blob: c6c007d3678939e84b1ffbef55816eec271ee69c [file] [log] [blame]
use std::cell::UnsafeCell;
use std::mem::MaybeUninit;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use crate::{PopError, PushError};
const LOCKED: usize = 1 << 0;
const PUSHED: usize = 1 << 1;
const CLOSED: usize = 1 << 2;
/// A single-element queue.
pub struct Single<T> {
state: AtomicUsize,
slot: UnsafeCell<MaybeUninit<T>>,
}
impl<T> Single<T> {
/// Creates a new single-element queue.
pub fn new() -> Single<T> {
Single {
state: AtomicUsize::new(0),
slot: UnsafeCell::new(MaybeUninit::uninit()),
}
}
/// Attempts to push an item into the queue.
pub fn push(&self, value: T) -> Result<(), PushError<T>> {
// Lock and fill the slot.
let state = self
.state
.compare_and_swap(0, LOCKED | PUSHED, Ordering::SeqCst);
if state == 0 {
// Write the value and unlock.
unsafe { self.slot.get().write(MaybeUninit::new(value)) }
self.state.fetch_and(!LOCKED, Ordering::Release);
Ok(())
} else if state & CLOSED != 0 {
Err(PushError::Closed(value))
} else {
Err(PushError::Full(value))
}
}
/// Attempts to pop an item from the queue.
pub fn pop(&self) -> Result<T, PopError> {
let mut state = PUSHED;
loop {
// Lock and empty the slot.
let prev =
self.state
.compare_and_swap(state, (state | LOCKED) & !PUSHED, Ordering::SeqCst);
if prev == state {
// Read the value and unlock.
let value = unsafe { self.slot.get().read().assume_init() };
self.state.fetch_and(!LOCKED, Ordering::Release);
return Ok(value);
}
if prev & PUSHED == 0 {
if prev & CLOSED == 0 {
return Err(PopError::Empty);
} else {
return Err(PopError::Closed);
}
}
if prev & LOCKED == 0 {
state = prev;
} else {
thread::yield_now();
state = prev & !LOCKED;
}
}
}
/// Returns the number of items in the queue.
pub fn len(&self) -> usize {
if self.state.load(Ordering::SeqCst) & PUSHED == 0 {
0
} else {
1
}
}
/// Returns `true` if the queue is empty.
pub fn is_empty(&self) -> bool {
self.len() == 0
}
/// Returns `true` if the queue is full.
pub fn is_full(&self) -> bool {
self.len() == 1
}
/// Closes the queue.
///
/// Returns `true` if this call closed the queue.
pub fn close(&self) -> bool {
let state = self.state.fetch_or(CLOSED, Ordering::SeqCst);
state & CLOSED == 0
}
/// Returns `true` if the queue is closed.
pub fn is_closed(&self) -> bool {
self.state.load(Ordering::SeqCst) & CLOSED != 0
}
}
impl<T> Drop for Single<T> {
fn drop(&mut self) {
// Drop the value in the slot.
if *self.state.get_mut() & PUSHED != 0 {
let value = unsafe { self.slot.get().read().assume_init() };
drop(value);
}
}
}