1 files changed, 48 insertions, 0 deletions

diff --git a/2021/day06/src/main.rs b/2021/day06/src/main.rs
new file mode 100644
index 0000000..e3ef79c
--- /dev/null
+++ b/2021/day06/src/main.rs
@@ -0,0 +1,48 @@
+use std::fs;
+use std::path::Path;
+
+/* AOC21 Day 6: https://adventofcode.com/2021/day/6 */
+fn main() {
+    let input = Path::new("resources").join("input.txt");
+    let content = fs::read_to_string(input).expect("Unable to read input file");
+    println!("Ex1: The number of lanternfishes is {}", evolution(parse_input(&content), 80));
+    println!("Ex2: The number of lanternfishes is {}", evolution(parse_input(&content), 256));
+}
+
+fn parse_input(s: &str) -> Vec<u64> {
+    let mut lfs = vec![0;9];
+    s.split(",").for_each(|n| lfs[n.parse::<usize>().expect("Malformed input")] += 1);
+    lfs
+}
+
+fn evolution(mut lfs: Vec<u64>, days: usize) -> u64 {
+    (1..days).for_each(|i| lfs[(i+7)%9] += lfs[i%9]);
+    lfs.iter().sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const LANTERNFISHES: &str = "3,4,3,1,2";
+
+    #[test]
+    fn input_parsing() {
+        assert_eq!(vec![0,1,1,2,1,0,0,0,0], parse_input("3,4,3,1,2"))
+    }
+
+    #[test]
+    fn evolution_18days() {
+        assert_eq!(26, evolution(parse_input(LANTERNFISHES), 18))
+    }
+
+    #[test]
+    fn evolution_80days() {
+        assert_eq!(5934, evolution(parse_input(LANTERNFISHES), 80))
+    }
+
+    #[test]
+    fn evolution_256days() {
+        assert_eq!(26984457539, evolution(parse_input(LANTERNFISHES), 256))
+    }
+}