1 files changed, 84 insertions, 0 deletions

diff --git a/crates/profile/src/tree.rs b/crates/profile/src/tree.rs
new file mode 100644
index 000000000..096f58511
--- /dev/null
+++ b/crates/profile/src/tree.rs
@@ -0,0 +1,84 @@
+//! A simple tree implementation which tries to not allocate all over the place.
+use std::ops;
+
+use arena::Arena;
+
+#[derive(Default)]
+pub struct Tree<T> {
+    nodes: Arena<Node<T>>,
+    current_path: Vec<(Idx<T>, Option<Idx<T>>)>,
+}
+
+pub type Idx<T> = arena::Idx<Node<T>>;
+
+impl<T> Tree<T> {
+    pub fn start(&mut self)
+    where
+        T: Default,
+    {
+        let me = self.nodes.alloc(Node::new(T::default()));
+        if let Some((parent, last_child)) = self.current_path.last_mut() {
+            let slot = match *last_child {
+                Some(last_child) => &mut self.nodes[last_child].next_sibling,
+                None => &mut self.nodes[*parent].first_child,
+            };
+            let prev = slot.replace(me);
+            assert!(prev.is_none());
+            *last_child = Some(me);
+        }
+
+        self.current_path.push((me, None));
+    }
+
+    pub fn finish(&mut self, data: T) {
+        let (me, _last_child) = self.current_path.pop().unwrap();
+        self.nodes[me].data = data;
+    }
+
+    pub fn root(&self) -> Option<Idx<T>> {
+        self.nodes.iter().next().map(|(idx, _)| idx)
+    }
+
+    pub fn children(&self, idx: Idx<T>) -> impl Iterator<Item = Idx<T>> + '_ {
+        NodeIter { nodes: &self.nodes, next: self.nodes[idx].first_child }
+    }
+    pub fn clear(&mut self) {
+        self.nodes.clear();
+        self.current_path.clear();
+    }
+}
+
+impl<T> ops::Index<Idx<T>> for Tree<T> {
+    type Output = T;
+    fn index(&self, index: Idx<T>) -> &T {
+        &self.nodes[index].data
+    }
+}
+
+pub struct Node<T> {
+    data: T,
+    first_child: Option<Idx<T>>,
+    next_sibling: Option<Idx<T>>,
+}
+
+impl<T> Node<T> {
+    fn new(data: T) -> Node<T> {
+        Node { data, first_child: None, next_sibling: None }
+    }
+}
+
+struct NodeIter<'a, T> {
+    nodes: &'a Arena<Node<T>>,
+    next: Option<Idx<T>>,
+}
+
+impl<'a, T> Iterator for NodeIter<'a, T> {
+    type Item = Idx<T>;
+
+    fn next(&mut self) -> Option<Idx<T>> {
+        self.next.map(|next| {
+            self.next = self.nodes[next].next_sibling;
+            next
+        })
+    }
+}