Struct CursorMut 

pub struct CursorMut<'a, K: TrieKey, V> { /* private fields */ }

Mutable counterpart to Cursor: a tree-walk iterator that lends out &mut V borrows over the stored values, in sorted (DFS) key order.

Unlike Cursor, the value reference is tied to &mut self (a lending cursor), not to the trie lifetime 'a. This is a soundness requirement, not a stylistic choice: a cursor is re-positionable — current(), seek(), first(), last() can all revisit a slot already visited. An 'a-tied &mut V (as the immutable cursor hands out &'a V) would let two such calls return &mut V to the same element simultaneously — aliasing undefined behavior. Tying the borrow to &mut self makes the borrow checker enforce “one live &mut V at a time,” which is the only sound rule for a re-positionable mutable cursor. The practical consequence: you cannot collect the &mut V into a Vec or hold two at once; each must be released before the next next()/prev()/current()/seek() call. In-place mutation loops (while let Some((k, v)) = c.next() { *v += 1; }) work as expected.

The key is returned as &[u8] borrowing the trie’s key store (zero allocation, matching the immutable cursor’s borrowed key). Both the key and the &mut V are tied to &mut self. Only the stored value is mutated; the cursor never alters key bytes, node structure, or slot occupancy, so trie invariants are preserved.

Implementations

impl<'a, K: TrieKey, V> CursorMut<'a, K, V>

pub fn new(trie: &'a mut BitTrie<K, V>) -> Self

Forward mutable cursor parked before the first key.

pub fn new_last(trie: &'a mut BitTrie<K, V>) -> Self

Reverse mutable cursor parked on the last key (or empty if the trie is empty).

pub fn current(&mut self) -> Option<(&[u8], &mut V)>

The key/value the cursor is parked on, or None if not parked (before first, or exhausted). The key borrows the trie’s key store and the &mut V reborrows the stored value — both tied to &mut self.

keys and values are disjoint fields of the trie, so the shared key borrow (via key_bytes) and the mutable value borrow coexist without aliasing.

pub fn current_index(&self) -> Option<usize>

The key index the cursor is parked on, or None if not parked.

pub fn first(&mut self) -> Option<(&[u8], &mut V)>

Jump to the first key (smallest in sorted order). Returns its key/value, or None if the trie is empty.

pub fn last(&mut self) -> Option<(&[u8], &mut V)>

Jump to the last key (largest in sorted order). Returns its key/value, or None if the trie is empty.

pub fn next(&mut self) -> Option<(&[u8], &mut V)>

pub fn prev(&mut self) -> Option<(&[u8], &mut V)>

pub fn next_index(&mut self) -> Option<usize>

pub fn prev_index(&mut self) -> Option<usize>

pub fn seek(&mut self, key: &[u8]) -> Option<(&[u8], &mut V)>