diff --git a/2021/Cargo.toml b/2021/Cargo.toml
index 1e77142..23a5cba 100644
--- a/2021/Cargo.toml
+++ b/2021/Cargo.toml
@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+fnv = "1.0.7"
 impl_ops = "0.1.1"
 itertools = "0.10"
 paste = "1.0"
diff --git a/2021/inputs/day14 b/2021/inputs/day14
new file mode 100644
index 0000000..7c05136
--- /dev/null
+++ b/2021/inputs/day14
@@ -0,0 +1,102 @@
+PBVHVOCOCFFNBCNCCBHK
+
+FV -> C
+SS -> B
+SC -> B
+BP -> K
+VP -> S
+HK -> K
+FS -> F
+CC -> V
+VB -> P
+OP -> B
+FO -> N
+FH -> O
+VK -> N
+PV -> S
+HV -> O
+PF -> F
+HH -> F
+NK -> S
+NC -> S
+FC -> H
+FK -> K
+OO -> N
+HP -> C
+NN -> H
+BB -> H
+CN -> P
+PS -> N
+VF -> S
+CB -> B
+OH -> S
+CF -> C
+OK -> P
+CV -> V
+CS -> H
+KN -> B
+OV -> S
+HB -> C
+OS -> V
+PC -> B
+CK -> S
+PP -> K
+SN -> O
+VV -> C
+NS -> F
+PN -> K
+HS -> P
+VO -> B
+VC -> B
+NV -> P
+VS -> N
+FP -> F
+HO -> S
+KS -> O
+BN -> F
+VN -> P
+OC -> K
+SF -> P
+PO -> P
+SB -> O
+FN -> F
+OF -> F
+CP -> C
+HC -> O
+PH -> O
+BC -> O
+NO -> C
+BH -> C
+VH -> S
+KK -> O
+SV -> K
+KB -> K
+BS -> S
+HF -> B
+NH -> S
+PB -> N
+HN -> K
+SK -> B
+FB -> F
+KV -> S
+BF -> S
+ON -> S
+BV -> P
+KC -> S
+NB -> S
+NP -> B
+BK -> K
+NF -> C
+BO -> K
+KF -> B
+KH -> N
+SP -> O
+CO -> S
+KO -> V
+SO -> B
+CH -> C
+KP -> C
+FF -> K
+PK -> F
+OB -> H
+SH -> C
diff --git a/2021/src/bin/day14.rs b/2021/src/bin/day14.rs
new file mode 100644
index 0000000..c66250f
--- /dev/null
+++ b/2021/src/bin/day14.rs
@@ -0,0 +1,90 @@
+#![feature(array_windows)]
+#![feature(test)]
+extern crate test;
+use aoc2021::common::*;
+use fnv::FnvHashMap;
+
+const DAY: usize = 14;
+type Parsed = (FnvHashMap<[u8; 2], u8>, String);
+
+fn parse_input(raw: &str) -> Parsed {
+    let (state, rules) = raw.split_once("\n\n").unwrap();
+    let rules = rules.lines().map(|line| line.as_bytes()).map(|bytes| ([bytes[0], bytes[1]], bytes[bytes.len() - 1])).collect();
+    (rules, state.to_owned())
+}
+
+fn grow_polys((rules, raw_state): &Parsed, generations: usize) -> usize {
+    let mut state = FnvHashMap::default();
+    for polymer in raw_state.as_bytes().array_windows() {
+        *state.entry(*polymer).or_insert(0) += 1;
+    }
+    for _ in 0..generations {
+        for ([p1, p2], quantity) in state.clone() {
+            let output = rules[&[p1, p2]];
+            *state.entry([p1, output]).or_insert(0) += quantity;
+            *state.entry([output, p2]).or_insert(0) += quantity;
+            *state.get_mut(&[p1, p2]).unwrap() -= quantity;
+        }
+    }
+    let mut charcounts = FnvHashMap::default();
+    for (p, q) in state.into_iter().flat_map(|([p1, p2], q)| [(p1, q), (p2, q)]) {
+        *charcounts.entry(p).or_insert(0) += q;
+    }
+    for (&p, q) in charcounts.iter_mut() {
+        // This implementation counts each element except the very first and very last twice.
+        // We add 1 for those and then divide by 2.
+        *q += (p == raw_state.bytes().next().unwrap()) as usize;
+        *q += (p == raw_state.bytes().last().unwrap()) as usize;
+        *q /= 2;
+    }
+    charcounts.values().max().unwrap() - charcounts.values().filter(|&&q| q != 0).min().unwrap()
+}
+
+fn part1(parsed: &Parsed) -> usize {
+    grow_polys(parsed, 10)
+}
+
+fn part2(parsed: &Parsed) -> usize {
+    grow_polys(parsed, 40)
+}
+
+fn main() {
+    let input = parse_input(&read_file(DAY));
+    println!("Part 1: {}", part1(&input));
+    println!("Part 2: {}", part2(&input));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use aoc2021::*;
+
+    const TEST_INPUT: &str = "NNCB
+
+CH -> B
+HH -> N
+CB -> H
+NH -> C
+HB -> C
+HC -> B
+HN -> C
+NN -> C
+BH -> H
+NC -> B
+NB -> B
+BN -> B
+BB -> N
+BC -> B
+CC -> N
+CN -> C";
+
+    test!(part1() == 1588);
+    test!(part2() == 2188189693529);
+    bench!(part1() == 3587);
+    bench!(part2() == 3906445077999);
+    bench_input!(input_size => 120);
+
+    fn input_size((m, s): &Parsed) -> usize {
+        m.len() + s.len()
+    }
+}