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use std::{
fmt,
num::NonZeroI32,
sync::atomic::{AtomicUsize, Ordering},
};
use hibitset::{AtomicBitSet, BitSet, BitSetOr};
use shred::Read;
#[nougat::gat(Type)]
use crate::join::LendJoin;
#[cfg(feature = "parallel")]
use crate::join::ParJoin;
use crate::{
error::WrongGeneration,
join::{Join, RepeatableLendGet},
storage::WriteStorage,
world::Component,
};
/// An index is basically the id of an `Entity`.
pub type Index = u32;
/// A wrapper for a read `Entities` resource.
/// Note that this is just `Read<Entities>`, so
/// you can easily use it in your system:
///
/// ```
/// # use specs::prelude::*;
/// # struct Sys;
/// # impl<'a> System<'a> for Sys {
/// type SystemData = (Entities<'a> /* ... */,);
/// # fn run(&mut self, _: Self::SystemData) {}
/// # }
/// ```
///
/// Please note that you should call `World::maintain`
/// after creating / deleting entities with this resource.
///
/// When `.join`ing on `Entities`, you will need to do it like this:
///
/// ```
/// use specs::prelude::*;
///
/// # struct Pos; impl Component for Pos { type Storage = VecStorage<Self>; }
/// # let mut world = World::new(); world.register::<Pos>();
/// # let entities = world.entities(); let positions = world.write_storage::<Pos>();
/// for (e, pos) in (&entities, &positions).join() {
/// // Do something
/// # let _ = e;
/// # let _ = pos;
/// }
/// ```
pub type Entities<'a> = Read<'a, EntitiesRes>;
/// Internally used structure for `Entity` allocation.
#[derive(Default, Debug)]
pub(crate) struct Allocator {
generations: Vec<ZeroableGeneration>,
alive: BitSet,
raised: AtomicBitSet,
killed: AtomicBitSet,
cache: EntityCache,
max_id: AtomicUsize,
}
impl Allocator {
/// Kills a list of entities immediately.
///
/// If an entity with an outdated generation is encountered, the index of
/// that entity within the provided slice is returned (entities after this
/// index are not killed).
pub fn kill(&mut self, delete: &[Entity]) -> Result<(), (WrongGeneration, usize)> {
for (index, &entity) in delete.iter().enumerate() {
let id = entity.id() as usize;
if !self.is_alive(entity) {
return Err((self.del_err(entity), index));
}
self.alive.remove(entity.id());
// If the `Entity` was killed by `kill_atomic`, remove the bit set by it.
self.killed.remove(entity.id());
self.update_generation_length(id);
if self.raised.remove(entity.id()) {
self.generations[id].raise();
}
self.generations[id].die();
}
self.cache.extend(delete.iter().map(|e| e.0));
Ok(())
}
/// Kills an entity atomically (will be updated when the allocator is
/// maintained).
pub fn kill_atomic(&self, e: Entity) -> Result<(), WrongGeneration> {
if !self.is_alive(e) {
return Err(self.del_err(e));
}
self.killed.add_atomic(e.id());
Ok(())
}
pub(crate) fn del_err(&self, e: Entity) -> WrongGeneration {
WrongGeneration {
action: "delete",
actual_gen: self.generations[e.id() as usize]
.0
.unwrap_or_else(Generation::one),
entity: e,
}
}
/// Return `true` if the entity is alive.
pub fn is_alive(&self, e: Entity) -> bool {
e.gen()
== match self.generations.get(e.id() as usize) {
Some(g) if !g.is_alive() && self.raised.contains(e.id()) => g.raised(),
Some(g) => g.0.unwrap_or_else(Generation::one),
None => Generation::one(),
}
}
/// Returns the `Generation` of the given `Index`, if any.
pub fn generation(&self, id: Index) -> Option<Generation> {
self.generations
.get(id as usize)
.cloned()
.and_then(|gen| gen.0)
}
/// Returns the current alive entity with the given `Index`.
pub fn entity(&self, id: Index) -> Entity {
let gen = match self.generations.get(id as usize) {
Some(g) if !g.is_alive() && self.raised.contains(id) => g.raised(),
Some(g) => g.0.unwrap_or_else(Generation::one),
None => Generation::one(),
};
Entity(id, gen)
}
/// Allocate a new entity
pub fn allocate_atomic(&self) -> Entity {
let id = self.cache.pop_atomic().unwrap_or_else(|| {
atomic_increment(&self.max_id).expect("No entity left to allocate") as Index
});
self.raised.add_atomic(id);
let gen = self
.generation(id)
.map(|gen| if gen.is_alive() { gen } else { gen.raised() })
.unwrap_or_else(Generation::one);
Entity(id, gen)
}
/// Allocate a new entity
pub fn allocate(&mut self) -> Entity {
let id = self.cache.pop().unwrap_or_else(|| {
let id = *self.max_id.get_mut();
*self.max_id.get_mut() = id.checked_add(1).expect("No entity left to allocate");
id as Index
});
self.update_generation_length(id as usize);
self.alive.add(id as Index);
let gen = self.generations[id as usize].raise();
Entity(id as Index, gen)
}
/// Maintains the allocated entities, mainly dealing with atomically
/// allocated or killed entities.
pub fn merge(&mut self) -> Vec<Entity> {
use hibitset::BitSetLike;
let mut deleted = vec![];
let max_id = *self.max_id.get_mut();
self.update_generation_length(max_id + 1);
for i in (&self.raised).iter() {
self.generations[i as usize].raise();
self.alive.add(i);
}
self.raised.clear();
for i in (&self.killed).iter() {
self.alive.remove(i);
deleted.push(Entity(i, self.generations[i as usize].0.unwrap()));
self.generations[i as usize].die();
}
self.killed.clear();
self.cache.extend(deleted.iter().map(|e| e.0));
deleted
}
fn update_generation_length(&mut self, i: usize) {
if self.generations.len() <= i {
self.generations.resize(i + 1, ZeroableGeneration(None));
}
}
}
/// An iterator for entity creation.
/// Please note that you have to consume
/// it because iterators are lazy.
///
/// Returned from `Entities::create_iter`.
pub struct CreateIterAtomic<'a>(&'a Allocator);
impl<'a> Iterator for CreateIterAtomic<'a> {
type Item = Entity;
fn next(&mut self) -> Option<Entity> {
Some(self.0.allocate_atomic())
}
}
/// `Entity` type, as seen by the user.
#[derive(Clone, Copy, Debug, Hash, Eq, Ord, PartialEq, PartialOrd)]
pub struct Entity(Index, Generation);
impl Entity {
/// Creates a new entity (externally from ECS).
#[cfg(test)]
pub fn new(index: Index, gen: Generation) -> Self {
Self(index, gen)
}
/// Returns the index of the `Entity`.
#[inline]
pub fn id(self) -> Index {
self.0
}
/// Returns the `Generation` of the `Entity`.
#[inline]
pub fn gen(self) -> Generation {
self.1
}
}
/// The entities of this ECS. This is a resource, stored in the `World`.
/// If you just want to access it in your system, you can also use the
/// `Entities` type def.
///
/// **Please note that you should never get
/// this mutably in a system, because it would
/// block all the other systems.**
///
/// You need to call `World::maintain` after creating / deleting
/// entities with this struct.
#[derive(Debug, Default)]
pub struct EntitiesRes {
pub(crate) alloc: Allocator,
}
impl EntitiesRes {
/// Creates a new entity atomically.
/// This will be persistent as soon
/// as you call `World::maintain`.
///
/// If you want a lazy entity builder, take a look
/// at `LazyUpdate::create_entity`.
///
/// In case you have access to the `World`,
/// you can also use `World::create_entity` which
/// creates the entity and the components immediately.
pub fn create(&self) -> Entity {
self.alloc.allocate_atomic()
}
/// Returns an iterator which creates
/// new entities atomically.
/// They will be persistent as soon
/// as you call `World::maintain`.
pub fn create_iter(&self) -> CreateIterAtomic {
CreateIterAtomic(&self.alloc)
}
/// Similar to the `create` method above this
/// creates an entity atomically, and then returns a
/// builder which can be used to insert components into
/// various storages if available.
pub fn build_entity(&self) -> EntityResBuilder {
let entity = self.create();
EntityResBuilder {
entity,
entities: self,
built: false,
}
}
/// Deletes an entity atomically.
/// The associated components will be
/// deleted as soon as you call `World::maintain`.
pub fn delete(&self, e: Entity) -> Result<(), WrongGeneration> {
self.alloc.kill_atomic(e)
}
/// Returns an entity with a given `id`. There's no guarantee for validity,
/// meaning the entity could be not alive.
pub fn entity(&self, id: Index) -> Entity {
self.alloc.entity(id)
}
/// Returns `true` if the specified entity is alive.
#[inline]
pub fn is_alive(&self, e: Entity) -> bool {
self.alloc.is_alive(e)
}
}
// SAFETY: It is safe to retrieve elements with any `id` regardless of the mask.
#[nougat::gat]
unsafe impl<'a> LendJoin for &'a EntitiesRes {
type Mask = BitSetOr<&'a BitSet, &'a AtomicBitSet>;
type Type<'next> = Entity;
type Value = Self;
unsafe fn open(self) -> (Self::Mask, Self::Value) {
(BitSetOr(&self.alloc.alive, &self.alloc.raised), self)
}
unsafe fn get<'next>(v: &'next mut &'a EntitiesRes, id: Index) -> Entity {
let gen = v
.alloc
.generation(id)
.map(|gen| if gen.is_alive() { gen } else { gen.raised() })
.unwrap_or_else(Generation::one);
Entity(id, gen)
}
}
// SAFETY: <EntitiesRes as LendJoin>::get does not rely on only being called
// once with a particular ID.
unsafe impl<'a> RepeatableLendGet for &'a EntitiesRes {}
// SAFETY: It is safe to retrieve elements with any `id` regardless of the mask.
unsafe impl<'a> Join for &'a EntitiesRes {
type Mask = BitSetOr<&'a BitSet, &'a AtomicBitSet>;
type Type = Entity;
type Value = Self;
unsafe fn open(self) -> (Self::Mask, Self::Value) {
(BitSetOr(&self.alloc.alive, &self.alloc.raised), self)
}
unsafe fn get(v: &mut &'a EntitiesRes, id: Index) -> Entity {
let gen = v
.alloc
.generation(id)
.map(|gen| if gen.is_alive() { gen } else { gen.raised() })
.unwrap_or_else(Generation::one);
Entity(id, gen)
}
}
// SAFETY: No unsafe code is used and it is safe to call `get` from multiple
// threads at once.
//
// It is safe to retrieve elements with any `id` regardless of the mask.
#[cfg(feature = "parallel")]
unsafe impl<'a> ParJoin for &'a EntitiesRes {
type Mask = BitSetOr<&'a BitSet, &'a AtomicBitSet>;
type Type = Entity;
type Value = Self;
unsafe fn open(self) -> (Self::Mask, Self::Value) {
(BitSetOr(&self.alloc.alive, &self.alloc.raised), self)
}
unsafe fn get(v: &&'a EntitiesRes, id: Index) -> Entity {
let gen = v
.alloc
.generation(id)
.map(|gen| if gen.is_alive() { gen } else { gen.raised() })
.unwrap_or_else(Generation::one);
Entity(id, gen)
}
}
/// An entity builder from `EntitiesRes`. Allows building an entity with its
/// components if you have mutable access to the component storages.
#[must_use = "Please call .build() on this to finish building it."]
pub struct EntityResBuilder<'a> {
/// The entity being built
pub entity: Entity,
/// The active borrow to `EntitiesRes`, used to delete the entity if the
/// builder is dropped without called `build()`.
pub entities: &'a EntitiesRes,
built: bool,
}
impl<'a> EntityResBuilder<'a> {
/// Appends a component and associates it with the entity.
pub fn with<T: Component>(self, c: T, storage: &mut WriteStorage<T>) -> Self {
storage.insert(self.entity, c).unwrap();
self
}
/// Finishes the building and returns the entity.
pub fn build(mut self) -> Entity {
self.built = true;
self.entity
}
}
impl<'a> Drop for EntityResBuilder<'a> {
fn drop(&mut self) {
if !self.built {
self.entities.delete(self.entity).unwrap();
}
}
}
/// Index generation. When a new entity is placed at an old index,
/// it bumps the `Generation` by 1. This allows to avoid using components
/// from the entities that were deleted.
#[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
pub struct Generation(NonZeroI32);
// Show the inner value as i32 instead of u32.
impl fmt::Debug for Generation {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_tuple("Generation").field(&self.id()).finish()
}
}
impl Generation {
pub(crate) fn one() -> Self {
// SAFETY: `1` is not zero.
Generation(unsafe { NonZeroI32::new_unchecked(1) })
}
#[cfg(test)]
pub fn new(v: i32) -> Self {
Generation(NonZeroI32::new(v).expect("generation id must be non-zero"))
}
/// Returns the id of the generation.
#[inline]
pub fn id(self) -> i32 {
self.0.get()
}
/// Returns `true` if entities of this `Generation` are alive.
#[inline]
pub fn is_alive(self) -> bool {
self.id() > 0
}
/// Revives and increments a dead `Generation`.
///
/// # Panics
///
/// Panics if it is alive.
fn raised(self) -> Generation {
assert!(!self.is_alive());
// SAFETY: Since `self` is not alive, `self.id()` will be negative so
// subtracting it from `1` will give us a value `>= 2`. If this
// overflows it will at most wrap to `i32::MIN + 1` (so it will never be
// zero).
unsafe { Generation(NonZeroI32::new_unchecked(1 - self.id())) }
}
}
/// Convenience wrapper around Option<Generation>
#[derive(Clone, Copy, Debug, Hash, Eq, PartialEq)]
struct ZeroableGeneration(Option<Generation>);
impl ZeroableGeneration {
/// Returns the id of the generation.
#[inline]
pub fn id(self) -> i32 {
// should optimise to a noop.
self.0.map(|gen| gen.id()).unwrap_or(0)
}
/// Returns `true` if entities of this `Generation` are alive.
#[inline]
fn is_alive(self) -> bool {
self.id() > 0
}
/// Kills this `Generation`.
///
/// # Panics
///
/// Panics in debug mode if it's not alive.
fn die(&mut self) {
debug_assert!(self.is_alive());
self.0 = NonZeroI32::new(-self.id()).map(Generation);
}
/// Revives and increments a dead `Generation`.
///
/// # Panics
///
/// Panics if it is alive.
fn raised(self) -> Generation {
assert!(!self.is_alive());
let gen = 1i32.checked_sub(self.id()).expect("generation overflow");
// SAFETY: Since `self` is not alive, `self.id()` will be negative so
// subtracting it from `1` will give us a value `>= 2`.
Generation(unsafe { NonZeroI32::new_unchecked(gen) })
}
/// Revives and increments a dead `ZeroableGeneration`.
///
/// # Panics
///
/// Panics if it is alive.
fn raise(&mut self) -> Generation {
let gen = self.raised();
self.0 = Some(gen);
gen
}
}
#[derive(Default, Debug)]
struct EntityCache {
cache: Vec<Index>,
len: AtomicUsize,
}
impl EntityCache {
fn pop_atomic(&self) -> Option<Index> {
atomic_decrement(&self.len).map(|x| self.cache[x - 1])
}
fn pop(&mut self) -> Option<Index> {
self.maintain();
let x = self.cache.pop();
*self.len.get_mut() = self.cache.len();
x
}
fn maintain(&mut self) {
self.cache.truncate(*(self.len.get_mut()));
}
}
impl Extend<Index> for EntityCache {
fn extend<T: IntoIterator<Item = Index>>(&mut self, iter: T) {
self.maintain();
self.cache.extend(iter);
*self.len.get_mut() = self.cache.len();
}
}
/// Increments `i` atomically without wrapping on overflow.
/// Resembles a `fetch_add(1, Ordering::Relaxed)` with
/// checked overflow, returning `None` instead.
fn atomic_increment(i: &AtomicUsize) -> Option<usize> {
use std::usize;
let mut prev = i.load(Ordering::Relaxed);
while prev != usize::MAX {
match i.compare_exchange_weak(prev, prev + 1, Ordering::Relaxed, Ordering::Relaxed) {
Ok(x) => return Some(x),
Err(next_prev) => prev = next_prev,
}
}
None
}
/// Increments `i` atomically without wrapping on overflow.
/// Resembles a `fetch_sub(1, Ordering::Relaxed)` with
/// checked underflow, returning `None` instead.
fn atomic_decrement(i: &AtomicUsize) -> Option<usize> {
let mut prev = i.load(Ordering::Relaxed);
while prev != 0 {
match i.compare_exchange_weak(prev, prev - 1, Ordering::Relaxed, Ordering::Relaxed) {
Ok(x) => return Some(x),
Err(next_prev) => prev = next_prev,
}
}
None
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_nonzero_optimization() {
use std::mem::size_of;
assert_eq!(size_of::<Option<Generation>>(), size_of::<Generation>());
assert_eq!(size_of::<Option<Entity>>(), size_of::<Entity>());
}
#[test]
fn kill_atomic_create_merge() {
let mut allocator = Allocator::default();
let entity = allocator.allocate();
assert_eq!(entity.id(), 0);
allocator.kill_atomic(entity).unwrap();
assert_ne!(allocator.allocate(), entity);
assert_eq!(allocator.killed.contains(entity.id()), true);
assert_eq!(allocator.merge(), vec![entity]);
}
#[test]
fn kill_atomic_kill_now_create_merge() {
let mut allocator = Allocator::default();
let entity = allocator.allocate();
allocator.kill_atomic(entity).unwrap();
assert_ne!(allocator.allocate(), entity);
allocator.kill(&[entity]).unwrap();
allocator.allocate();
assert_eq!(allocator.killed.contains(entity.id()), false);
assert_eq!(allocator.merge(), vec![]);
}
}