2 files changed, 136 insertions, 0 deletions
diff --git a/2021/rust/day06/Cargo.toml b/2021/rust/day06/Cargo.toml
new file mode 100644
index 0000000..93feea9
--- /dev/null
+++ b/2021/rust/day06/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "day06"
+version = "0.1.0"
+authors = ["cos <cos>"]
+edition = "2021"
+
+[dependencies]
+aoc = { path = "../../../common/rust/aoc" }
+anyhow = "1.0"
diff --git a/2021/rust/day06/src/main.rs b/2021/rust/day06/src/main.rs
new file mode 100644
index 0000000..b17e746
--- /dev/null
+++ b/2021/rust/day06/src/main.rs
@@ -0,0 +1,127 @@
+use {
+    anyhow::{
+        anyhow,
+        Context,
+        Result,
+    },
+    std::{
+        env::args,
+        fs::File,
+        io::{
+            BufRead,
+            BufReader,
+        },
+        path::Path,
+    },
+};
+
+#[derive(Clone,Copy,Debug)]
+struct LanternFish {
+    days: u8,
+}
+
+impl LanternFish {
+    pub fn new(days: u8) -> Self {
+        Self {
+            days,
+        }
+    }
+
+    fn step(&mut self) -> bool {
+        if self.days == 0 {
+            self.days = 6;
+            true
+        } else {
+            self.days -= 1;
+            false
+        }
+    }
+}
+
+impl Default for LanternFish {
+    fn default() -> Self {
+        Self::new(8)
+    }
+}
+
+struct Population {
+    count: Vec<usize>,
+}
+
+impl Population {
+    fn new<'a, I: IntoIterator<Item = &'a LanternFish>>(input: I) -> Self {
+        let mut count = vec![0; 9];
+
+        for fish in input {
+            count[fish.days as usize] += 1;
+        }
+
+        Self {
+            count,
+        }
+    }
+
+    fn step(&mut self) {
+        let iter = &mut self.count.iter_mut();
+        let spawn = iter.next().unwrap();
+
+        let mut new_count: Vec<_> = iter.map(|v| *v).collect();
+        new_count.push(0);
+        new_count[6] += *spawn;
+        new_count[8] = *spawn;
+        self.count = new_count;
+    }
+
+    fn sum(&self) -> usize {
+        self.count.iter().sum()
+    }
+}
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<LanternFish>> {
+    let reader = BufReader::new(File::open(filename)?);
+
+    let line = reader.lines().next().ok_or(anyhow!("Could not read input"))??;
+    line.split(',').map(|v| {
+        let days = v.parse().map_err(|err| anyhow!("{}", err))?;
+        Ok(LanternFish::new(days))
+    }).collect()
+}
+
+fn part1<'a, I: IntoIterator<Item = &'a LanternFish>>(input: I) -> Result<usize> {
+    let mut population: Vec<_> = input.into_iter().copied().collect();
+    for _ in 0..80 {
+        let mut children = vec![];
+        for fish in &mut population {
+            if fish.step() {
+                children.push(LanternFish::default())
+            }
+        }
+        population.append(&mut children);
+    }
+    Ok(population.len())
+}
+
+fn part2<'a, I: IntoIterator<Item = &'a LanternFish>>(input: I) -> Result<usize> {
+    let mut population = Population::new(input);
+    for _ in 0..256 {
+        population.step();
+    }
+
+    Ok(population.sum())
+}
+
+fn main() -> Result<()> {
+    let ( do_part_1, do_part_2 ) = aoc::do_parts();
+
+    let filename = args().nth(1).ok_or(anyhow!("Missing input filename"))?;
+    let input = read_input(filename).context("Could not read input")?;
+    if do_part_1 {
+        let solution = part1(&input).context("No solution for part 1")?;
+        println!("Part1, solution found to be: {}", solution);
+    }
+    if do_part_2 {
+        let solution = part2(&input).context("No solution for part 2")?;
+        println!("Part2, solution found to be: {}", solution);
+    }
+    Ok(())
+}