diff --git a/src/bin/21.rs b/src/bin/21.rs
new file mode 100644
index 0000000..2900f36
--- /dev/null
+++ b/src/bin/21.rs
@@ -0,0 +1,152 @@
+use aoc::*;
+use cached::proc_macro::cached;
+use itertools::Itertools;
+use std::collections::VecDeque;
+use std::iter;
+
+const INPUT: &str = include_str!("../../input/21");
+
+fn main() {
+    assert_example!(part1, "21-test", 126384);
+    println!("Part 1: {}", part1(INPUT));
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    let pads = vec![Pad::Num, Pad::Dir, Pad::Dir];
+    input
+        .lines()
+        .map(|code| complexity(code, pads.clone()))
+        .sum()
+}
+
+fn part2(input: &str) -> usize {
+    let robots = iter::repeat(Pad::Dir).take(25);
+    let pads = iter::once(Pad::Num).chain(robots).collect_vec();
+    input
+        .lines()
+        .map(|code| complexity(code, pads.clone()))
+        .sum()
+}
+
+fn complexity(code: &str, pads: Vec<Pad>) -> usize {
+    let sequence = expand(code.to_string(), pads);
+    let num: usize = code[0..3].parse().unwrap();
+    sequence * num
+}
+
+/// Return all shortest possible expansions.
+#[cached]
+fn expand(input: String, mut pads: Vec<Pad>) -> usize {
+    let pad = if pads.is_empty() {
+        return input.len();
+    } else {
+        pads.remove(0)
+    };
+
+    let mut output = 0;
+    let mut current = 'A';
+
+    for target in input.chars() {
+        let expansion = shortest_paths(current, target, pad)
+            .into_iter()
+            .map(|path| {
+                let mut required_moves: String = path.into_iter().map(|(_, dir)| dir).collect();
+                required_moves.push('A');
+                expand(required_moves, pads.clone())
+            })
+            .min()
+            .unwrap();
+        output += expansion;
+        current = target;
+    }
+
+    output
+}
+
+#[cached]
+fn shortest_paths(start: char, end: char, pad: Pad) -> Vec<Vec<(char, char)>> {
+    if start == end {
+        return vec![vec![]];
+    }
+
+    let next_chars = pad.next_moves();
+
+    let initial_work = next_chars(start).iter().map(|&w| vec![w]);
+    let mut work: VecDeque<Vec<(char, char)>> = VecDeque::from_iter(initial_work);
+
+    let mut shortest_paths = Vec::new();
+    let mut shortest_length = usize::MAX;
+
+    while let Some(path) = work.pop_front() {
+        if path.len() > shortest_length {
+            // BFS can only get worse now
+            break;
+        }
+
+        let head = path.last().copied().unwrap();
+
+        if head.0 == end {
+            shortest_length = path.len();
+            shortest_paths.push(path);
+            continue;
+        }
+
+        for &next in next_chars(head.0) {
+            if path.iter().any(|&(c, _)| c == next.0) {
+                // Already been there
+                continue;
+            }
+            let mut path = path.clone();
+            path.push(next);
+            work.push_back(path);
+        }
+    }
+
+    shortest_paths
+}
+
+type Moves = &'static [(char, char)];
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+enum Pad {
+    Num,
+    Dir,
+}
+
+impl Pad {
+    fn next_moves(self) -> fn(char) -> Moves {
+        match self {
+            Self::Num => numpad_next,
+            Self::Dir => dirpad_next,
+        }
+    }
+}
+
+fn numpad_next(current: char) -> Moves {
+    match current {
+        'A' => &[('0', '<'), ('3', '^')],
+        '0' => &[('A', '>'), ('2', '^')],
+        '1' => &[('2', '>'), ('4', '^')],
+        '2' => &[('0', 'v'), ('1', '<'), ('3', '>'), ('5', '^')],
+        '3' => &[('A', 'v'), ('2', '<'), ('6', '^')],
+        '4' => &[('1', 'v'), ('5', '>'), ('7', '^')],
+        '5' => &[('2', 'v'), ('4', '<'), ('6', '>'), ('8', '^')],
+        '6' => &[('3', 'v'), ('5', '<'), ('9', '^')],
+        '7' => &[('4', 'v'), ('8', '>')],
+        '8' => &[('5', 'v'), ('7', '<'), ('9', '>')],
+        '9' => &[('6', 'v'), ('8', '<')],
+        other => unreachable!("unknown numpad char: '{other}'"),
+    }
+}
+
+fn dirpad_next(current: char) -> Moves {
+    match current {
+        'A' => &[('>', 'v'), ('^', '<')],
+        '^' => &[('v', 'v'), ('A', '>')],
+        'v' => &[('<', '<'), ('>', '>'), ('^', '^')],
+        '<' => &[('v', '>')],
+        '>' => &[('v', '<'), ('A', '^')],
+        other => unreachable!("unknown dirpad char: '{other}'"),
+    }
+}