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//! Cache computations and efficiently reuse them.
use std::cell::RefCell;
use std::fmt;
use std::mem;
use std::sync::atomic::{self, AtomicU64};
/// A simple cache that stores generated values to avoid recomputation.
///
/// Keeps track of the last generated value after clearing.
pub struct Cache<T> {
group: Group,
state: RefCell<State<T>>,
}
impl<T> Cache<T> {
/// Creates a new empty [`Cache`].
pub fn new() -> Self {
Cache {
group: Group::singleton(),
state: RefCell::new(State::Empty { previous: None }),
}
}
/// Creates a new empty [`Cache`] with the given [`Group`].
///
/// Caches within the same group may reuse internal rendering storage.
///
/// You should generally group caches that are likely to change
/// together.
pub fn with_group(group: Group) -> Self {
assert!(
!group.is_singleton(),
"The group {group:?} cannot be shared!"
);
Cache {
group,
state: RefCell::new(State::Empty { previous: None }),
}
}
/// Returns the [`Group`] of the [`Cache`].
pub fn group(&self) -> Group {
self.group
}
/// Puts the given value in the [`Cache`].
///
/// Notice that, given this is a cache, a mutable reference is not
/// necessary to call this method. You can safely update the cache in
/// rendering code.
pub fn put(&self, value: T) {
*self.state.borrow_mut() = State::Filled { current: value };
}
/// Returns a reference cell to the internal [`State`] of the [`Cache`].
pub fn state(&self) -> &RefCell<State<T>> {
&self.state
}
/// Clears the [`Cache`].
pub fn clear(&self) {
let mut state = self.state.borrow_mut();
let previous =
mem::replace(&mut *state, State::Empty { previous: None });
let previous = match previous {
State::Empty { previous } => previous,
State::Filled { current } => Some(current),
};
*state = State::Empty { previous };
}
}
/// A cache group.
///
/// Caches that share the same group generally change together.
///
/// A cache group can be used to implement certain performance
/// optimizations during rendering, like batching or sharing atlases.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Group {
id: u64,
is_singleton: bool,
}
impl Group {
/// Generates a new unique cache [`Group`].
pub fn unique() -> Self {
static NEXT: AtomicU64 = AtomicU64::new(0);
Self {
id: NEXT.fetch_add(1, atomic::Ordering::Relaxed),
is_singleton: false,
}
}
/// Returns `true` if the [`Group`] can only ever have a
/// single [`Cache`] in it.
///
/// This is the default kind of [`Group`] assigned when using
/// [`Cache::new`].
///
/// Knowing that a [`Group`] will never be shared may be
/// useful for rendering backends to perform additional
/// optimizations.
pub fn is_singleton(self) -> bool {
self.is_singleton
}
fn singleton() -> Self {
Self {
is_singleton: true,
..Self::unique()
}
}
}
impl<T> fmt::Debug for Cache<T>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
use std::ops::Deref;
let state = self.state.borrow();
match state.deref() {
State::Empty { previous } => {
write!(f, "Cache::Empty {{ previous: {previous:?} }}")
}
State::Filled { current } => {
write!(f, "Cache::Filled {{ current: {current:?} }}")
}
}
}
}
impl<T> Default for Cache<T> {
fn default() -> Self {
Self::new()
}
}
/// The state of a [`Cache`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum State<T> {
/// The [`Cache`] is empty.
Empty {
/// The previous value of the [`Cache`].
previous: Option<T>,
},
/// The [`Cache`] is filled.
Filled {
/// The current value of the [`Cache`]
current: T,
},
}
/// A piece of data that can be cached.
pub trait Cached: Sized {
/// The type of cache produced.
type Cache: Clone;
/// Loads the [`Cache`] into a proper instance.
///
/// [`Cache`]: Self::Cache
fn load(cache: &Self::Cache) -> Self;
/// Caches this value, producing its corresponding [`Cache`].
///
/// [`Cache`]: Self::Cache
fn cache(self, group: Group, previous: Option<Self::Cache>) -> Self::Cache;
}
#[cfg(debug_assertions)]
impl Cached for () {
type Cache = ();
fn load(_cache: &Self::Cache) -> Self {}
fn cache(
self,
_group: Group,
_previous: Option<Self::Cache>,
) -> Self::Cache {
}
}