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use super::ebr::{Arc, AtomicArc, Barrier, Ptr, Tag};
use std::sync::atomic::Ordering::{self, Relaxed, Release};
/// [`LinkedList`] is a type trait implementing a lock-free singly linked list.
pub trait LinkedList: 'static + Sized {
/// Returns a reference to the forward link.
///
/// The pointer value may be tagged if [`Self::mark`] or [`Self::delete_self`] has been
/// invoked.
fn link_ref(&self) -> &AtomicArc<Self>;
/// Returns `true` if `self` is reachable and not marked.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::{AtomicArc, Tag};
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let head: L = L::default();
/// assert!(head.is_clear(Relaxed));
/// assert!(head.mark(Relaxed));
/// assert!(!head.is_clear(Relaxed));
/// assert!(head.delete_self(Relaxed));
/// assert!(!head.is_clear(Relaxed));
/// ```
fn is_clear(&self, order: Ordering) -> bool {
self.link_ref().tag(order) == Tag::None
}
/// Marks `self` with an internal flag to denote that `self` is in a special state.
///
/// It returns `false` if a flag has already been marked on `self`.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::AtomicArc;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let head: L = L::default();
/// assert!(head.mark(Relaxed));
/// ```
fn mark(&self, order: Ordering) -> bool {
self.link_ref()
.update_tag_if(Tag::First, |t| t == Tag::None, order)
}
/// Removes the mark from `self`.
///
/// It returns `false` if no flag has been marked on `self`.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::AtomicArc;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let head: L = L::default();
/// assert!(!head.unmark(Relaxed));
/// assert!(head.mark(Relaxed));
/// assert!(head.unmark(Relaxed));
/// assert!(!head.is_marked(Relaxed));
/// ```
fn unmark(&self, order: Ordering) -> bool {
self.link_ref()
.update_tag_if(Tag::None, |t| t == Tag::First, order)
}
/// Returns `true` if `self` has a mark on it.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::AtomicArc;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let head: L = L::default();
/// assert!(!head.is_marked(Relaxed));
/// assert!(head.mark(Relaxed));
/// assert!(head.is_marked(Relaxed));
/// ```
fn is_marked(&self, order: Ordering) -> bool {
self.link_ref().tag(order) == Tag::First
}
/// Deletes `self`.
///
/// It returns `false` if `self` already has `deleted` marked on it.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::{Arc, AtomicArc, Barrier};
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let barrier = Barrier::new();
///
/// let head: L = L::default();
/// let tail: Arc<L> = Arc::new(L::default());
/// assert!(head.push_back(tail.clone(), false, Relaxed, &barrier).is_ok());
///
/// tail.delete_self(Relaxed);
/// assert!(head.next_ptr(Relaxed, &barrier).as_ref().is_none());
/// ```
fn delete_self(&self, order: Ordering) -> bool {
self.link_ref()
.update_tag_if(Tag::Second, |t| t != Tag::Second, order)
}
/// Returns `true` if `self` has been deleted.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::AtomicArc;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let entry: L = L::default();
/// assert!(!entry.is_deleted(Relaxed));
/// entry.delete_self(Relaxed);
/// assert!(entry.is_deleted(Relaxed));
/// ```
fn is_deleted(&self, order: Ordering) -> bool {
self.link_ref().tag(order) == Tag::Second
}
/// Appends the given entry after `self` and returns a pointer to the entry.
///
/// If `mark` is given `true`, it atomically marks an internal flag on `self` when updating
/// the linked list, otherwise it removes marks.
///
/// # Errors
///
/// It returns the supplied [`Arc`] when it finds `self` deleted.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::{Arc, AtomicArc, Barrier};
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let barrier = Barrier::new();
///
/// let head: L = L::default();
/// assert!(head.push_back(Arc::new(L::default()), true, Relaxed, &barrier).is_ok());
/// assert!(head.is_marked(Relaxed));
/// assert!(head.push_back(Arc::new(L::default()), false, Relaxed, &barrier).is_ok());
/// assert!(!head.is_marked(Relaxed));
///
/// head.delete_self(Relaxed);
/// assert!(!head.is_marked(Relaxed));
/// assert!(head.push_back(Arc::new(L::default()), false, Relaxed, &barrier).is_err());
/// ```
fn push_back<'b>(
&self,
mut entry: Arc<Self>,
mark: bool,
order: Ordering,
barrier: &'b Barrier,
) -> Result<Ptr<'b, Self>, Arc<Self>> {
let new_tag = if mark { Tag::First } else { Tag::None };
let mut next_ptr = self.link_ref().load(Relaxed, barrier);
while next_ptr.tag() != Tag::Second {
entry
.link_ref()
.swap((next_ptr.get_arc(), Tag::None), Relaxed);
match self.link_ref().compare_exchange(
next_ptr,
(Some(entry), new_tag),
order,
Relaxed,
barrier,
) {
Ok((_, updated)) => {
return Ok(updated);
}
Err((passed, actual)) => {
entry = passed.unwrap();
next_ptr = actual;
}
}
}
// `self` has been deleted.
Err(entry)
}
/// Returns the closest next valid entry.
///
/// It unlinks deleted entries until it reaches a valid one.
///
/// # Examples
///
/// ```
/// use scc::LinkedList;
/// use scc::ebr::{Arc, AtomicArc, Barrier};
/// use std::sync::atomic::Ordering::Relaxed;
///
/// #[derive(Default)]
/// struct L(AtomicArc<L>, usize);
/// impl LinkedList for L {
/// fn link_ref(&self) -> &AtomicArc<L> {
/// &self.0
/// }
/// }
///
/// let barrier = Barrier::new();
///
/// let head: L = L::default();
/// assert!(head.push_back(Arc::new(L(AtomicArc::null(), 1)), false, Relaxed, &barrier).is_ok());
/// head.mark(Relaxed);
///
/// let next_ptr = head.next_ptr(Relaxed, &barrier);
/// assert_eq!(next_ptr.as_ref().unwrap().1, 1);
/// assert!(head.is_marked(Relaxed));
/// ```
fn next_ptr<'b>(&self, order: Ordering, barrier: &'b Barrier) -> Ptr<'b, Self> {
let self_next_ptr = self.link_ref().load(order, barrier);
let self_tag = self_next_ptr.tag();
let mut next_ptr = self_next_ptr;
let mut update_self = false;
let next_valid_ptr = loop {
if let Some(next_ref) = next_ptr.as_ref() {
let next_next_ptr = next_ref.link_ref().load(order, barrier);
if next_next_ptr.tag() != Tag::Second {
break next_ptr;
}
update_self = true;
next_ptr = next_next_ptr;
} else {
break Ptr::null();
}
};
// Updates its link if an invalid entry has been found, and `self` is a valid one.
if update_self && self_tag != Tag::Second {
if let Ok((Some(prev), _)) = self.link_ref().compare_exchange(
self_next_ptr,
(next_valid_ptr.get_arc(), self_tag),
Release,
Relaxed,
barrier,
) {
barrier.reclaim(prev);
}
}
next_valid_ptr
}
}