diff --git a/crates/quadtree/src/lib.rs b/crates/quadtree/src/lib.rs
index 95095a50..4a7f4d18 100644
--- a/crates/quadtree/src/lib.rs
+++ b/crates/quadtree/src/lib.rs
@@ -1,7 +1,10 @@
 use glam::Vec2;
 use quadrants::{Quadrants, Rect};
+use tree::Tree;
 
+mod packed;
 mod quadrants;
+mod tree;
 
 pub struct Tree<T>
 where
diff --git a/crates/quadtree/src/packed.rs b/crates/quadtree/src/packed.rs
new file mode 100644
index 00000000..db36552e
--- /dev/null
+++ b/crates/quadtree/src/packed.rs
@@ -0,0 +1,37 @@
+use std::ops::{Deref, DerefMut};
+
+pub(super) struct Packed<T>(Vec<T>);
+
+impl<T> Packed<T> {
+    pub(super) fn new() -> Self {
+        Self(Vec::new())
+    }
+
+    /// Removes an item from the Vec with O(1) efficiency.
+    ///
+    /// It can move another item to the new position as a side effect. The
+    /// original index of the moved item is returned in such a case.
+    pub(super) fn swap_remove(&mut self, index: usize) -> (T, Option<usize>) {
+        let removed = self.0.swap_remove(index);
+        let moved = if index < self.0.len() {
+            Some(self.0.len())
+        } else {
+            None
+        };
+        (removed, moved)
+    }
+}
+
+impl<T> Deref for Packed<T> {
+    type Target = Vec<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T> DerefMut for Packed<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
diff --git a/crates/quadtree/src/tree.rs b/crates/quadtree/src/tree.rs
new file mode 100644
index 00000000..fc1c6a04
--- /dev/null
+++ b/crates/quadtree/src/tree.rs
@@ -0,0 +1,116 @@
+use crate::{packed::Packed, quadrants::Quadrants};
+
+pub(super) struct Tree<S>
+where
+    S: Default,
+{
+    inner: Packed<Inner>,
+    leafs: Packed<Leaf<S>>,
+}
+
+impl<S> Tree<S>
+where
+    S: Default,
+{
+    pub(super) fn new() -> Self {
+        let mut inner = Packed::new();
+        // Add empty root.
+        inner.push(Inner::new(usize::MAX, Quadrants::default()));
+        Self {
+            inner,
+            leafs: Packed::new(),
+        }
+    }
+
+    pub(super) fn get_leaf_mut(&self, index: usize) -> Option<&mut S> {
+        self.leafs.get_mut(index).map(|l| &mut l.item)
+    }
+
+    // TODO remove
+    // TODO replace (auto crate the replacement)
+
+    fn replace(&mut self, node: Node, replacement: Option<Node>) -> Option<S> {
+        let (removed_parent, moved, item) = match node {
+            Node::Inner(index) => {
+                if index == 0 {
+                    panic!("Cannot remove or replace root node.");
+                }
+
+                let (removed, moved) = self.inner.swap_remove(index);
+                (removed.parent, moved.map(|index| Node::Inner(index)), None)
+            }
+            Node::Leaf(index) => {
+                let (removed, moved) = self.leafs.swap_remove(index);
+                (
+                    removed.parent,
+                    moved.map(|index| Node::Leaf(index)),
+                    Some(removed.item),
+                )
+            }
+        };
+
+        self.inner
+            .get_mut(removed_parent)
+            .unwrap()
+            .replace_child(node, replacement);
+
+        if let Some(moved) = moved {
+            let moved_parent = match node {
+                Node::Inner(index) => self.inner.get(index).unwrap().parent,
+                Node::Leaf(index) => self.leafs.get(index).unwrap().parent,
+            };
+
+            self.inner
+                .get_mut(moved_parent)
+                .unwrap()
+                .replace_child(moved, Some(node));
+        }
+
+        item
+    }
+}
+
+// TODO rename to Node
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum Node {
+    // TODO consider compressing usize to something smaller & saving the extra
+    // byte for enum
+    Inner(usize),
+    Leaf(usize),
+}
+
+struct Inner {
+    // TODO consider using MAX value for no parent
+    // TODO consider using something smaller than usize
+    parent: usize,
+    children: Quadrants<Node>,
+}
+
+impl Inner {
+    fn new(parent: usize, children: Quadrants<Node>) -> Self {
+        Self { parent, children }
+    }
+
+    fn replace_child(&mut self, old: Node, new: Option<Node>) {
+        self.children.replace(&old, new);
+    }
+}
+
+struct Leaf<S> {
+    // TODO consider using something smaller than usize
+    parent: usize,
+    // TODO consider using different array len
+    item: S,
+}
+
+impl<S> Leaf<S>
+where
+    S: Default,
+{
+    fn new(parent: usize) -> Self {
+        Self {
+            parent,
+            item: S::default(),
+        }
+    }
+}