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use super::MaybeJoin;
use hibitset::{BitIter, BitSetLike};
use crate::world::{Entities, Entity, Index};
/// Like the [`Join`](super::Join) trait except this is similar to a [lending
/// iterator](https://blog.rust-lang.org/2021/08/03/GATs-stabilization-push.html#so-what-are-gats)
/// in that only one item can be accessed at once.
///
/// The type returned from [`.lend_join()`](LendJoin::lend_join),
/// [`JoinLendIter`] does not implement `Iterator` like
/// [`JoinIter`](super::JoinIter) does. Instead, it provides a
/// [`next`](JoinLendIter::next) method that exclusively borrows the
/// `JoinLendIter` for the lifetime of the returned value.
///
/// This limitation allows freedom for more patterns to be soundly implemented.
/// Thus, `LendJoin` acts as the "lowest common denominator" of the
/// `Join`-like traits (i.e. if something can implement `Join` it can also
/// implement `LendJoin`).
///
/// In particular, [`Entries`](crate::storage::Entries) only implements
/// `LendJoin`. As another example,
/// [`RestrictedStorage`](crate::storage::RestrictedStorage) implements both
/// `Join` and `LendJoin`. However, for joining mutably, lend join variant
/// produces
/// [`PairedStorageWriteExclusive`](crate::storage::PairedStorageWriteExclusive)
/// values which have `get_other`/`get_other_mut` methods that aren't provided
/// by [`PairedStorageWriteShared`](crate::storage::PairedStorageWriteShared).
///
/// Finally, these limitations allow providing the [`JoinLendIter::get`] method
/// which can be useful to get a set of components from an entity without
/// calling `get` individually on each storage (see the example in that method's
/// docs).
///
/// Also see the `lend_join` example.
///
/// # Safety
///
/// The `Self::Mask` value returned with the `Self::Value` must correspond such
/// that it is safe to retrieve items from `Self::Value` whose presence is
/// indicated in the mask. As part of this, `BitSetLike::iter` must not produce
/// an iterator that repeats an `Index` value if the `LendJoin::get` impl relies
/// on not being called twice with the same `Index`.
#[nougat::gat]
pub unsafe trait LendJoin {
/// Type of joined components.
///
/// # Note
///
/// This type is using macro magic to emulate GATs on stable. So to refer to
/// it you need to use the [`LendJoinType<'next, J>`](LendJoinType) type
/// alias.
type Type<'next>
where
Self: 'next;
/// Type of joined storages.
type Value;
/// Type of joined bit mask.
type Mask: BitSetLike;
/// Create a joined lending iterator over the contents.
fn lend_join(self) -> JoinLendIter<Self>
where
Self: Sized,
{
JoinLendIter::new(self)
}
/// Returns a structure that implements `Join`/`LendJoin`/`MaybeJoin` if the
/// contained `T` does and that yields all indices, returning `None` for all
/// missing elements and `Some(T)` for found elements.
///
/// To join over and optional component mutably this pattern can be used:
/// `(&mut storage).maybe()`.
///
/// WARNING: Do not have a join of only `MaybeJoin`s. Otherwise the join
/// will iterate over every single index of the bitset. If you want a
/// join with all `MaybeJoin`s, add an `EntitiesRes` to the join as well
/// to bound the join to all entities that are alive.
///
/// ```
/// # use specs::prelude::*;
/// # #[derive(Debug, PartialEq)]
/// # struct Pos { x: i32, y: i32 } impl Component for Pos { type Storage = VecStorage<Self>; }
/// # #[derive(Debug, PartialEq)]
/// # struct Vel { x: i32, y: i32 } impl Component for Vel { type Storage = VecStorage<Self>; }
/// struct ExampleSystem;
/// impl<'a> System<'a> for ExampleSystem {
/// type SystemData = (
/// WriteStorage<'a, Pos>,
/// ReadStorage<'a, Vel>,
/// );
/// fn run(&mut self, (mut positions, velocities): Self::SystemData) {
/// let mut join = (&mut positions, velocities.maybe()).lend_join();
/// while let Some ((mut position, maybe_velocity)) = join.next() {
/// if let Some(velocity) = maybe_velocity {
/// position.x += velocity.x;
/// position.y += velocity.y;
/// }
/// }
/// }
/// }
///
/// fn main() {
/// let mut world = World::new();
/// let mut dispatcher = DispatcherBuilder::new()
/// .with(ExampleSystem, "example_system", &[])
/// .build();
///
/// dispatcher.setup(&mut world);
///
/// let e1 = world.create_entity()
/// .with(Pos { x: 0, y: 0 })
/// .with(Vel { x: 5, y: 2 })
/// .build();
///
/// let e2 = world.create_entity()
/// .with(Pos { x: 0, y: 0 })
/// .build();
///
/// dispatcher.dispatch(&mut world);
///
/// let positions = world.read_storage::<Pos>();
/// assert_eq!(positions.get(e1), Some(&Pos { x: 5, y: 2 }));
/// assert_eq!(positions.get(e2), Some(&Pos { x: 0, y: 0 }));
/// }
/// ```
fn maybe(self) -> MaybeJoin<Self>
where
Self: Sized,
{
MaybeJoin(self)
}
/// Open this join by returning the mask and the storages.
///
/// # Safety
///
/// This is unsafe because implementations of this trait can permit the
/// `Value` to be mutated independently of the `Mask`. If the `Mask` does
/// not correctly report the status of the `Value` then illegal memory
/// access can occur.
unsafe fn open(self) -> (Self::Mask, Self::Value);
/// Get a joined component value by a given index.
///
/// # Safety
///
/// * A call to `get` must be preceded by a check if `id` is part of
/// `Self::Mask`
/// * Multiple calls with the same `id` are not allowed, for a particular
/// instance of the values from [`open`](LendJoin::open). Unless this type
/// implements the unsafe trait [`RepeatableLendGet`].
unsafe fn get<'next>(value: &'next mut Self::Value, id: Index) -> Self::Type<'next>;
/// If this `LendJoin` typically returns all indices in the mask, then
/// iterating over only it or combined with other joins that are also
/// dangerous will cause the `JoinLendIter` to go through all indices which
/// is usually not what is wanted and will kill performance.
#[inline]
fn is_unconstrained() -> bool {
false
}
}
/// # Safety
///
/// Implementing this trait guarantees that `<Self as LendJoin>::get` can
/// soundly be called multiple times with the same ID.
pub unsafe trait RepeatableLendGet: LendJoin {}
/// Type alias to refer to the `<J as LendJoin>::Type<'next>` (except this
/// doesn't actually exist in this form so the `nougat::Gat!` macro is needed).
pub type LendJoinType<'next, J> = nougat::Gat!(<J as LendJoin>::Type<'next>);
/// `JoinLendIter` is an is a lending/streaming iterator over components from a
/// group of storages.
#[must_use]
pub struct JoinLendIter<J: LendJoin> {
keys: BitIter<J::Mask>,
values: J::Value,
}
impl<J: LendJoin> JoinLendIter<J> {
/// Create a new lending join iterator.
pub fn new(j: J) -> Self {
if <J as LendJoin>::is_unconstrained() {
log::warn!(
"`LendJoin` possibly iterating through all indices, \
you might've made a join with all `MaybeJoin`s, \
which is unbounded in length."
);
}
// SAFETY: We do not swap out the mask or the values, nor do we allow it
// by exposing them.
let (keys, values) = unsafe { j.open() };
JoinLendIter {
keys: keys.iter(),
values,
}
}
}
impl<J: LendJoin> JoinLendIter<J> {
/// Lending `next`.
///
/// Can be used to iterate with this pattern:
///
/// `while let Some(components) = join_lending_iter.next() {`
#[allow(clippy::should_implement_trait)] // we want this to look like iterator
pub fn next(&mut self) -> Option<LendJoinType<'_, J>> {
// SAFETY: Since `idx` is yielded from `keys` (the mask), it is
// necessarily a part of it. `LendJoin` requires that the iterator
// doesn't repeat indices and we advance the iterator for each `get`
// call in all methods that don't require `RepeatableLendGet`.
self.keys
.next()
.map(|idx| unsafe { J::get(&mut self.values, idx) })
}
/// Calls a closure on each entity in the join.
pub fn for_each(mut self, mut f: impl FnMut(LendJoinType<'_, J>)) {
self.keys.for_each(|idx| {
// SAFETY: Since `idx` is yielded from `keys` (the mask), it is
// necessarily a part of it. `LendJoin` requires that the iterator
// doesn't repeat indices and we advance the iterator for each `get`
// call in all methods that don't require `RepeatableLendGet`.
let item = unsafe { J::get(&mut self.values, idx) };
f(item);
})
}
/// Allows getting joined values for specific entity.
///
/// ## Example
///
/// ```
/// # use specs::prelude::*;
/// # #[derive(Debug, PartialEq)]
/// # struct Pos; impl Component for Pos { type Storage = VecStorage<Self>; }
/// # #[derive(Debug, PartialEq)]
/// # struct Vel; impl Component for Vel { type Storage = VecStorage<Self>; }
/// let mut world = World::new();
///
/// world.register::<Pos>();
/// world.register::<Vel>();
///
/// // This entity could be stashed anywhere (into `Component`, `Resource`, `System`s data, etc.) as it's just a number.
/// let entity = world
/// .create_entity()
/// .with(Pos)
/// .with(Vel)
/// .build();
///
/// // Later
/// {
/// let mut pos = world.write_storage::<Pos>();
/// let vel = world.read_storage::<Vel>();
///
/// assert_eq!(
/// Some((&mut Pos, &Vel)),
/// (&mut pos, &vel).lend_join().get(entity, &world.entities()),
/// "The entity that was stashed still has the needed components and is alive."
/// );
/// }
///
/// // The entity has found nice spot and doesn't need to move anymore.
/// world.write_storage::<Vel>().remove(entity);
///
/// // Even later
/// {
/// let mut pos = world.write_storage::<Pos>();
/// let vel = world.read_storage::<Vel>();
///
/// assert_eq!(
/// None,
/// (&mut pos, &vel).lend_join().get(entity, &world.entities()),
/// "The entity doesn't have velocity anymore."
/// );
/// }
/// ```
pub fn get(&mut self, entity: Entity, entities: &Entities) -> Option<LendJoinType<'_, J>>
where
J: RepeatableLendGet,
{
if self.keys.contains(entity.id()) && entities.is_alive(entity) {
// SAFETY: the mask (`keys`) is checked as specified in the docs of
// `get`. We require `J: RepeatableJoinGet` so this can be safely
// called multiple time with the same ID.
Some(unsafe { J::get(&mut self.values, entity.id()) })
} else {
None
}
}
/// Allows getting joined values for specific raw index.
///
/// The raw index for an `Entity` can be retrieved using `Entity::id`
/// method.
///
/// As this method operates on raw indices, there is no check to see if the
/// entity is still alive, so the caller should ensure it instead.
///
/// Note: Not checking is still sound (thus this method is safe to call),
/// but this can return data from deleted entities!
pub fn get_unchecked(&mut self, index: Index) -> Option<LendJoinType<'_, J>>
where
J: RepeatableLendGet,
{
if self.keys.contains(index) {
// SAFETY: the mask (`keys`) is checked as specified in the docs of
// `get`. We require `J: RepeatableJoinGet` so this can be safely
// called multiple time with the same ID.
Some(unsafe { J::get(&mut self.values, index) })
} else {
None
}
}
}