diff --git a/aoc2021/input/day06.txt b/aoc2021/input/day06.txt
new file mode 100644
index 0000000..156bc49
--- /dev/null
+++ b/aoc2021/input/day06.txt
@@ -0,0 +1 @@
+4,1,4,1,3,3,1,4,3,3,2,1,1,3,5,1,3,5,2,5,1,5,5,1,3,2,5,3,1,3,4,2,3,2,3,3,2,1,5,4,1,1,1,2,1,4,4,4,2,1,2,1,5,1,5,1,2,1,4,4,5,3,3,4,1,4,4,2,1,4,4,3,5,2,5,4,1,5,1,1,1,4,5,3,4,3,4,2,2,2,2,4,5,3,5,2,4,2,3,4,1,4,4,1,4,5,3,4,2,2,2,4,3,3,3,3,4,2,1,2,5,5,3,2,3,5,5,5,4,4,5,5,4,3,4,1,5,1,3,4,4,1,3,1,3,1,1,2,4,5,3,1,2,4,3,3,5,4,4,5,4,1,3,1,1,4,4,4,4,3,4,3,1,4,5,1,2,4,3,5,1,1,2,1,1,5,4,2,1,5,4,5,2,4,4,1,5,2,2,5,3,3,2,3,1,5,5,5,4,3,1,1,5,1,4,5,2,1,3,1,2,4,4,1,1,2,5,3,1,5,2,4,5,1,2,3,1,2,2,1,2,2,1,4,1,3,4,2,1,1,5,4,1,5,4,4,3,1,3,3,1,1,3,3,4,2,3,4,2,3,1,4,1,5,3,1,1,5,3,2,3,5,1,3,1,1,3,5,1,5,1,1,3,1,1,1,1,3,3,1
diff --git a/aoc2021/src/day06.rs b/aoc2021/src/day06.rs
new file mode 100644
index 0000000..876252c
--- /dev/null
+++ b/aoc2021/src/day06.rs
@@ -0,0 +1,126 @@
+use std::fmt::Write;
+use std::str;
+
+use anyhow::Result;
+
+const INPUT: &str = include_str!("../input/day06.txt");
+
+const SPAWNING_DELAY: u8 = 7;
+const TURNS_PART_1: usize = 80;
+const TURNS_PART_2: usize = 256;
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let mut school = input
+        .trim()
+        .split(',')
+        .map(str::parse::<LanternFish>)
+        .collect::<Result<Vec<_>>>()?;
+
+    for _ in 0..TURNS_PART_1 {
+        let mut newly_spawned = 0;
+
+        for fish in &mut school {
+            if fish.next_turn() {
+                newly_spawned += 1;
+            }
+        }
+
+        school.resize_with(school.len() + newly_spawned, LanternFish::default)
+    }
+
+    Ok(school.len())
+}
+
+fn part2(input: &str) -> Result<usize> {
+    let mut school = input.trim().parse::<School>()?;
+
+    for _ in 0..TURNS_PART_2 {
+        school.next_turn();
+    }
+
+    Ok(school.size())
+}
+
+struct LanternFish {
+    timer: u8,
+}
+
+impl LanternFish {
+    fn spawn() -> Self {
+        LanternFish {
+            timer: SPAWNING_DELAY + 1,
+        }
+    }
+
+    fn next_turn(&mut self) -> bool {
+        if self.timer == 0 {
+            self.timer = SPAWNING_DELAY - 1;
+            true
+        } else {
+            self.timer -= 1;
+            false
+        }
+    }
+}
+
+impl Default for LanternFish {
+    fn default() -> Self {
+        Self::spawn()
+    }
+}
+
+struct School {
+    fish_timers: [usize; SPAWNING_DELAY as usize + 2],
+}
+
+impl School {
+    fn next_turn(&mut self) {
+        let newly_spawned = self.fish_timers[0];
+
+        for i in 1..self.fish_timers.len() {
+            self.fish_timers[i - 1] = self.fish_timers[i];
+        }
+
+        self.fish_timers[SPAWNING_DELAY as usize - 1] += newly_spawned;
+        *self.fish_timers.last_mut().unwrap() = newly_spawned;
+    }
+
+    fn size(&self) -> usize {
+        self.fish_timers.iter().sum()
+    }
+}
+
+impl std::str::FromStr for School {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let mut fish_timers = [0usize; SPAWNING_DELAY as usize + 2];
+
+        for fish in s.split(',').map(str::parse::<usize>) {
+            let fish = fish?;
+            fish_timers[fish] += 1;
+        }
+
+        Ok(School { fish_timers })
+    }
+}
+
+impl std::str::FromStr for LanternFish {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        Ok(LanternFish { timer: s.parse()? })
+    }
+}
+
+#[cfg(test)]
+mod tests {}
diff --git a/aoc2021/src/lib.rs b/aoc2021/src/lib.rs
index e0126f6..f236974 100644
--- a/aoc2021/src/lib.rs
+++ b/aoc2021/src/lib.rs
@@ -5,3 +5,4 @@ pub mod day02;
 pub mod day03;
 pub mod day04;
 pub mod day05;
+pub mod day06;
diff --git a/aoc2021/src/main.rs b/aoc2021/src/main.rs
index 20a1ce0..c712420 100644
--- a/aoc2021/src/main.rs
+++ b/aoc2021/src/main.rs
@@ -7,9 +7,17 @@ use aoc2021::day02;
 use aoc2021::day03;
 use aoc2021::day04;
 use aoc2021::day05;
+use aoc2021::day06;
 
 fn main() -> Result<()> {
-    let days: &[DayFunc] = &[day01::run, day02::run, day03::run, day04::run, day05::run];
+    let days: &[DayFunc] = &[
+        day01::run,
+        day02::run,
+        day03::run,
+        day04::run,
+        day05::run,
+        day06::run,
+    ];
 
     aoc::run(days)
 }