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Diffstat (limited to 'library/alloc/src/collections/btree/remove.rs')
-rw-r--r-- | library/alloc/src/collections/btree/remove.rs | 95 |
1 files changed, 95 insertions, 0 deletions
diff --git a/library/alloc/src/collections/btree/remove.rs b/library/alloc/src/collections/btree/remove.rs new file mode 100644 index 000000000..090429925 --- /dev/null +++ b/library/alloc/src/collections/btree/remove.rs @@ -0,0 +1,95 @@ +use super::map::MIN_LEN; +use super::node::{marker, ForceResult::*, Handle, LeftOrRight::*, NodeRef}; +use core::alloc::Allocator; + +impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV> { + /// Removes a key-value pair from the tree, and returns that pair, as well as + /// the leaf edge corresponding to that former pair. It's possible this empties + /// a root node that is internal, which the caller should pop from the map + /// holding the tree. The caller should also decrement the map's length. + pub fn remove_kv_tracking<F: FnOnce(), A: Allocator + Clone>( + self, + handle_emptied_internal_root: F, + alloc: A, + ) -> ((K, V), Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>) { + match self.force() { + Leaf(node) => node.remove_leaf_kv(handle_emptied_internal_root, alloc), + Internal(node) => node.remove_internal_kv(handle_emptied_internal_root, alloc), + } + } +} + +impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV> { + fn remove_leaf_kv<F: FnOnce(), A: Allocator + Clone>( + self, + handle_emptied_internal_root: F, + alloc: A, + ) -> ((K, V), Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>) { + let (old_kv, mut pos) = self.remove(); + let len = pos.reborrow().into_node().len(); + if len < MIN_LEN { + let idx = pos.idx(); + // We have to temporarily forget the child type, because there is no + // distinct node type for the immediate parents of a leaf. + let new_pos = match pos.into_node().forget_type().choose_parent_kv() { + Ok(Left(left_parent_kv)) => { + debug_assert!(left_parent_kv.right_child_len() == MIN_LEN - 1); + if left_parent_kv.can_merge() { + left_parent_kv.merge_tracking_child_edge(Right(idx), alloc.clone()) + } else { + debug_assert!(left_parent_kv.left_child_len() > MIN_LEN); + left_parent_kv.steal_left(idx) + } + } + Ok(Right(right_parent_kv)) => { + debug_assert!(right_parent_kv.left_child_len() == MIN_LEN - 1); + if right_parent_kv.can_merge() { + right_parent_kv.merge_tracking_child_edge(Left(idx), alloc.clone()) + } else { + debug_assert!(right_parent_kv.right_child_len() > MIN_LEN); + right_parent_kv.steal_right(idx) + } + } + Err(pos) => unsafe { Handle::new_edge(pos, idx) }, + }; + // SAFETY: `new_pos` is the leaf we started from or a sibling. + pos = unsafe { new_pos.cast_to_leaf_unchecked() }; + + // Only if we merged, the parent (if any) has shrunk, but skipping + // the following step otherwise does not pay off in benchmarks. + // + // SAFETY: We won't destroy or rearrange the leaf where `pos` is at + // by handling its parent recursively; at worst we will destroy or + // rearrange the parent through the grandparent, thus change the + // link to the parent inside the leaf. + if let Ok(parent) = unsafe { pos.reborrow_mut() }.into_node().ascend() { + if !parent.into_node().forget_type().fix_node_and_affected_ancestors(alloc) { + handle_emptied_internal_root(); + } + } + } + (old_kv, pos) + } +} + +impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::KV> { + fn remove_internal_kv<F: FnOnce(), A: Allocator + Clone>( + self, + handle_emptied_internal_root: F, + alloc: A, + ) -> ((K, V), Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>) { + // Remove an adjacent KV from its leaf and then put it back in place of + // the element we were asked to remove. Prefer the left adjacent KV, + // for the reasons listed in `choose_parent_kv`. + let left_leaf_kv = self.left_edge().descend().last_leaf_edge().left_kv(); + let left_leaf_kv = unsafe { left_leaf_kv.ok().unwrap_unchecked() }; + let (left_kv, left_hole) = left_leaf_kv.remove_leaf_kv(handle_emptied_internal_root, alloc); + + // The internal node may have been stolen from or merged. Go back right + // to find where the original KV ended up. + let mut internal = unsafe { left_hole.next_kv().ok().unwrap_unchecked() }; + let old_kv = internal.replace_kv(left_kv.0, left_kv.1); + let pos = internal.next_leaf_edge(); + (old_kv, pos) + } +} |