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BTreeMap: admit the existence of leaf edges in comments #77395

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Oct 5, 2020
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BTreeMap: admit the existence of leaf edges in comments #77395

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34 changes: 12 additions & 22 deletions library/alloc/src/collections/btree/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,11 +9,8 @@
// struct Node<K, V, height: usize> {
// keys: [K; 2 * B - 1],
// vals: [V; 2 * B - 1],
// edges: if height > 0 {
// [Box<Node<K, V, height - 1>>; 2 * B]
// } else { () },
// parent: Option<NonNull<Node<K, V, height + 1>>>,
// parent_idx: u16,
// edges: [if height > 0 { Box<Node<K, V, height - 1>> } else { () }; 2 * B],
// parent: Option<(NonNull<Node<K, V, height + 1>>, u16)>,
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parent is unrelated to leaf edges, but the fact that parent_idx is not part of the option is not "the ideal", it's probably the only way we can have the compiler merge the two u16 fields. I was going to undo that but it's in here now,

// len: u16,
// }
// ```
Expand All @@ -28,8 +25,8 @@
//
// - Trees must have uniform depth/height. This means that every path down to a leaf from a
// given node has exactly the same length.
// - A node of length `n` has `n` keys, `n` values, and (in an internal node) `n + 1` edges.
// This implies that even an empty internal node has at least one edge.
// - A node of length `n` has `n` keys, `n` values, and `n + 1` edges.
// This implies that even an empty node has at least one edge.

use core::cmp::Ordering;
use core::marker::PhantomData;
Expand Down Expand Up @@ -298,9 +295,8 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
}

impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
/// Finds the length of the node. This is the number of keys or values. In an
/// internal node, the number of edges is `len() + 1`.
/// For any node, the number of possible edge handles is also `len() + 1`.
/// Finds the length of the node. This is the number of keys or values.
/// The number of edges is `len() + 1`.
/// Note that, despite being safe, calling this function can have the side effect
/// of invalidating mutable references that unsafe code has created.
pub fn len(&self) -> usize {
Expand All @@ -321,9 +317,6 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
}

/// Exposes the leaf portion of any leaf or internal node.
/// If the node is a leaf, this function simply opens up its data.
/// If the node is an internal node, so not a leaf, it does have all the data a leaf has
/// (header, keys and values), and this function exposes that.
Comment on lines -324 to -326
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There hasn't been a separate header for quite a while, so pruning my own comment.

///
/// Returns a raw ptr to avoid invalidating other references to this node,
/// which is possible when BorrowType is marker::ValMut.
Expand Down Expand Up @@ -471,9 +464,6 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
}

/// Exposes the leaf portion of any leaf or internal node for writing.
/// If the node is a leaf, this function simply opens up its data.
/// If the node is an internal node, so not a leaf, it does have all the data a leaf has
/// (header, keys and values), and this function exposes that.
///
/// We don't need to return a raw ptr because we have unique access to the entire node.
fn as_leaf_mut(&mut self) -> &'a mut LeafNode<K, V> {
Expand Down Expand Up @@ -679,9 +669,9 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
}

impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
/// Removes a key/value pair from the end of this node and returns the pair.
/// If this is an internal node, also removes the edge that was to the right
/// of that pair and returns the orphaned node that this edge owned.
/// Removes a key/value pair from the end of the node and returns the pair.
/// Also removes the edge that was to the right of that pair and, if the node
/// is internal, returns the orphaned subtree that this edge owned.
fn pop(&mut self) -> (K, V, Option<Root<K, V>>) {
debug_assert!(self.len() > 0);

Expand All @@ -705,9 +695,9 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
}
}

/// Removes a key/value pair from the beginning of this node and returns the pair.
/// If this is an internal node, also removes the edge that was to the left
/// of that pair and returns the orphaned node that this edge owned.
/// Removes a key/value pair from the beginning of the node and returns the pair.
/// Also removes the edge that was to the left of that pair and, if the node is
/// internal, returns the orphaned subtree that this edge owned.
fn pop_front(&mut self) -> (K, V, Option<Root<K, V>>) {
debug_assert!(self.len() > 0);

Expand Down