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Keep track of position when deleting from a BTreeMap

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This improves the performance of drain_filter and is needed for
future Cursor support for BTreeMap.
This commit is contained in:
Amanieu d'Antras 2020-04-05 01:58:25 +01:00
parent 537ccdf3ac
commit 7365748c0c
2 changed files with 59 additions and 39 deletions
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93
src/liballoc/collections/btree/map.rs
View File

@ -1780,18 +1780,12 @@ pub(super) fn next<F>(&mut self, pred: &mut F) -> Option<(K, V)>
where
F: FnMut(&K, &mut V) -> bool,
{
while let Some(kv) = unsafe { self.next_kv() } {
let (k, v) = unsafe { ptr::read(&kv) }.into_kv_mut();
while let Some(mut kv) = unsafe { self.next_kv() } {
let (k, v) = kv.kv_mut();
if pred(k, v) {
*self.length -= 1;
let (k, v, leaf_edge_location) = kv.remove_kv_tracking();
// `remove_kv_tracking` has either preserved or invalidated `self.cur_leaf_edge`
if let Some(node) = leaf_edge_location {
match search::search_tree(node, &k) {
search::SearchResult::Found(_) => unreachable!(),
search::SearchResult::GoDown(leaf) => self.cur_leaf_edge = Some(leaf),
}
};
self.cur_leaf_edge = Some(leaf_edge_location);
return Some((k, v));
}
self.cur_leaf_edge = Some(kv.next_leaf_edge());
@ -2698,28 +2692,26 @@ fn remove_kv(self) -> (K, V) {
impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV> {
/// Removes a key/value-pair from the map, and returns that pair, as well as
/// the whereabouts of the leaf edge corresponding to that former pair:
/// if None is returned, the leaf edge is still the left leaf edge of the KV handle;
/// if a node is returned, it heads the subtree where the leaf edge may be found.
/// the leaf edge corresponding to that former pair.
fn remove_kv_tracking(
self,
) -> (K, V, Option<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>>) {
let mut levels_down_handled: isize;
let (small_leaf, old_key, old_val) = match self.force() {
) -> (K, V, Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>) {
let (mut pos, old_key, old_val, was_internal) = match self.force() {
Leaf(leaf) => {
levels_down_handled = 1; // handled at same level, but affects only the right side
let (hole, old_key, old_val) = leaf.remove();
(hole.into_node(), old_key, old_val)
(hole, old_key, old_val, false)
}
Internal(mut internal) => {
// Replace the location freed in the internal node with the next KV,
// and remove that next KV from its leaf.
levels_down_handled = unsafe { ptr::read(&internal).into_node().height() } as isize;
let key_loc = internal.kv_mut().0 as *mut K;
let val_loc = internal.kv_mut().1 as *mut V;
let to_remove = internal.right_edge().descend().first_leaf_edge().right_kv().ok();
// Deleting from the left side is typically faster since we can
// just pop an element from the end of the KV array without
// needing to shift the other values.
let to_remove = internal.left_edge().descend().last_leaf_edge().left_kv().ok();
let to_remove = unsafe { unwrap_unchecked(to_remove) };
let (hole, key, val) = to_remove.remove();
@ -2727,50 +2719,72 @@ fn remove_kv_tracking(
let old_key = unsafe { mem::replace(&mut *key_loc, key) };
let old_val = unsafe { mem::replace(&mut *val_loc, val) };
(hole.into_node(), old_key, old_val)
(hole, old_key, old_val, true)
}
};
// Handle underflow
let mut cur_node = small_leaf.forget_type();
let mut cur_node = unsafe { ptr::read(&pos).into_node().forget_type() };
let mut at_leaf = true;
while cur_node.len() < node::MIN_LEN {
match handle_underfull_node(cur_node) {
AtRoot(root) => {
cur_node = root;
break;
}
AtRoot(_) => break,
EmptyParent(_) => unreachable!(),
Merged(parent) => {
levels_down_handled -= 1;
Merged(edge, merged_with_left, offset) => {
// If we merged with our right sibling then our tracked
// position has not changed. However if we merged with our
// left sibling then our tracked position is now dangling.
if at_leaf && merged_with_left {
let idx = pos.idx() + offset;
let node = match unsafe { ptr::read(&edge).descend().force() } {
Leaf(leaf) => leaf,
Internal(_) => unreachable!(),
};
debug_assert!(idx <= node.len());
pos = unsafe { Handle::new_edge(node, idx) };
}
let parent = edge.into_node();
if parent.len() == 0 {
// We must be at the root
let root = parent.into_root_mut();
root.pop_level();
cur_node = root.as_mut();
parent.into_root_mut().pop_level();
break;
} else {
cur_node = parent.forget_type();
at_leaf = false;
}
}
Stole(internal_node) => {
levels_down_handled -= 1;
cur_node = internal_node.forget_type();
Stole(_, stole_from_left) => {
// Adjust the tracked position if we stole from a left sibling
if stole_from_left && at_leaf {
// SAFETY: This is safe since we just added an element to our node.
unsafe {
pos.next_unchecked();
}
}
// This internal node might be underfull, but only if it's the root.
break;
}
}
}
let leaf_edge_location = if levels_down_handled > 0 { None } else { Some(cur_node) };
(old_key, old_val, leaf_edge_location)
// If we deleted from an internal node then we need to compensate for
// the earlier swap and adjust the tracked position to point to the
// next element.
if was_internal {
pos = unsafe { unwrap_unchecked(pos.next_kv().ok()).next_leaf_edge() };
}
(old_key, old_val, pos)
}
}
enum UnderflowResult<'a, K, V> {
AtRoot(NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>),
EmptyParent(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
Merged(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
Stole(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
Merged(Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::Edge>, bool, usize),
Stole(NodeRef<marker::Mut<'a>, K, V, marker::Internal>, bool),
}
fn handle_underfull_node<K, V>(
@ -2792,14 +2806,15 @@ fn handle_underfull_node<K, V>(
};
if handle.can_merge() {
Merged(handle.merge().into_node())
let offset = if is_left { handle.reborrow().left_edge().descend().len() + 1 } else { 0 };
Merged(handle.merge(), is_left, offset)
} else {
if is_left {
handle.steal_left();
} else {
handle.steal_right();
}
Stole(handle.into_node())
Stole(handle.into_node(), is_left)
}
}

5
src/liballoc/collections/btree/node.rs
View File

@ -723,6 +723,11 @@ impl<Node, Type> Handle<Node, Type> {
pub fn into_node(self) -> Node {
self.node
}
/// Returns the position of this handle in the node.
pub fn idx(&self) -> usize {
self.idx
}
}
impl<BorrowType, K, V, NodeType> Handle<NodeRef<BorrowType, K, V, NodeType>, marker::KV> {