1 files changed, 179 insertions, 0 deletions
diff --git a/library/alloc/src/collections/btree/fix.rs b/library/alloc/src/collections/btree/fix.rs
new file mode 100644
index 000000000..91b612180
--- /dev/null
+++ b/library/alloc/src/collections/btree/fix.rs
@@ -0,0 +1,179 @@
+use super::map::MIN_LEN;
+use super::node::{marker, ForceResult::*, Handle, LeftOrRight::*, NodeRef, Root};
+use core::alloc::Allocator;
+
+impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
+    /// Stocks up a possibly underfull node by merging with or stealing from a
+    /// sibling. If successful but at the cost of shrinking the parent node,
+    /// returns that shrunk parent node. Returns an `Err` if the node is
+    /// an empty root.
+    fn fix_node_through_parent<A: Allocator + Clone>(
+        self,
+        alloc: A,
+    ) -> Result<Option<NodeRef<marker::Mut<'a>, K, V, marker::Internal>>, Self> {
+        let len = self.len();
+        if len >= MIN_LEN {
+            Ok(None)
+        } else {
+            match self.choose_parent_kv() {
+                Ok(Left(mut left_parent_kv)) => {
+                    if left_parent_kv.can_merge() {
+                        let parent = left_parent_kv.merge_tracking_parent(alloc);
+                        Ok(Some(parent))
+                    } else {
+                        left_parent_kv.bulk_steal_left(MIN_LEN - len);
+                        Ok(None)
+                    }
+                }
+                Ok(Right(mut right_parent_kv)) => {
+                    if right_parent_kv.can_merge() {
+                        let parent = right_parent_kv.merge_tracking_parent(alloc);
+                        Ok(Some(parent))
+                    } else {
+                        right_parent_kv.bulk_steal_right(MIN_LEN - len);
+                        Ok(None)
+                    }
+                }
+                Err(root) => {
+                    if len > 0 {
+                        Ok(None)
+                    } else {
+                        Err(root)
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
+    /// Stocks up a possibly underfull node, and if that causes its parent node
+    /// to shrink, stocks up the parent, recursively.
+    /// Returns `true` if it fixed the tree, `false` if it couldn't because the
+    /// root node became empty.
+    ///
+    /// This method does not expect ancestors to already be underfull upon entry
+    /// and panics if it encounters an empty ancestor.
+    pub fn fix_node_and_affected_ancestors<A: Allocator + Clone>(mut self, alloc: A) -> bool {
+        loop {
+            match self.fix_node_through_parent(alloc.clone()) {
+                Ok(Some(parent)) => self = parent.forget_type(),
+                Ok(None) => return true,
+                Err(_) => return false,
+            }
+        }
+    }
+}
+
+impl<K, V> Root<K, V> {
+    /// Removes empty levels on the top, but keeps an empty leaf if the entire tree is empty.
+    pub fn fix_top<A: Allocator + Clone>(&mut self, alloc: A) {
+        while self.height() > 0 && self.len() == 0 {
+            self.pop_internal_level(alloc.clone());
+        }
+    }
+
+    /// Stocks up or merge away any underfull nodes on the right border of the
+    /// tree. The other nodes, those that are not the root nor a rightmost edge,
+    /// must already have at least MIN_LEN elements.
+    pub fn fix_right_border<A: Allocator + Clone>(&mut self, alloc: A) {
+        self.fix_top(alloc.clone());
+        if self.len() > 0 {
+            self.borrow_mut().last_kv().fix_right_border_of_right_edge(alloc.clone());
+            self.fix_top(alloc);
+        }
+    }
+
+    /// The symmetric clone of `fix_right_border`.
+    pub fn fix_left_border<A: Allocator + Clone>(&mut self, alloc: A) {
+        self.fix_top(alloc.clone());
+        if self.len() > 0 {
+            self.borrow_mut().first_kv().fix_left_border_of_left_edge(alloc.clone());
+            self.fix_top(alloc);
+        }
+    }
+
+    /// Stocks up any underfull nodes on the right border of the tree.
+    /// The other nodes, those that are neither the root nor a rightmost edge,
+    /// must be prepared to have up to MIN_LEN elements stolen.
+    pub fn fix_right_border_of_plentiful(&mut self) {
+        let mut cur_node = self.borrow_mut();
+        while let Internal(internal) = cur_node.force() {
+            // Check if right-most child is underfull.
+            let mut last_kv = internal.last_kv().consider_for_balancing();
+            debug_assert!(last_kv.left_child_len() >= MIN_LEN * 2);
+            let right_child_len = last_kv.right_child_len();
+            if right_child_len < MIN_LEN {
+                // We need to steal.
+                last_kv.bulk_steal_left(MIN_LEN - right_child_len);
+            }
+
+            // Go further down.
+            cur_node = last_kv.into_right_child();
+        }
+    }
+}
+
+impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV> {
+    fn fix_left_border_of_left_edge<A: Allocator + Clone>(mut self, alloc: A) {
+        while let Internal(internal_kv) = self.force() {
+            self = internal_kv.fix_left_child(alloc.clone()).first_kv();
+            debug_assert!(self.reborrow().into_node().len() > MIN_LEN);
+        }
+    }
+
+    fn fix_right_border_of_right_edge<A: Allocator + Clone>(mut self, alloc: A) {
+        while let Internal(internal_kv) = self.force() {
+            self = internal_kv.fix_right_child(alloc.clone()).last_kv();
+            debug_assert!(self.reborrow().into_node().len() > MIN_LEN);
+        }
+    }
+}
+
+impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::KV> {
+    /// Stocks up the left child, assuming the right child isn't underfull, and
+    /// provisions an extra element to allow merging its children in turn
+    /// without becoming underfull.
+    /// Returns the left child.
+    fn fix_left_child<A: Allocator + Clone>(
+        self,
+        alloc: A,
+    ) -> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
+        let mut internal_kv = self.consider_for_balancing();
+        let left_len = internal_kv.left_child_len();
+        debug_assert!(internal_kv.right_child_len() >= MIN_LEN);
+        if internal_kv.can_merge() {
+            internal_kv.merge_tracking_child(alloc)
+        } else {
+            // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
+            let count = (MIN_LEN + 1).saturating_sub(left_len);
+            if count > 0 {
+                internal_kv.bulk_steal_right(count);
+            }
+            internal_kv.into_left_child()
+        }
+    }
+
+    /// Stocks up the right child, assuming the left child isn't underfull, and
+    /// provisions an extra element to allow merging its children in turn
+    /// without becoming underfull.
+    /// Returns wherever the right child ended up.
+    fn fix_right_child<A: Allocator + Clone>(
+        self,
+        alloc: A,
+    ) -> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
+        let mut internal_kv = self.consider_for_balancing();
+        let right_len = internal_kv.right_child_len();
+        debug_assert!(internal_kv.left_child_len() >= MIN_LEN);
+        if internal_kv.can_merge() {
+            internal_kv.merge_tracking_child(alloc)
+        } else {
+            // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
+            let count = (MIN_LEN + 1).saturating_sub(right_len);
+            if count > 0 {
+                internal_kv.bulk_steal_left(count);
+            }
+            internal_kv.into_right_child()
+        }
+    }
+}