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use std::cell::UnsafeCell;
use std::mem::MaybeUninit;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use cache_padded::CachePadded;
use crate::{PopError, PushError};
/// A slot in a queue.
struct Slot<T> {
/// The current stamp.
stamp: AtomicUsize,
/// The value in this slot.
value: UnsafeCell<MaybeUninit<T>>,
}
/// A bounded queue.
pub struct Bounded<T> {
/// The head of the queue.
///
/// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
/// packed into a single `usize`. The lower bits represent the index, while the upper bits
/// represent the lap. The mark bit in the head is always zero.
///
/// Values are popped from the head of the queue.
head: CachePadded<AtomicUsize>,
/// The tail of the queue.
///
/// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
/// packed into a single `usize`. The lower bits represent the index, while the upper bits
/// represent the lap. The mark bit indicates that the queue is closed.
///
/// Values are pushed into the tail of the queue.
tail: CachePadded<AtomicUsize>,
/// The buffer holding slots.
buffer: Box<[Slot<T>]>,
/// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`.
one_lap: usize,
/// If this bit is set in the tail, that means the queue is closed.
mark_bit: usize,
}
impl<T> Bounded<T> {
/// Creates a new bounded queue.
pub fn new(cap: usize) -> Bounded<T> {
assert!(cap > 0, "capacity must be positive");
// Head is initialized to `{ lap: 0, mark: 0, index: 0 }`.
let head = 0;
// Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`.
let tail = 0;
// Allocate a buffer of `cap` slots initialized with stamps.
let mut buffer = Vec::with_capacity(cap);
for i in 0..cap {
// Set the stamp to `{ lap: 0, mark: 0, index: i }`.
buffer.push(Slot {
stamp: AtomicUsize::new(i),
value: UnsafeCell::new(MaybeUninit::uninit()),
});
}
// Compute constants `mark_bit` and `one_lap`.
let mark_bit = (cap + 1).next_power_of_two();
let one_lap = mark_bit * 2;
Bounded {
buffer: buffer.into(),
one_lap,
mark_bit,
head: CachePadded::new(AtomicUsize::new(head)),
tail: CachePadded::new(AtomicUsize::new(tail)),
}
}
/// Attempts to push an item into the queue.
pub fn push(&self, value: T) -> Result<(), PushError<T>> {
let mut tail = self.tail.load(Ordering::Relaxed);
loop {
// Check if the queue is closed.
if tail & self.mark_bit != 0 {
return Err(PushError::Closed(value));
}
// Deconstruct the tail.
let index = tail & (self.mark_bit - 1);
let lap = tail & !(self.one_lap - 1);
// Inspect the corresponding slot.
let slot = &self.buffer[index];
let stamp = slot.stamp.load(Ordering::Acquire);
// If the tail and the stamp match, we may attempt to push.
if tail == stamp {
let new_tail = if index + 1 < self.buffer.len() {
// Same lap, incremented index.
// Set to `{ lap: lap, mark: 0, index: index + 1 }`.
tail + 1
} else {
// One lap forward, index wraps around to zero.
// Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
lap.wrapping_add(self.one_lap)
};
// Try moving the tail.
match self.tail.compare_exchange_weak(
tail,
new_tail,
Ordering::SeqCst,
Ordering::Relaxed,
) {
Ok(_) => {
// Write the value into the slot and update the stamp.
unsafe {
slot.value.get().write(MaybeUninit::new(value));
}
slot.stamp.store(tail + 1, Ordering::Release);
return Ok(());
}
Err(t) => {
tail = t;
}
}
} else if stamp.wrapping_add(self.one_lap) == tail + 1 {
crate::full_fence();
let head = self.head.load(Ordering::Relaxed);
// If the head lags one lap behind the tail as well...
if head.wrapping_add(self.one_lap) == tail {
// ...then the queue is full.
return Err(PushError::Full(value));
}
tail = self.tail.load(Ordering::Relaxed);
} else {
// Yield because we need to wait for the stamp to get updated.
thread::yield_now();
tail = self.tail.load(Ordering::Relaxed);
}
}
}
/// Attempts to pop an item from the queue.
pub fn pop(&self) -> Result<T, PopError> {
let mut head = self.head.load(Ordering::Relaxed);
loop {
// Deconstruct the head.
let index = head & (self.mark_bit - 1);
let lap = head & !(self.one_lap - 1);
// Inspect the corresponding slot.
let slot = &self.buffer[index];
let stamp = slot.stamp.load(Ordering::Acquire);
// If the the stamp is ahead of the head by 1, we may attempt to pop.
if head + 1 == stamp {
let new = if index + 1 < self.buffer.len() {
// Same lap, incremented index.
// Set to `{ lap: lap, mark: 0, index: index + 1 }`.
head + 1
} else {
// One lap forward, index wraps around to zero.
// Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
lap.wrapping_add(self.one_lap)
};
// Try moving the head.
match self.head.compare_exchange_weak(
head,
new,
Ordering::SeqCst,
Ordering::Relaxed,
) {
Ok(_) => {
// Read the value from the slot and update the stamp.
let value = unsafe { slot.value.get().read().assume_init() };
slot.stamp
.store(head.wrapping_add(self.one_lap), Ordering::Release);
return Ok(value);
}
Err(h) => {
head = h;
}
}
} else if stamp == head {
crate::full_fence();
let tail = self.tail.load(Ordering::Relaxed);
// If the tail equals the head, that means the queue is empty.
if (tail & !self.mark_bit) == head {
// Check if the queue is closed.
if tail & self.mark_bit != 0 {
return Err(PopError::Closed);
} else {
return Err(PopError::Empty);
}
}
head = self.head.load(Ordering::Relaxed);
} else {
// Yield because we need to wait for the stamp to get updated.
thread::yield_now();
head = self.head.load(Ordering::Relaxed);
}
}
}
/// Returns the number of items in the queue.
pub fn len(&self) -> usize {
loop {
// Load the tail, then load the head.
let tail = self.tail.load(Ordering::SeqCst);
let head = self.head.load(Ordering::SeqCst);
// If the tail didn't change, we've got consistent values to work with.
if self.tail.load(Ordering::SeqCst) == tail {
let hix = head & (self.mark_bit - 1);
let tix = tail & (self.mark_bit - 1);
return if hix < tix {
tix - hix
} else if hix > tix {
self.buffer.len() - hix + tix
} else if (tail & !self.mark_bit) == head {
0
} else {
self.buffer.len()
};
}
}
}
/// Returns `true` if the queue is empty.
pub fn is_empty(&self) -> bool {
let head = self.head.load(Ordering::SeqCst);
let tail = self.tail.load(Ordering::SeqCst);
// Is the tail equal to the head?
//
// Note: If the head changes just before we load the tail, that means there was a moment
// when the queue was not empty, so it is safe to just return `false`.
(tail & !self.mark_bit) == head
}
/// Returns `true` if the queue is full.
pub fn is_full(&self) -> bool {
let tail = self.tail.load(Ordering::SeqCst);
let head = self.head.load(Ordering::SeqCst);
// Is the head lagging one lap behind tail?
//
// Note: If the tail changes just before we load the head, that means there was a moment
// when the queue was not full, so it is safe to just return `false`.
head.wrapping_add(self.one_lap) == tail & !self.mark_bit
}
/// Returns the capacity of the queue.
pub fn capacity(&self) -> usize {
self.buffer.len()
}
/// Closes the queue.
///
/// Returns `true` if this call closed the queue.
pub fn close(&self) -> bool {
let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
tail & self.mark_bit == 0
}
/// Returns `true` if the queue is closed.
pub fn is_closed(&self) -> bool {
self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
}
}
impl<T> Drop for Bounded<T> {
fn drop(&mut self) {
// Get the index of the head.
let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1);
// Loop over all slots that hold a value and drop them.
for i in 0..self.len() {
// Compute the index of the next slot holding a value.
let index = if hix + i < self.buffer.len() {
hix + i
} else {
hix + i - self.buffer.len()
};
// Drop the value in the slot.
let slot = &self.buffer[index];
unsafe {
let value = slot.value.get().read().assume_init();
drop(value);
}
}
}
}