diff --git a/2022/src/bin/day21.rs b/2022/src/bin/day21.rs
new file mode 100644
index 0000000..7730c07
--- /dev/null
+++ b/2022/src/bin/day21.rs
@@ -0,0 +1,137 @@
+use std::collections::{HashMap, VecDeque};
+
+const INPUT: &str = include_str!("../../input/day21.txt");
+
+fn main() {
+    let monkeys = parse(INPUT);
+    println!("{:?}", part1(monkeys.clone()));
+}
+
+fn parse(input: &str) -> HashMap<Name, Expr> {
+    input
+        .lines()
+        .map(|line| {
+            let words: Vec<&str> = line.split(": ").collect();
+            let name = words[0].to_string();
+            let expr = Expr::parse(words[1]);
+
+            (name, expr)
+        })
+        .collect()
+}
+
+type Name = String;
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+enum Expr {
+    Const(u64),
+    Monkey(Name),
+    Add(Box<Expr>, Box<Expr>),
+    Sub(Box<Expr>, Box<Expr>),
+    Mul(Box<Expr>, Box<Expr>),
+    Div(Box<Expr>, Box<Expr>),
+}
+
+impl Expr {
+    fn parse(s: &str) -> Self {
+        let w: Vec<&str> = s.split(' ').collect();
+
+        if w.len() == 1 {
+            Expr::parse_atom(w[0])
+        } else {
+            Expr::parse_binary(w)
+        }
+    }
+
+    fn parse_atom(s: &str) -> Self {
+        match s.parse::<u64>() {
+            Ok(u) => Expr::Const(u),
+            Err(_) => Expr::Monkey(s.to_string()),
+        }
+    }
+
+    fn parse_binary(w: Vec<&str>) -> Self {
+        assert_eq!(w.len(), 3);
+
+        let l = Self::parse_atom(w[0]);
+        let r = Self::parse_atom(w[2]);
+
+        match w[1] {
+            "+" => Expr::Add(Box::new(l), Box::new(r)),
+            "-" => Expr::Sub(Box::new(l), Box::new(r)),
+            "*" => Expr::Mul(Box::new(l), Box::new(r)),
+            "/" => Expr::Div(Box::new(l), Box::new(r)),
+            other => panic!("{}", other),
+        }
+    }
+}
+
+fn part1(monkeys: HashMap<Name, Expr>) -> u64 {
+    let mut ev = Evaluator::new(monkeys);
+    ev.eval("root".to_string())
+}
+
+impl Expr {
+    fn bindings(&self) -> Vec<Name> {
+        match self {
+            Expr::Const(_) => vec![],
+            Expr::Monkey(name) => vec![name.clone()],
+            Expr::Add(l, r) => [l.bindings(), r.bindings()].concat(),
+            Expr::Sub(l, r) => [l.bindings(), r.bindings()].concat(),
+            Expr::Mul(l, r) => [l.bindings(), r.bindings()].concat(),
+            Expr::Div(l, r) => [l.bindings(), r.bindings()].concat(),
+        }
+    }
+
+    fn value(&self, memo: &HashMap<Name, u64>) -> u64 {
+        match self {
+            Expr::Const(v) => *v,
+            Expr::Monkey(name) => memo[name],
+            Expr::Add(l, r) => l.value(memo) + r.value(memo),
+            Expr::Sub(l, r) => l.value(memo) - r.value(memo),
+            Expr::Mul(l, r) => l.value(memo) * r.value(memo),
+            Expr::Div(l, r) => l.value(memo) / r.value(memo),
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Evaluator {
+    memo: HashMap<Name, u64>,
+    monkeys: HashMap<Name, Expr>,
+}
+
+impl Evaluator {
+    fn new(monkeys: HashMap<Name, Expr>) -> Self {
+        Self {
+            memo: Default::default(),
+            monkeys,
+        }
+    }
+
+    fn eval(&mut self, name: Name) -> u64 {
+        let mut queue: VecDeque<Name> = Default::default();
+        queue.push_front(name.clone());
+
+        let mut needed: VecDeque<Name> = Default::default();
+
+        while let Some(want) = queue.pop_front() {
+            if needed.contains(&want) {
+                continue;
+            }
+            needed.push_front(want.clone());
+
+            for name in self.monkeys[&want].bindings() {
+                queue.push_front(name.to_string());
+            }
+        }
+
+        for name in needed {
+            let expr = &self.monkeys[&name];
+            let v = expr.value(&self.memo);
+            self.memo.insert(name, v);
+        }
+
+        self.memo[&name]
+    }
+}