diff options
| -rw-r--r-- | rust/kernel/list.rs | 82 |
1 files changed, 82 insertions, 0 deletions
diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs index a215f77a9de4..6017f2e4ba3e 100644 --- a/rust/kernel/list.rs +++ b/rust/kernel/list.rs @@ -440,6 +440,20 @@ impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> { other.first = ptr::null_mut(); } + /// Returns a cursor to the first element of the list. + /// + /// If the list is empty, this returns `None`. + pub fn cursor_front(&mut self) -> Option<Cursor<'_, T, ID>> { + if self.first.is_null() { + None + } else { + Some(Cursor { + current: self.first, + list: self, + }) + } + } + /// Creates an iterator over the list. pub fn iter(&self) -> Iter<'_, T, ID> { // INVARIANT: If the list is empty, both pointers are null. Otherwise, both pointers point @@ -514,6 +528,74 @@ impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Iterator for Iter<'a, T, ID> { } } +/// A cursor into a [`List`]. +/// +/// # Invariants +/// +/// The `current` pointer points a value in `list`. +pub struct Cursor<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> { + current: *mut ListLinksFields, + list: &'a mut List<T, ID>, +} + +impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Cursor<'a, T, ID> { + /// Access the current element of this cursor. + pub fn current(&self) -> ArcBorrow<'_, T> { + // SAFETY: The `current` pointer points a value in the list. + let me = unsafe { T::view_value(ListLinks::from_fields(self.current)) }; + // SAFETY: + // * All values in a list are stored in an `Arc`. + // * The value cannot be removed from the list for the duration of the lifetime annotated + // on the returned `ArcBorrow`, because removing it from the list would require mutable + // access to the cursor or the list. However, the `ArcBorrow` holds an immutable borrow + // on the cursor, which in turn holds a mutable borrow on the list, so any such + // mutable access requires first releasing the immutable borrow on the cursor. + // * Values in a list never have a `UniqueArc` reference, because the list has a `ListArc` + // reference, and `UniqueArc` references must be unique. + unsafe { ArcBorrow::from_raw(me) } + } + + /// Move the cursor to the next element. + pub fn next(self) -> Option<Cursor<'a, T, ID>> { + // SAFETY: The `current` field is always in a list. + let next = unsafe { (*self.current).next }; + + if next == self.list.first { + None + } else { + // INVARIANT: Since `self.current` is in the `list`, its `next` pointer is also in the + // `list`. + Some(Cursor { + current: next, + list: self.list, + }) + } + } + + /// Move the cursor to the previous element. + pub fn prev(self) -> Option<Cursor<'a, T, ID>> { + // SAFETY: The `current` field is always in a list. + let prev = unsafe { (*self.current).prev }; + + if self.current == self.list.first { + None + } else { + // INVARIANT: Since `self.current` is in the `list`, its `prev` pointer is also in the + // `list`. + Some(Cursor { + current: prev, + list: self.list, + }) + } + } + + /// Remove the current element from the list. + pub fn remove(self) -> ListArc<T, ID> { + // SAFETY: The `current` pointer always points at a member of the list. + unsafe { self.list.remove_internal(self.current) } + } +} + impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> FusedIterator for Iter<'a, T, ID> {} impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> IntoIterator for &'a List<T, ID> { |