diff --git a/ti-basic-optimizer/src/analyze/control_flow/graph.rs b/ti-basic-optimizer/src/analyze/control_flow/graph.rs
new file mode 100644
index 0000000..54de14d
--- /dev/null
+++ b/ti-basic-optimizer/src/analyze/control_flow/graph.rs
@@ -0,0 +1,113 @@
+//! # Graph Implementation
+//! Existing graph libraries won't do for the sorts of invasive manipulations we need to be able to
+//! do efficiently.
+
+use std::cell::{Ref, RefCell, RefMut};
+
+/// Opaque, immutable "name" for a node in a [`Digraph`]. Can be used to obtain a mutable or immutable
+/// reference to the associated data with [`Digraph::node`] and [`Digraph::node_mut`].
+#[derive(Debug, Eq, PartialEq, Clone, Copy)]
+pub struct NodeIndex(usize);
+
+/// Implements a directed cyclic graph.
+pub struct Digraph<T> {
+    nodes: Vec<RefCell<T>>,
+    out_arcs: Vec<Vec<usize>>,
+    in_arcs: Vec<Vec<usize>>,
+}
+
+impl<T> Digraph<T> {
+    pub fn new() -> Self {
+        Self {
+            nodes: vec![],
+            out_arcs: vec![],
+            in_arcs: vec![],
+        }
+    }
+
+    /// Returns a Vec with the direct successors of the provided block.
+    pub fn out_arcs(&self, idx: NodeIndex) -> Vec<NodeIndex> {
+        self.out_arcs[idx.0].iter().map(|&x| NodeIndex(x)).collect()
+    }
+
+    /// Returns a Vec with the direct predecessors of the provided block.
+    pub fn in_arcs(&self, idx: NodeIndex) -> Vec<NodeIndex> {
+        self.in_arcs[idx.0].iter().map(|&x| NodeIndex(x)).collect()
+    }
+
+    pub fn out_degree(&self, idx: NodeIndex) -> usize {
+        self.out_arcs[idx.0].len()
+    }
+
+    pub fn in_degree(&self, idx: NodeIndex) -> usize {
+        self.in_arcs[idx.0].len()
+    }
+
+    /// Access and mutate the data associated with the provided [`NodeIndex`].
+    pub fn node_mut(&self, idx: NodeIndex) -> RefMut<T> {
+        self.nodes[idx.0].borrow_mut()
+    }
+
+    /// Access the data associated with the provided [`NodeIndex`].
+    pub fn node(&self, idx: NodeIndex) -> Ref<T> {
+        self.nodes[idx.0].borrow()
+    }
+
+    /// Insert a node into the digraph. No edges are created.
+    ///
+    /// Returns a [`NodeIndex`] for the node which was just inserted.
+    pub fn insert_node(&mut self, node: T) -> NodeIndex {
+        self.nodes.push(RefCell::new(node));
+        self.out_arcs.push(vec![]);
+        self.in_arcs.push(vec![]);
+
+        NodeIndex(self.nodes.len() - 1)
+    }
+
+    pub fn insert_arc(&mut self, from: NodeIndex, to: NodeIndex) {
+        self.out_arcs[from.0].push(to.0);
+        self.in_arcs[to.0].push(from.0);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::analyze::control_flow::graph::Digraph;
+
+    #[test]
+    #[should_panic]
+    fn cant_have_two_refmut_to_same_block() {
+        struct Node(u8);
+
+        let mut g = Digraph::new();
+
+        let a = g.insert_node(Node(1));
+        let b = g.insert_node(Node(2));
+        g.insert_arc(a, b);
+
+        let a_mut = g.node_mut(a);
+        let a_mut_2 = g.node_mut(a);
+    }
+
+    #[test]
+    fn interior_mutability() {
+        struct Node(u8);
+
+        let mut g = Digraph::new();
+
+        let a = g.insert_node(Node(1));
+        let b = g.insert_node(Node(2));
+        g.insert_arc(a, b);
+
+        {
+            let mut a_mut = g.node_mut(a);
+            let mut b_mut = g.node_mut(b);
+
+            a_mut.0 += 1;
+            b_mut.0 += 1;
+        }
+
+        assert_eq!(g.node(a).0, 2);
+        assert_eq!(g.node(b).0, 3);
+    }
+}
diff --git a/ti-basic-optimizer/src/analyze/control_flow/mod.rs b/ti-basic-optimizer/src/analyze/control_flow/mod.rs
index 61b36d5..e3b94ce 100644
--- a/ti-basic-optimizer/src/analyze/control_flow/mod.rs
+++ b/ti-basic-optimizer/src/analyze/control_flow/mod.rs
@@ -2,3 +2,6 @@
 
 mod failure_paths;
 mod labels;
+mod graph;
+
+pub use graph::Digraph;