1 files changed, 79 insertions, 0 deletions
diff --git a/2021/src/bin/day_6.rs b/2021/src/bin/day_6.rs
new file mode 100644
index 0000000..9a40f9e
--- /dev/null
+++ b/2021/src/bin/day_6.rs
@@ -0,0 +1,79 @@
+use nom::{
+    bytes::complete::tag,
+    character::complete::u32 as nom_u32,
+    combinator::{map, map_res},
+    multi::separated_list1,
+    IResult, ToUsize,
+};
+use std::{collections::VecDeque, fs};
+use thiserror::Error;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("inputs/day_6.txt")?;
+    let mut swarm = parse_swarm(&input).unwrap().1;
+    for _ in 0..80 {
+        swarm.grow();
+    }
+    dbg!(swarm.fish_sum());
+
+    for _ in 80..256 {
+        swarm.grow();
+    }
+    dbg!(swarm.fish_sum());
+
+    Ok(())
+}
+
+#[derive(
+    Default, Debug, Clone, Copy, derive_more::Add, derive_more::AddAssign, derive_more::Sum,
+)]
+struct FishCount(u64);
+
+const FISH_INITIAL_SPAWN_COUNTDOWN: usize = 9;
+const FISH_REPEAT_SPAWN_COUNTDOWN: usize = 7;
+#[derive(Debug)]
+struct Swarm {
+    fish: VecDeque<FishCount>,
+}
+
+#[derive(Debug, Error)]
+enum SwarmParseError {
+    #[error("input was out of range")]
+    OutOfRange,
+}
+
+impl Swarm {
+    fn new(fish_counters: Vec<usize>) -> Result<Swarm, SwarmParseError> {
+        let mut fish = VecDeque::with_capacity(FISH_INITIAL_SPAWN_COUNTDOWN);
+        for _ in 0..FISH_INITIAL_SPAWN_COUNTDOWN {
+            fish.push_back(FishCount::default());
+        }
+        for fish_counter in fish_counters {
+            if fish_counter > fish.len() {
+                return Err(SwarmParseError::OutOfRange);
+            }
+            fish[fish_counter] += FishCount(1);
+        }
+        Ok(Swarm { fish })
+    }
+
+    fn grow(&mut self) {
+        let spawning = self
+            .fish
+            .pop_front()
+            .expect("Fish buffer should maintain exactly 9 entries");
+        self.fish[FISH_REPEAT_SPAWN_COUNTDOWN - 1] += spawning;
+        self.fish.push_back(spawning);
+    }
+
+    fn fish_sum(&self) -> FishCount {
+        self.fish.iter().copied().sum()
+    }
+}
+
+fn parse_swarm(input: &str) -> IResult<&str, Swarm> {
+    map_res(
+        separated_list1(tag(","), map(nom_u32, |n| n.to_usize())),
+        Swarm::new,
+    )(input)
+}