Re: [PATCH] rust: rbtree: add immutable cursor

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Sep 04, 2025 at 04:25:52PM +0200, Vitaly Wool wrote:
[...]
> +
> +    /// Returns a cursor over the tree nodes based on the given key.
> +    ///
> +    /// If the given key exists, the cursor starts there.
> +    /// Otherwise it starts with the first larger key in sort order.
> +    /// If there is no larger key, it returns [`None`].
> +    pub fn cursor_lower_bound(&self, key: &K) -> Option<Cursor<'_, K, V>>

I think you can make a helper function that returns a
`Option<NonNull<..>>` and `cursor_lower_bound()` and
`cursor_lower_bound_mut()` could share the searching logic in the helper
function.

> +    where
> +        K: Ord,
> +    {
> +        let mut node = self.root.rb_node;
> +        let mut best_match: Option<NonNull<Node<K, V>>> = None;
> +        while !node.is_null() {
> +            // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in
> +            // `self` point to the links field of `Node<K, V>` objects.
> +            let this = unsafe { container_of!(node, Node<K, V>, links) };
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
> +            let this_key = unsafe { &(*this).key };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let left_child = unsafe { (*node).rb_left };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let right_child = unsafe { (*node).rb_right };
> +            match key.cmp(this_key) {
> +                Ordering::Equal => {
> +                    best_match = NonNull::new(this);
> +                    break;
> +                }
> +                Ordering::Greater => {
> +                    node = right_child;
> +                }
> +                Ordering::Less => {
> +                    let is_better_match = match best_match {
> +                        None => true,
> +                        Some(best) => {
> +                            // SAFETY: `best` is a non-null node so it is valid by the type
> +                            // invariants.
> +                            let best_key = unsafe { &(*best.as_ptr()).key };
> +                            best_key > this_key
> +                        }
> +                    };
> +                    if is_better_match {
> +                        best_match = NonNull::new(this);
> +                    }
> +                    node = left_child;
> +                }
> +            };
> +        }
> +
> +        let best = best_match?;
> +
> +        // SAFETY: `best` is a non-null node so it is valid by the type invariants.
> +        let links = unsafe { addr_of_mut!((*best.as_ptr()).links) };
> +
> +        NonNull::new(links).map(|current| {
> +            // INVARIANT:
> +            // - `current` is a valid node in the [`RBTree`] pointed to by `self`.
> +            Cursor {
> +                current,
> +                _tree: PhantomData,
> +            }
> +        })
> +    }
>  }
>  
[...]