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use std::{
ptr::null_mut,
slice::from_raw_parts_mut,
sync::{
Mutex,
atomic::{AtomicPtr, Ordering},
},
};
const BUCKETS: usize = (usize::BITS + 1) as usize;
/// An `Option`-like type that guarantees that a fully zeroed value is a valid
/// `None` variant.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[repr(u8)]
enum COption<T> {
// TODO(alexkirsz) We need a way to guarantee that a fully zeroed value is a
// valid `None` variant. This is theoretically possible when the wrapped
// type has no valid value that can be represented by all zeros, but there
// is no way to enforce this at the type level. For now, we just use a custom
// option type with explicit discriminant for the `None` variant.
// The issue with this implementation is that it disables niche optimization.
None = 0,
Some(T),
}
impl<T> Default for COption<T> {
fn default() -> Self {
Self::None
}
}
impl<T> COption<T> {
/// Returns a slice of the given size filled with the `None` variant.
fn new_none_slice(size: usize) -> Box<[Self]> {
let slice = Box::<[COption<T>]>::new_zeroed_slice(size);
// Safety:
// We know that a zeroed COption<T> is a valid COption::None value.
unsafe { slice.assume_init() }
}
/// Returns a reference to the contained value, or `None` if it is `None`.
fn as_option_ref(&self) -> Option<&T> {
match self {
COption::None => None,
COption::Some(t) => Some(t),
}
}
}
pub struct NoMoveVec<T, const INITIAL_CAPACITY_BITS: u32 = 6> {
buckets: [(AtomicPtr<COption<T>>, Mutex<()>); BUCKETS],
}
fn get_bucket_index<const INITIAL_CAPACITY_BITS: u32>(idx: usize) -> u32 {
(usize::BITS - idx.leading_zeros()).saturating_sub(INITIAL_CAPACITY_BITS)
}
fn get_bucket_size<const INITIAL_CAPACITY_BITS: u32>(bucket_index: u32) -> usize {
if bucket_index != 0 {
1 << (bucket_index + INITIAL_CAPACITY_BITS - 1)
} else {
1 << INITIAL_CAPACITY_BITS
}
}
fn get_index_in_bucket<const INITIAL_CAPACITY_BITS: u32>(idx: usize, bucket_index: u32) -> usize {
if bucket_index != 0 {
idx ^ (1 << (bucket_index + INITIAL_CAPACITY_BITS - 1))
} else {
idx
}
}
/// Allocates a new bucket of `COption<T>`s, all initialized to `None`.
fn allocate_bucket<const INITIAL_CAPACITY_BITS: u32, T>(bucket_index: u32) -> *mut COption<T> {
let size = get_bucket_size::<INITIAL_CAPACITY_BITS>(bucket_index);
let slice = COption::<T>::new_none_slice(size);
Box::into_raw(slice) as *mut COption<T>
}
impl<T, const INITIAL_CAPACITY_BITS: u32> Default for NoMoveVec<T, INITIAL_CAPACITY_BITS> {
fn default() -> Self {
Self::new()
}
}
impl<T, const INITIAL_CAPACITY_BITS: u32> NoMoveVec<T, INITIAL_CAPACITY_BITS> {
pub fn new() -> Self {
let mut buckets = [null_mut(); BUCKETS];
buckets[0] = allocate_bucket::<INITIAL_CAPACITY_BITS, T>(0);
let buckets = buckets.map(|p| (AtomicPtr::new(p), Mutex::new(())));
NoMoveVec { buckets }
}
pub fn get(&self, idx: usize) -> Option<&T> {
let bucket_idx = get_bucket_index::<INITIAL_CAPACITY_BITS>(idx);
let bucket_ptr = unsafe { self.buckets.get_unchecked(bucket_idx as usize) }
.0
.load(Ordering::Acquire);
if bucket_ptr.is_null() {
return None;
}
let index = get_index_in_bucket::<INITIAL_CAPACITY_BITS>(idx, bucket_idx);
unsafe { &*bucket_ptr.add(index) }.as_option_ref()
}
/// # Safety
/// There must not be a concurrent operation to this idx
pub unsafe fn insert(&self, idx: usize, value: T) -> &T {
let bucket_idx = get_bucket_index::<INITIAL_CAPACITY_BITS>(idx);
let bucket = unsafe { self.buckets.get_unchecked(bucket_idx as usize) };
// SAFETY: This is safe to be relaxed as the bucket will never become null
// again. We perform a acquire load when it's null.
let mut bucket_ptr = bucket.0.load(Ordering::Relaxed);
if bucket_ptr.is_null() {
bucket_ptr = bucket.0.load(Ordering::Acquire);
if bucket_ptr.is_null() {
let lock = bucket.1.lock();
let guarded_bucket_ptr = bucket.0.load(Ordering::Acquire);
if guarded_bucket_ptr.is_null() {
let new_bucket = allocate_bucket::<INITIAL_CAPACITY_BITS, T>(bucket_idx);
bucket_ptr = match bucket.0.compare_exchange(
null_mut(),
new_bucket,
Ordering::AcqRel,
Ordering::Relaxed,
) {
Ok(_) => new_bucket,
Err(current_bucket) => {
drop(unsafe { Box::from_raw(new_bucket) });
current_bucket
}
};
drop(lock);
} else {
bucket_ptr = guarded_bucket_ptr;
}
}
}
let index = get_index_in_bucket::<INITIAL_CAPACITY_BITS>(idx, bucket_idx);
let item = unsafe { &mut *bucket_ptr.add(index) };
*item = COption::Some(value);
// To sync with any acquire load of the bucket ptr
bucket.0.store(bucket_ptr, Ordering::Release);
item.as_option_ref().unwrap()
}
}
impl<T, const INITIAL_CAPACITY_BITS: u32> Drop for NoMoveVec<T, INITIAL_CAPACITY_BITS> {
fn drop(&mut self) {
for (bucket_index, (bucket, _)) in self.buckets.iter_mut().enumerate() {
if bucket_index < (usize::BITS + 1 - INITIAL_CAPACITY_BITS) as usize {
let bucket_size = get_bucket_size::<INITIAL_CAPACITY_BITS>(bucket_index as u32);
let bucket_ptr = *bucket.get_mut();
if !bucket_ptr.is_null() {
drop(unsafe { Box::from_raw(from_raw_parts_mut(bucket_ptr, bucket_size)) });
}
}
}
}
}
#[cfg(test)]
mod tests {
use super::NoMoveVec;
#[test]
fn basic_operations() {
let v = NoMoveVec::<(usize, usize)>::new();
assert_eq!(v.get(0), None);
assert_eq!(v.get(1), None);
assert_eq!(v.get(8), None);
assert_eq!(v.get(9), None);
assert_eq!(v.get(15), None);
assert_eq!(v.get(16), None);
assert_eq!(v.get(100), None);
assert_eq!(v.get(1000), None);
for i in 0..1000 {
unsafe {
v.insert(i, (i, i));
}
assert_eq!(v.get(i), Some(&(i, i)));
}
for i in 0..1000 {
assert_eq!(v.get(i), Some(&(i, i)));
}
assert_eq!(v.get(1001), None);
unsafe {
v.insert(1000000, (0, 0));
}
assert_eq!(v.get(1000000), Some(&(0, 0)));
assert_eq!(v.get(10000), None);
}
}
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