let loc = if entity.id as usize >= self.meta.len() {
self.pending.extend((self.meta.len() as u32)..entity.id);
let new_free_cursor = self.pending.len() as isize;
*self.free_cursor.get_mut() = new_free_cursor;
self.meta.resize(entity.id as usize + 1, EntityMeta::EMPTY);
self.len += 1;
None
} else if let Some(index) = self.pending.iter().position(|item| *item == entity.id) {
self.pending.swap_remove(index);
let new_free_cursor = self.pending.len() as isize;
*self.free_cursor.get_mut() = new_free_cursor;
self.len += 1;
None
} else {
Some(mem::replace(
&mut self.meta[entity.id as usize].location,
EntityMeta::EMPTY.location,
))
};
self.meta[entity.id as usize].generation = entity.generation;
loc
}
/// Destroy an entity, allowing it to be reused
///
/// Must not be called while reserved entities are awaiting `flush()`.
pub fn free(&mut self, entity: Entity) -> Result<Location, NoSuchEntity> {
self.verify_flushed();
let meta = self.meta.get_mut(entity.id as usize).ok_or(NoSuchEntity)?;
if meta.generation != entity.generation || meta.location.index == u32::MAX {
return Err(NoSuchEntity);
}
meta.generation = NonZeroU32::new(u32::from(meta.generation).wrapping_add(1))
.unwrap_or_else(|| NonZeroU32::new(1).unwrap());
let loc = mem::replace(&mut meta.location, EntityMeta::EMPTY.location);
self.pending.push(entity.id);
let new_free_cursor = self.pending.len() as isize;
*self.free_cursor.get_mut() = new_free_cursor;
self.len -= 1;
Ok(loc)
}
/// Ensure at least `n` allocations can succeed without reallocating
pub fn reserve(&mut self, additional: u32) {
self.verify_flushed();
let freelist_size = *self.free_cursor.get_mut();
let shortfall = additional as isize - freelist_size;
if shortfall > 0 {
self.meta.reserve(shortfall as usize);
}
}
pub fn contains(&self, entity: Entity) -> bool {
match self.meta.get(entity.id as usize) {
Some(meta) => {
meta.generation == entity.generation
&& (meta.location.index != u32::MAX
|| self.pending[self.free_cursor.load(Ordering::Relaxed).max(0) as usize..]
.contains(&entity.id))
}
None => {
// Check if this could have been obtained from `reserve_entity`
let free = self.free_cursor.load(Ordering::Relaxed);
entity.generation.get() == 1
&& free < 0
&& (entity.id as isize) < (free.abs() + self.meta.len() as isize)
}
}
}
pub fn clear(&mut self) {
self.meta.clear();
self.pending.clear();
*self.free_cursor.get_mut() = 0;
self.len = 0;
}
/// Access the location storage of an entity
///
/// Must not be called on pending entities.
pub fn get_mut(&mut self, entity: Entity) -> Result<&mut Location, NoSuchEntity> {
let meta = self.meta.get_mut(entity.id as usize).ok_or(NoSuchEntity)?;
if meta.generation == entity.generation && meta.location.index != u32::MAX {
Ok(&mut meta.location)
} else {
Err(NoSuchEntity)
}
}
/// Returns `Ok(Location { archetype: 0, index: undefined })` for pending entities
pub fn get(&self, entity: Entity) -> Result<Location, NoSuchEntity> {
if self.meta.len() <= entity.id as usize {
// Check if this could have been obtained from `reserve_entity`
let free = self.free_cursor.load(Ordering::Relaxed);
if entity.generation.get() == 1
&& free < 0
&& (entity.id as isize) < (free.abs() + self.meta.len() as isize)
{
return Ok(Location {
archetype: 0,
index: u32::max_value(),
});
} else {
return Err(NoSuchEntity);
}
}
let meta = &self.meta[entity.id as usize];
if meta.generation != entity.generation || meta.location.index == u32::MAX {
return Err(NoSuchEntity);
}
Ok(meta.location)
}
/// Panics if the given id would represent an index outside of `meta`.
///
/// # Safety
/// Must only be called for currently allocated `id`s.
pub unsafe fn resolve_unknown_gen(&self, id: u32) -> Entity {
let meta_len = self.meta.len();
if meta_len > id as usize {
let meta = &self.meta[id as usize];
Entity {
generation: meta.generation,
id,
}
} else {
// See if it's pending, but not yet flushed.
let free_cursor = self.free_cursor.load(Ordering::Relaxed);
let num_pending = cmp::max(-free_cursor, 0) as usize;
if meta_len + num_pending > id as usize {
// Pending entities will have the first generation.
Entity {
generation: NonZeroU32::new(1).unwrap(),
id,
}
} else {
panic!("entity id is out of range");
}
}
}
fn needs_flush(&mut self) -> bool {
*self.free_cursor.get_mut() != self.pending.len() as isize
}
/// Allocates space for entities previously reserved with `reserve_entity` or
/// `reserve_entities`, then initializes each one using the supplied function.
pub fn flush(&mut self, mut init: impl FnMut(u32, &mut Location)) {
let free_cursor = *self.free_cursor.get_mut();
let new_free_cursor = if free_cursor >= 0 {
free_cursor as usize
} else {
let old_meta_len = self.meta.len();
let new_meta_len = old_meta_len + -free_cursor as usize;
self.meta.resize(new_meta_len, EntityMeta::EMPTY);
self.len += -free_cursor as u32;
for (id, meta) in self.meta.iter_mut().enumerate().skip(old_meta_len) {
init(id as u32, &mut meta.location);
}
*self.free_cursor.get_mut() = 0;
0
};
self.len += (self.pending.len() - new_free_cursor) as u32;
for id in self.pending.drain(new_free_cursor..) {
init(id, &mut self.meta[id as usize].location);
}
}
#[inline]
pub fn len(&self) -> u32 {
self.len
}
}
#[derive(Copy, Clone)]
pub(crate) struct EntityMeta {
pub generation: NonZeroU32,
pub location: Location,
}
impl EntityMeta {
const EMPTY: EntityMeta = EntityMeta {
generation: match NonZeroU32::new(1) {
Some(x) => x,
None => unreachable!(),
},
location: Location {
archetype: 0,
index: u32::max_value(), // dummy value, to be filled in
},
};
}
#[derive(Copy, Clone)]
pub(crate) struct Location {
pub archetype: u32,
pub index: u32,
}
/// Error indicating that no entity with a particular ID exists
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct NoSuchEntity;
impl fmt::Display for NoSuchEntity {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("no such entity")
}
}
#[cfg(feature = "std")]
impl Error for NoSuchEntity {}
#[derive(Clone)]
pub(crate) struct AllocManyState {
pub pending_end: usize,
fresh: Range<u32>,
}
impl AllocManyState {
pub fn next(&mut self, entities: &Entities) -> Option<u32> {
if self.pending_end < entities.pending.len() {
let id = entities.pending[self.pending_end];
self.pending_end += 1;
Some(id)
} else {
self.fresh.next()
}
}
pub fn len(&self, entities: &Entities) -> usize {
self.fresh.len() + (entities.pending.len() - self.pending_end)
}
}
#[cfg(test)]
mod tests {
use super::*;
use hashbrown::{HashMap, HashSet};
use rand::{rngs::StdRng, Rng, SeedableRng};
#[test]
fn entity_bits_roundtrip() {
let e = Entity {
generation: NonZeroU32::new(0xDEADBEEF).unwrap(),
id: 0xBAADF00D,
};
assert_eq!(Entity::from_bits(e.to_bits().into()).unwrap(), e);
}
#[test]
fn alloc_and_free() {
let mut rng = StdRng::seed_from_u64(0xFEEDFACEDEADF00D);
let mut e = Entities::default();
let mut first_unused = 0u32;
let mut id_to_gen: HashMap<u32, u32> = Default::default();
let mut free_set: HashSet<u32> = Default::default();
let mut len = 0;
for _ in 0..100 {
let alloc = rng.gen_bool(0.7);
if alloc || first_unused == 0 {
let entity = e.alloc();
e.meta[entity.id as usize].location.index = 0;
len += 1;
let id = entity.id;
if !free_set.is_empty() {
// This should have come from the freelist.
assert!(free_set.remove(&id));
} else if id >= first_unused {
first_unused = id + 1;
}
e.get_mut(entity).unwrap().index = 37;
assert!(id_to_gen.insert(id, entity.generation.get()).is_none());
} else {
// Free a random ID, whether or not it's in use, and check for errors.
let id = rng.gen_range(0..first_unused);
let generation = id_to_gen.remove(&id);
let entity = Entity {
id,
generation: NonZeroU32::new(
generation.unwrap_or_else(|| NonZeroU32::new(1).unwrap().get()),
)
.unwrap(),
};
assert_eq!(e.free(entity).is_ok(), generation.is_some());
if generation.is_some() {
len -= 1;
}
free_set.insert(id);
}
assert_eq!(e.len(), len);
}
}
#[test]
fn alloc_at() {
let mut e = Entities::default();
let mut old = Vec::new();
for _ in 0..2 {
let entity = e.alloc();
e.meta[entity.id as usize].location.index = 0;
old.push(entity);
e.free(entity).unwrap();
}
assert_eq!(e.len(), 0);
let id = old.first().unwrap().id();
assert!(old.iter().all(|entity| entity.id() == id));
let entity = *old.last().unwrap();
// The old ID shouldn't exist at this point, and should exist
// in the pending list.
assert!(!e.contains(entity));
assert!(e.pending.contains(&entity.id()));
// Allocating an entity at an unused location should not cause a location to be returned.
assert!(e.alloc_at(entity).is_none());
e.meta[entity.id as usize].location.index = 0;
assert!(e.contains(entity));
// The entity in question should not exist in the free-list once allocated.
assert!(!e.pending.contains(&entity.id()));
assert_eq!(e.len(), 1);
// Allocating at the same id again should cause a location to be returned
// this time around.
assert!(e.alloc_at(entity).is_some());
e.meta[entity.id as usize].location.index = 0;
assert!(e.contains(entity));
assert_eq!(e.len(), 1);
// Allocating an Entity should cause the new empty locations
// to be located in the free list.
assert_eq!(e.meta.len(), 1);
assert!(e
.alloc_at(Entity {
id: 3,
generation: NonZeroU32::new(2).unwrap(),
})
.is_none());
e.meta[entity.id as usize].location.index = 0;
assert_eq!(e.pending.len(), 2);
assert_eq!(&e.pending, &[1, 2]);
assert_eq!(e.meta.len(), 4);
}
#[test]
fn contains() {
let mut e = Entities::default();
for _ in 0..2 {
let entity = e.alloc();
e.meta[entity.id as usize].location.index = 0;
assert!(e.contains(entity));
e.free(entity).unwrap();
assert!(!e.contains(entity));
}
// Reserved but not flushed are still "contained".
for _ in 0..3 {
let entity = e.reserve_entity();
assert!(e.contains(entity));
assert!(!e.contains(Entity {
id: entity.id,
generation: NonZeroU32::new(2).unwrap(),
}));
assert!(!e.contains(Entity {
id: entity.id + 1,
generation: NonZeroU32::new(1).unwrap(),
}));
}
}
// Shared test code parameterized by how we want to allocate an Entity block.
fn reserve_test_helper(reserve_n: impl FnOnce(&mut Entities, u32) -> Vec<Entity>) {
let mut e = Entities::default();
// Allocate 10 items.
let mut v1: Vec<Entity> = (0..10).map(|_| e.alloc()).collect();
for &entity in &v1 {
e.meta[entity.id as usize].location.index = 0;
}
assert_eq!(v1.iter().map(|e| e.id).max(), Some(9));
for &entity in v1.iter() {
assert!(e.contains(entity));
e.get_mut(entity).unwrap().index = 37;
}
// Put the last 4 on the freelist.
for entity in v1.drain(6..) {
e.free(entity).unwrap();
}
assert_eq!(*e.free_cursor.get_mut(), 4);
// Reserve 10 entities, so 4 will come from the freelist.
// This means we will have allocated 10 + 10 - 4 total items, so max id is 15.
let v2 = reserve_n(&mut e, 10);
assert_eq!(v2.iter().map(|e| e.id).max(), Some(15));
// Reserved IDs still count as "contained".
assert!(v2.iter().all(|&entity| e.contains(entity)));
// We should have exactly IDs 0..16
let mut v3: Vec<Entity> = v1.iter().chain(v2.iter()).copied().collect();
assert_eq!(v3.len(), 16);
v3.sort_by_key(|entity| entity.id);
for (i, entity) in v3.into_iter().enumerate() {
assert_eq!(entity.id, i as u32);
}
// 6 will come from pending.
assert_eq!(*e.free_cursor.get_mut(), -6);
let mut flushed = Vec::new();
e.flush(|id, loc| {
loc.index = 0;
flushed.push(id);
});
flushed.sort_unstable();
assert_eq!(flushed, (6..16).collect::<Vec<_>>());
}
#[test]
fn reserve_entity() {
reserve_test_helper(|e, n| (0..n).map(|_| e.reserve_entity()).collect())
}
#[test]
fn reserve_entities() {
reserve_test_helper(|e, n| e.reserve_entities(n).collect())
}
#[test]
fn reserve_grows() {
let mut e = Entities::default();
let _ = e.reserve_entity();
e.flush(|_, l| {
l.index = 0;
});
assert_eq!(e.len(), 1);
}
#[test]
fn reserve_grows_mixed() {
let mut e = Entities::default();
let a = e.alloc();
e.meta[a.id as usize].location.index = 0;
let b = e.alloc();
e.meta[b.id as usize].location.index = 0;
e.free(a).unwrap();
let _ = e.reserve_entities(3);
e.flush(|_, l| {
l.index = 0;
});
assert_eq!(e.len(), 4);
}
#[test]
fn alloc_at_regression() {
let mut e = Entities::default();
assert!(e
.alloc_at(Entity {
generation: NonZeroU32::new(1).unwrap(),
id: 1
})
.is_none());
assert!(!e.contains(Entity {
generation: NonZeroU32::new(1).unwrap(),
id: 0
}));
}
}