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Keep track of position when deleting from a BTreeMap #70795

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merged 2 commits into from
Apr 5, 2020
Merged

Keep track of position when deleting from a BTreeMap #70795

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Keep track of position when deleting from a BTreeMap
Amanieu Apr 5, 2020
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Amanieu Apr 5, 2020
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25 changes: 10 additions & 15 deletions src/liballoc/collections/btree/map.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2728,8 +2728,7 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
let mut at_leaf = true;
while cur_node.len() < node::MIN_LEN {
match handle_underfull_node(cur_node) {
AtRoot(_) => break,
EmptyParent(_) => unreachable!(),
AtRoot => break,
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
Expand All @@ -2740,7 +2739,6 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
Leaf(leaf) => leaf,
Internal(_) => unreachable!(),
};
debug_assert!(idx <= node.len());
pos = unsafe { Handle::new_edge(node, idx) };
}

Expand All @@ -2754,7 +2752,7 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
at_leaf = false;
}
}
Stole(_, stole_from_left) => {
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.
Expand All @@ -2781,28 +2779,25 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
}

enum UnderflowResult<'a, K, V> {
AtRoot(NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>),
EmptyParent(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
AtRoot,
Merged(Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::Edge>, bool, usize),
Stole(NodeRef<marker::Mut<'a>, K, V, marker::Internal>, bool),
Stole(bool),
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Nice, but now Stole is begging me to be split up into something like StoleFromLeft and StoleFromRight.

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But then I would have to do the same with MergedWithRight and MergedWithLeft, and that makes just makes the match above more complicated.

}

fn handle_underfull_node<K, V>(
node: NodeRef<marker::Mut<'_>, K, V, marker::LeafOrInternal>,
) -> UnderflowResult<'_, K, V> {
let parent = match node.ascend() {
Ok(parent) => parent,
Err(root) => return AtRoot(root),
Err(_) => return AtRoot,
};

let (is_left, mut handle) = match parent.left_kv() {
Ok(left) => (true, left),
Err(parent) => match parent.right_kv() {
Ok(right) => (false, right),
Err(parent) => {
return EmptyParent(parent.into_node());
}
},
Err(parent) => {
let right = unsafe { unwrap_unchecked(parent.right_kv().ok()) };
(false, right)
}
};

if handle.can_merge() {
Expand All @@ -2814,7 +2809,7 @@ fn handle_underfull_node<K, V>(
} else {
handle.steal_right();
}
Stole(handle.into_node(), is_left)
Stole(is_left)
}
}

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