1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286
//! Provides types used to statically intern immutable values.
//!
//! Interning is a pattern used to save memory by deduplicating identical values,
//! speed up code by shrinking the stack size of large types,
//! and make comparisons for any type as fast as integers.
use std::{
fmt::Debug,
hash::Hash,
ops::Deref,
sync::{OnceLock, PoisonError, RwLock},
};
use bevy_utils::HashSet;
/// An interned value. Will stay valid until the end of the program and will not drop.
///
/// For details on interning, see [the module level docs](self).
///
/// # Comparisons
///
/// Interned values use reference equality, meaning they implement [`Eq`]
/// and [`Hash`] regardless of whether `T` implements these traits.
/// Two interned values are only guaranteed to compare equal if they were interned using
/// the same [`Interner`] instance.
// NOTE: This type must NEVER implement Borrow since it does not obey that trait's invariants.
///
/// ```
/// # use bevy_ecs::intern::*;
/// #[derive(PartialEq, Eq, Hash, Debug)]
/// struct Value(i32);
/// impl Internable for Value {
/// // ...
/// # fn leak(&self) -> &'static Self { Box::leak(Box::new(Value(self.0))) }
/// # fn ref_eq(&self, other: &Self) -> bool { std::ptr::eq(self, other ) }
/// # fn ref_hash<H: std::hash::Hasher>(&self, state: &mut H) { std::ptr::hash(self, state); }
/// }
/// let interner_1 = Interner::new();
/// let interner_2 = Interner::new();
/// // Even though both values are identical, their interned forms do not
/// // compare equal as they use different interner instances.
/// assert_ne!(interner_1.intern(&Value(42)), interner_2.intern(&Value(42)));
/// ```
pub struct Interned<T: ?Sized + 'static>(pub &'static T);
impl<T: ?Sized> Deref for Interned<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.0
}
}
impl<T: ?Sized> Clone for Interned<T> {
fn clone(&self) -> Self {
*self
}
}
impl<T: ?Sized> Copy for Interned<T> {}
// Two Interned<T> should only be equal if they are clones from the same instance.
// Therefore, we only use the pointer to determine equality.
impl<T: ?Sized + Internable> PartialEq for Interned<T> {
fn eq(&self, other: &Self) -> bool {
self.0.ref_eq(other.0)
}
}
impl<T: ?Sized + Internable> Eq for Interned<T> {}
// Important: This must be kept in sync with the PartialEq/Eq implementation
impl<T: ?Sized + Internable> Hash for Interned<T> {
fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
self.0.ref_hash(state);
}
}
impl<T: ?Sized + Debug> Debug for Interned<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
self.0.fmt(f)
}
}
impl<T> From<&Interned<T>> for Interned<T> {
fn from(value: &Interned<T>) -> Self {
*value
}
}
/// A trait for internable values.
///
/// This is used by [`Interner<T>`] to create static references for values that are interned.
pub trait Internable: Hash + Eq {
/// Creates a static reference to `self`, possibly leaking memory.
fn leak(&self) -> &'static Self;
/// Returns `true` if the two references point to the same value.
fn ref_eq(&self, other: &Self) -> bool;
/// Feeds the reference to the hasher.
fn ref_hash<H: std::hash::Hasher>(&self, state: &mut H);
}
impl Internable for str {
fn leak(&self) -> &'static Self {
let str = self.to_owned().into_boxed_str();
Box::leak(str)
}
fn ref_eq(&self, other: &Self) -> bool {
self.as_ptr() == other.as_ptr() && self.len() == other.len()
}
fn ref_hash<H: std::hash::Hasher>(&self, state: &mut H) {
self.len().hash(state);
self.as_ptr().hash(state);
}
}
/// A thread-safe interner which can be used to create [`Interned<T>`] from `&T`
///
/// For details on interning, see [the module level docs](self).
///
/// The implementation ensures that two equal values return two equal [`Interned<T>`] values.
///
/// To use an [`Interner<T>`], `T` must implement [`Internable`].
pub struct Interner<T: ?Sized + 'static>(OnceLock<RwLock<HashSet<&'static T>>>);
impl<T: ?Sized> Interner<T> {
/// Creates a new empty interner
pub const fn new() -> Self {
Self(OnceLock::new())
}
}
impl<T: Internable + ?Sized> Interner<T> {
/// Return the [`Interned<T>`] corresponding to `value`.
///
/// If it is called the first time for `value`, it will possibly leak the value and return an
/// [`Interned<T>`] using the obtained static reference. Subsequent calls for the same `value`
/// will return [`Interned<T>`] using the same static reference.
pub fn intern(&self, value: &T) -> Interned<T> {
let lock = self.0.get_or_init(Default::default);
{
let set = lock.read().unwrap_or_else(PoisonError::into_inner);
if let Some(value) = set.get(value) {
return Interned(*value);
}
}
{
let mut set = lock.write().unwrap_or_else(PoisonError::into_inner);
if let Some(value) = set.get(value) {
Interned(*value)
} else {
let leaked = value.leak();
set.insert(leaked);
Interned(leaked)
}
}
}
}
impl<T: ?Sized> Default for Interner<T> {
fn default() -> Self {
Self::new()
}
}
#[cfg(test)]
mod tests {
use std::{
collections::hash_map::DefaultHasher,
hash::{Hash, Hasher},
};
use crate::intern::{Internable, Interned, Interner};
#[test]
fn zero_sized_type() {
#[derive(PartialEq, Eq, Hash, Debug)]
pub struct A;
impl Internable for A {
fn leak(&self) -> &'static Self {
&A
}
fn ref_eq(&self, other: &Self) -> bool {
std::ptr::eq(self, other)
}
fn ref_hash<H: Hasher>(&self, state: &mut H) {
std::ptr::hash(self, state);
}
}
let interner = Interner::default();
let x = interner.intern(&A);
let y = interner.intern(&A);
assert_eq!(x, y);
}
#[test]
fn fieldless_enum() {
#[derive(PartialEq, Eq, Hash, Debug, Clone)]
pub enum A {
X,
Y,
}
impl Internable for A {
fn leak(&self) -> &'static Self {
match self {
A::X => &A::X,
A::Y => &A::Y,
}
}
fn ref_eq(&self, other: &Self) -> bool {
std::ptr::eq(self, other)
}
fn ref_hash<H: Hasher>(&self, state: &mut H) {
std::ptr::hash(self, state);
}
}
let interner = Interner::default();
let x = interner.intern(&A::X);
let y = interner.intern(&A::Y);
assert_ne!(x, y);
}
#[test]
fn static_sub_strings() {
let str = "ABC ABC";
let a = &str[0..3];
let b = &str[4..7];
// Same contents
assert_eq!(a, b);
let x = Interned(a);
let y = Interned(b);
// Different pointers
assert_ne!(x, y);
let interner = Interner::default();
let x = interner.intern(a);
let y = interner.intern(b);
// Same pointers returned by interner
assert_eq!(x, y);
}
#[test]
fn same_interned_instance() {
let a = Interned("A");
let b = a;
assert_eq!(a, b);
let mut hasher = DefaultHasher::default();
a.hash(&mut hasher);
let hash_a = hasher.finish();
let mut hasher = DefaultHasher::default();
b.hash(&mut hasher);
let hash_b = hasher.finish();
assert_eq!(hash_a, hash_b);
}
#[test]
fn same_interned_content() {
let a = Interned::<str>(Box::leak(Box::new("A".to_string())));
let b = Interned::<str>(Box::leak(Box::new("A".to_string())));
assert_ne!(a, b);
}
#[test]
fn different_interned_content() {
let a = Interned::<str>("A");
let b = Interned::<str>("B");
assert_ne!(a, b);
}
}