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