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#![doc = include_str!("../README.md")]
#![cfg_attr(feature = "const-new", feature(const_fn_trait_bound))]
use std::{
fmt::{Debug, Formatter},
marker::PhantomData,
sync::{
atomic::{AtomicUsize, Ordering},
Arc, Weak,
},
};
pub type ArcCell<T> = AtomicCell<Arc<T>>;
pub type WeakCell<T> = AtomicCell<Weak<T>>;
pub type OptionalArcCell<T> = AtomicCell<Option<Arc<T>>>;
pub type OptionalWeakCell<T> = AtomicCell<Option<Weak<T>>>;
pub struct AtomicCell<T: AtomicCellStorable> {
value: AtomicUsize,
_marker: PhantomData<T>,
}
impl<T: AtomicCellStorable> AtomicCell<T> {
pub fn new(value: T) -> Self {
AtomicCell {
value: AtomicUsize::new(value.into_value()),
_marker: PhantomData,
}
}
pub fn set(&self, value: T) -> T {
let old = self.internal_take();
self.internal_put(value);
old
}
fn internal_take(&self) -> T {
unsafe {
let mut current = self.value.load(Ordering::SeqCst);
T::from_value(loop {
match self.value.compare_exchange_weak(
current,
T::TAKEN_VALUE,
Ordering::SeqCst,
Ordering::SeqCst,
) {
Ok(val) if val != T::TAKEN_VALUE => break val,
Ok(_) => current = T::TAKEN_VALUE,
Err(new_val) => current = new_val,
}
core::hint::spin_loop();
})
}
}
fn internal_put(&self, value: T) {
let _old = self.value.swap(value.into_value(), Ordering::SeqCst);
debug_assert_eq!(_old, T::TAKEN_VALUE);
}
}
impl<T: AtomicCellStorable> Drop for AtomicCell<T> {
fn drop(&mut self) {
unsafe {
let _ = T::from_value(self.value.load(Ordering::SeqCst));
}
}
}
impl<T: AtomicCellStorable + Clone> AtomicCell<T> {
pub fn get(&self) -> T {
let value = self.internal_take();
let copy = value.clone();
self.internal_put(value);
copy
}
}
impl<T: AtomicCellStorable + Clone> Clone for AtomicCell<T> {
fn clone(&self) -> AtomicCell<T> {
AtomicCell::new(self.get())
}
}
impl<T: AtomicCellStorable + Default> AtomicCell<T> {
pub fn take(&self) -> T {
let new_value = T::default();
let value = self.internal_take();
self.internal_put(new_value);
value
}
}
impl<T: AtomicCellStorable + Default> Default for AtomicCell<T> {
fn default() -> Self {
AtomicCell::new(T::default())
}
}
#[cfg(feature = "const-new")]
impl<T: AtomicCellStorable + AtomicCellConstInit> AtomicCell<T> {
pub const fn const_new() -> Self {
AtomicCell {
value: AtomicUsize::new(T::DEFAULT_VALUE),
_marker: PhantomData,
}
}
}
impl<T> AtomicCell<Weak<T>> {
pub fn empty() -> Self {
AtomicCell::new(Weak::new())
}
pub fn upgrade(&self) -> Option<Arc<T>> {
self.get().upgrade()
}
pub fn store(&self, arc: &Arc<T>) {
self.set(Arc::downgrade(arc));
}
}
impl<T> AtomicCell<Option<Weak<T>>> {
pub fn upgrade(&self) -> Option<Arc<T>> {
self.get().and_then(|weak| weak.upgrade())
}
pub fn store(&self, arc: &Arc<T>) {
self.set(Some(Arc::downgrade(arc)));
}
}
impl<T: AtomicCellStorable + Clone + Debug> Debug for AtomicCell<T> {
fn fmt(&self, fmt: &mut Formatter) -> std::fmt::Result {
fmt.debug_tuple("AtomicCell").field(&self.get()).finish()
}
}
pub unsafe trait AtomicCellStorable {
const TAKEN_VALUE: usize;
fn into_value(self) -> usize;
unsafe fn from_value(value: usize) -> Self;
}
unsafe impl<T> AtomicCellStorable for Arc<T> {
const TAKEN_VALUE: usize = usize::MAX;
fn into_value(self) -> usize {
Arc::into_raw(self) as usize
}
unsafe fn from_value(value: usize) -> Self {
Arc::from_raw(value as *const T)
}
}
unsafe impl<T> AtomicCellStorable for Weak<T> {
const TAKEN_VALUE: usize = usize::MAX - 1;
fn into_value(self) -> usize {
Weak::into_raw(self) as usize
}
unsafe fn from_value(value: usize) -> Self {
Weak::from_raw(value as *const T)
}
}
const EMPTY_OPTION: usize = 0;
unsafe impl<T> AtomicCellStorable for Option<Arc<T>> {
const TAKEN_VALUE: usize = <Arc<T> as AtomicCellStorable>::TAKEN_VALUE;
fn into_value(self) -> usize {
match self {
None => EMPTY_OPTION,
Some(arc) => Arc::into_raw(arc) as usize,
}
}
unsafe fn from_value(value: usize) -> Self {
match value {
EMPTY_OPTION => None,
value => Some(Arc::from_raw(value as *const T)),
}
}
}
unsafe impl<T> AtomicCellStorable for Option<Weak<T>> {
const TAKEN_VALUE: usize = <Weak<T> as AtomicCellStorable>::TAKEN_VALUE;
fn into_value(self) -> usize {
match self {
None => EMPTY_OPTION,
Some(arc) => Weak::into_raw(arc) as usize,
}
}
unsafe fn from_value(value: usize) -> Self {
match value {
EMPTY_OPTION => None,
value => Some(Weak::from_raw(value as *const T)),
}
}
}
pub unsafe trait AtomicCellConstInit {
const DEFAULT_VALUE: usize;
}
unsafe impl<T> AtomicCellConstInit for Option<Arc<T>> {
const DEFAULT_VALUE: usize = EMPTY_OPTION;
}
unsafe impl<T> AtomicCellConstInit for Option<Weak<T>> {
const DEFAULT_VALUE: usize = EMPTY_OPTION;
}
#[cfg(test)]
mod tests {
use crate::{ArcCell, WeakCell};
use std::sync::{
atomic::{AtomicUsize, Ordering},
Arc,
};
#[test]
fn arc_cell() {
let data1 = Arc::new(5);
let data2 = Arc::new(6);
let cell = ArcCell::new(data1);
assert_eq!(*cell.get(), 5);
cell.set(data2);
assert_eq!(*cell.get(), 6);
}
#[test]
fn weak_cell() {
let data = Arc::new(5);
let cell = WeakCell::empty();
cell.store(&data);
assert_eq!(cell.upgrade(), Some(data.clone()));
drop(data);
assert_eq!(cell.upgrade(), None);
}
#[test]
fn cell_drops() {
static DROPS: AtomicUsize = AtomicUsize::new(0);
struct DropCount;
impl std::ops::Drop for DropCount {
fn drop(&mut self) {
DROPS.fetch_add(1, Ordering::SeqCst);
}
}
{
let _cell = ArcCell::new(Arc::new(DropCount));
}
assert_eq!(DROPS.load(Ordering::SeqCst), 1);
}
}