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//! B+-tree node pool.
#[cfg(test)]
use super::Comparator;
use super::{Forest, Node, NodeData};
use crate::entity::PrimaryMap;
#[cfg(test)]
use core::fmt;
use core::ops::{Index, IndexMut};
/// A pool of nodes, including a free list.
pub(super) struct NodePool<F: Forest> {
nodes: PrimaryMap<Node, NodeData<F>>,
freelist: Option<Node>,
}
impl<F: Forest> NodePool<F> {
/// Allocate a new empty pool of nodes.
pub fn new() -> Self {
Self {
nodes: PrimaryMap::new(),
freelist: None,
}
}
/// Free all nodes.
pub fn clear(&mut self) {
self.nodes.clear();
self.freelist = None;
}
/// Allocate a new node containing `data`.
pub fn alloc_node(&mut self, data: NodeData<F>) -> Node {
debug_assert!(!data.is_free(), "can't allocate free node");
match self.freelist {
Some(node) => {
// Remove this node from the free list.
match self.nodes[node] {
NodeData::Free { next } => self.freelist = next,
_ => panic!("Invalid {} on free list", node),
}
self.nodes[node] = data;
node
}
None => {
// The free list is empty. Allocate a new node.
self.nodes.push(data)
}
}
}
/// Free a node.
pub fn free_node(&mut self, node: Node) {
// Quick check for a double free.
debug_assert!(!self.nodes[node].is_free(), "{node} is already free");
self.nodes[node] = NodeData::Free {
next: self.freelist,
};
self.freelist = Some(node);
}
/// Free the entire tree rooted at `node`.
pub fn free_tree(&mut self, node: Node) {
if let NodeData::Inner { size, tree, .. } = self[node] {
// Note that we have to capture `tree` by value to avoid borrow checker trouble.
for i in 0..usize::from(size + 1) {
// Recursively free sub-trees. This recursion can never be deeper than `MAX_PATH`,
// and since most trees have less than a handful of nodes, it is worthwhile to
// avoid the heap allocation for an iterative tree traversal.
self.free_tree(tree[i]);
}
}
self.free_node(node);
}
}
#[cfg(test)]
impl<F: Forest> NodePool<F> {
/// Verify the consistency of the tree rooted at `node`.
pub fn verify_tree<C: Comparator<F::Key>>(&self, node: Node, comp: &C)
where
NodeData<F>: fmt::Display,
F::Key: fmt::Display,
{
use crate::entity::EntitySet;
use alloc::vec::Vec;
use core::borrow::Borrow;
use core::cmp::Ordering;
// The root node can't be an inner node with just a single sub-tree. It should have been
// pruned.
if let NodeData::Inner { size, .. } = self[node] {
assert!(size > 0, "Root must have more than one sub-tree");
}
let mut done = match self[node] {
NodeData::Inner { size, .. } | NodeData::Leaf { size, .. } => {
EntitySet::with_capacity(size.into())
}
_ => EntitySet::new(),
};
let mut todo = Vec::new();
// Todo-list entries are:
// 1. Optional LHS key which must be <= all node entries.
// 2. The node reference.
// 3. Optional RHS key which must be > all node entries.
todo.push((None, node, None));
while let Some((lkey, node, rkey)) = todo.pop() {
assert!(done.insert(node), "Node appears more than once in tree");
let mut lower = lkey;
match self[node] {
NodeData::Inner { size, keys, tree } => {
let size = size as usize;
let capacity = tree.len();
let keys = &keys[0..size];
// Verify occupancy.
// Right-most nodes can be small, but others must be at least half full.
assert!(
rkey.is_none() || (size + 1) * 2 >= capacity,
"Only {}/{} entries in {}:{}, upper={}",
size + 1,
capacity,
node,
self[node],
rkey.unwrap()
);
// Queue up the sub-trees, checking for duplicates.
for i in 0..size + 1 {
// Get an upper bound for node[i].
let upper = keys.get(i).cloned().or(rkey);
// Check that keys are strictly monotonic.
if let (Some(a), Some(b)) = (lower, upper) {
assert_eq!(
comp.cmp(a, b),
Ordering::Less,
"Key order {} < {} failed in {}: {}",
a,
b,
node,
self[node]
);
}
// Queue up the sub-tree.
todo.push((lower, tree[i], upper));
// Set a lower bound for the next tree.
lower = upper;
}
}
NodeData::Leaf { size, keys, .. } => {
let size = size as usize;
let capacity = keys.borrow().len();
let keys = &keys.borrow()[0..size];
// Verify occupancy.
// Right-most nodes can be small, but others must be at least half full.
assert!(size > 0, "Leaf {node} is empty");
assert!(
rkey.is_none() || size * 2 >= capacity,
"Only {}/{} entries in {}:{}, upper={}",
size,
capacity,
node,
self[node],
rkey.unwrap()
);
for i in 0..size + 1 {
let upper = keys.get(i).cloned().or(rkey);
// Check that keys are strictly monotonic.
if let (Some(a), Some(b)) = (lower, upper) {
let wanted = if i == 0 {
Ordering::Equal
} else {
Ordering::Less
};
assert_eq!(
comp.cmp(a, b),
wanted,
"Key order for {} - {} failed in {}: {}",
a,
b,
node,
self[node]
);
}
// Set a lower bound for the next key.
lower = upper;
}
}
NodeData::Free { .. } => panic!("Free {} reached", node),
}
}
}
}
impl<F: Forest> Index<Node> for NodePool<F> {
type Output = NodeData<F>;
fn index(&self, index: Node) -> &Self::Output {
self.nodes.index(index)
}
}
impl<F: Forest> IndexMut<Node> for NodePool<F> {
fn index_mut(&mut self, index: Node) -> &mut Self::Output {
self.nodes.index_mut(index)
}
}