1 files changed, 47 insertions, 0 deletions
diff --git a/src/bin/2021/day6.rs b/src/bin/2021/day6.rs
new file mode 100644
index 0000000..039209e
--- /dev/null
+++ b/src/bin/2021/day6.rs
@@ -0,0 +1,47 @@
+use advent_of_code::prelude::*;
+
+pub fn parse(fh: File) -> Result<Vec<usize>> {
+    Ok(parse::split(fh, b',').collect())
+}
+
+pub fn part1(mut fishes: Vec<usize>) -> Result<usize> {
+    for _ in 0..80 {
+        let mut new = 0;
+        for fish in fishes.iter_mut() {
+            if *fish == 0 {
+                *fish = 6;
+                new += 1;
+            } else {
+                *fish -= 1;
+            }
+        }
+        fishes.resize(fishes.len() + new, 8);
+    }
+    Ok(fishes.len())
+}
+
+pub fn part2(fishes: Vec<usize>) -> Result<usize> {
+    let mut by_age = VecDeque::new();
+    by_age.resize(9, 0);
+    for fish in fishes {
+        by_age[fish] += 1;
+    }
+    for _ in 0..256 {
+        let new = by_age.pop_front().unwrap();
+        by_age[6] += new;
+        by_age.push_back(new);
+    }
+    Ok(by_age.iter().sum())
+}
+
+#[test]
+fn test() {
+    assert_eq!(
+        part1(parse(parse::data(2021, 6).unwrap()).unwrap()).unwrap(),
+        379114
+    );
+    assert_eq!(
+        part2(parse(parse::data(2021, 6).unwrap()).unwrap()).unwrap(),
+        1702631502303
+    );
+}