diff --git a/2021/src/day6.rs b/2021/src/day6.rs
new file mode 100644
index 0000000..2840b66
--- /dev/null
+++ b/2021/src/day6.rs
@@ -0,0 +1,56 @@
+use color_eyre::eyre;
+
+
+struct Ocean {
+    fish: [usize; 9],
+}
+
+impl Ocean {
+    fn from(ages: &[usize]) -> Self {
+        let mut ocean = Ocean {fish: [0; 9]};
+        for n in ages {
+            ocean.fish[*n] += 1;
+        }
+        ocean
+    }
+
+    fn step(&mut self) {
+        let mut new = [0; 9];
+        
+        // 0 is a special case
+        new[8] += self.fish[0]; // newly-created fish
+        new[6] += self.fish[0]; // existing fish with timers reset
+        for i in 1..9 {
+            new[i - 1] += self.fish[i];
+        }
+
+        self.fish = new;
+    }
+
+    fn sum(&self) -> usize {
+        self.fish.iter().sum()
+    }
+
+    fn step_by(&mut self, n: usize) {
+        for _ in 0..n {
+            self.step();
+        }
+    }
+}
+
+
+pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
+    let mut ages = Vec::new();
+    for a in data.trim().split(',') {
+        ages.push(a.parse::<usize>()?);
+    }
+    let mut ocean = Ocean::from(&ages);
+
+    ocean.step_by(80);
+    let one = ocean.sum();
+
+    ocean.step_by(256 - 80);
+    let two = ocean.sum();
+
+    Ok((one, two))
+}
diff --git a/2021/src/main.rs b/2021/src/main.rs
index 931307a..979da0d 100644
--- a/2021/src/main.rs
+++ b/2021/src/main.rs
@@ -4,14 +4,14 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day5;
+mod day6;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/05.txt")?;
+    let data = load("data/06.txt")?;
 
     let start = Instant::now();
-    let (one, two) = day5::run(&data)?;
+    let (one, two) = day6::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
 
     let precision = 2.0 - dur.log10().floor();