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Simplify into_key_slice_mut #67725

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Jan 9, 2020
Merged

Simplify into_key_slice_mut #67725

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37b5cca
Simplify into_key_slice_mut and document bits and bobs
ssomers Jan 2, 2020
9b92bf8
Apply suggestions from code review
ssomers Jan 9, 2020
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23 changes: 10 additions & 13 deletions src/liballoc/collections/btree/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -397,6 +397,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {

/// Borrows a view into the values stored in the node.
/// The caller must ensure that the node is not the shared root.
/// This function is not public, so doesn't have to support shared roots like `keys` does.
fn vals(&self) -> &[V] {
self.reborrow().into_val_slice()
}
Expand Down Expand Up @@ -514,6 +515,7 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
}

/// The caller must ensure that the node is not the shared root.
/// This function is not public, so doesn't have to support shared roots like `keys` does.
fn keys_mut(&mut self) -> &mut [K] {
unsafe { self.reborrow_mut().into_key_slice_mut() }
}
Expand Down Expand Up @@ -589,20 +591,15 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
unsafe { &mut *(self.root as *mut Root<K, V>) }
}

/// The caller must ensure that the node is not the shared root.
fn into_key_slice_mut(mut self) -> &'a mut [K] {
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// Same as for `into_key_slice` above, we try to avoid a run-time check.
if (mem::align_of::<NodeHeader<K, V, K>>() > mem::align_of::<NodeHeader<K, V>>()
|| mem::size_of::<NodeHeader<K, V, K>>() != mem::size_of::<NodeHeader<K, V>>())
&& self.is_shared_root()
{
&mut []
} else {
unsafe {
slice::from_raw_parts_mut(
MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
self.len(),
)
}
debug_assert!(!self.is_shared_root());
// We cannot be the shared root, so `as_leaf_mut` is okay.
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We? I'd prefer something more objective like the slice or whatever it is

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@ssomers ssomers Dec 30, 2019 •
edited
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I didn't actually write that comment, and think it's somewhat superfluous, but please come up with a concrete suggestion here, like:

  • // self cannot be the shared root
  • // This NodeRef cannot be the shared root
  • // Our node reference cannot be the shared root
  • // This node cannot be the shared root

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I would go with:

self cannot be the shared root

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I don't think that's any better, on the contrary, it comes across as poor grammar and capitalization to me. Also, there are already at least 3 comments of this "we = self" style in the file, so at least one author and one reviewer didn't object.

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@ssomers ssomers Jan 4, 2020 •
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The style of the public API documentation of Option, Vector, BTreeSet, leads to (in unscientifically determined order of popularity):

  • // The node cannot be the shared root
  • // The NodeRef cannot be the shared root

If there are multiple instances involved, we explicitly use self (with backticks). Somehow, its awkward capitalization is carefully avoided, except Vec::split_off

unsafe {
slice::from_raw_parts_mut(
MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
self.len(),
)
}
}

Expand Down
11 changes: 11 additions & 0 deletions src/liballoc/collections/btree/search.rs
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Expand Up @@ -46,6 +46,11 @@ where
}
}

/// Returns the index in the node at which the key (or an equivalent) exists
/// or could exist, and whether it exists in the node itself. If it doesn't
/// exist in the node itself, it may exist in the subtree with that index
/// (if the node has subtrees). If the key doesn't exist in node or subtree,
/// the returned index is the position or subtree to insert at.
pub fn search_linear<BorrowType, K, V, Type, Q: ?Sized>(
node: &NodeRef<BorrowType, K, V, Type>,
key: &Q,
Expand All @@ -54,6 +59,12 @@ where
Q: Ord,
K: Borrow<Q>,
{
// This function is defined over all borrow types (immutable, mutable, owned),
// and may be called on the shared root in each case.
// Crucially, we use `keys()` here, i.e., we work with immutable data.
// `keys_mut()` does not support the shared root, so we cannot use it.
// Using `keys()` is fine here even if BorrowType is mutable, as all we return
// is an index -- not a reference.
for (i, k) in node.keys().iter().enumerate() {
match key.cmp(k.borrow()) {
Ordering::Greater => {}
Expand Down