Solve day 6!

source/day_06/mod.rs

@@ -0,0 +1,72 @@
+use std::collections::HashMap;
+
+use color_eyre::Result;
+
+type FishMap = HashMap<isize, isize>;
+
+pub fn solve() -> Result<()> {
+  let input_data = include_str!("../../data/day_06.txt").trim();
+  println!("Day 06 Part 1: {}", part_1(input_data)?);
+  println!("Day 06 Part 2: {}", part_2(input_data)?);
+  Ok(())
+}
+
+fn part_1(input: &str) -> Result<isize> {
+  let mut fishes = parse_fishes(input)?;
+
+  for _ in 0..80 {
+    fishes = simulate_fishes(fishes);
+  }
+
+  Ok(count_fishes(fishes))
+}
+
+fn part_2(input: &str) -> Result<isize> {
+  let mut fishes = parse_fishes(input)?;
+
+  for _ in 0..256 {
+    fishes = simulate_fishes(fishes);
+  }
+
+  Ok(count_fishes(fishes))
+}
+
+fn parse_fishes(input: &str) -> Result<FishMap> {
+  let mut fishes = FishMap::new();
+  let individual_fishes = input
+    .split(",")
+    .map(str::parse)
+    .collect::<Result<Vec<_>, _>>()?;
+
+  for fish in individual_fishes {
+    let amount = fishes.entry(fish).or_default();
+    *amount += 1;
+  }
+
+  Ok(fishes)
+}
+
+fn count_fishes(fishes: FishMap) -> isize {
+  fishes.into_iter().map(|(_, amount)| amount).sum()
+}
+
+fn simulate_fishes(fishes: FishMap) -> FishMap {
+  let mut new_fishes = HashMap::new();
+
+  for (mut timer, amount) in fishes {
+    timer -= 1;
+
+    if timer < 0 {
+      let new_fish = new_fishes.entry(8).or_default();
+      *new_fish += amount;
+
+      let reset_fish = new_fishes.entry(6).or_default();
+      *reset_fish += amount;
+    } else {
+      let current_fish = new_fishes.entry(timer).or_default();
+      *current_fish += amount;
+    }
+  }
+
+  new_fishes
+}

source/main.rs

@@ -7,6 +7,7 @@ mod day_02;
 mod day_03;
 mod day_04;
 mod day_05;
+mod day_06;
 
 fn main() -> Result<()> {
   color_eyre::install()?;
@@ -19,6 +20,7 @@ fn main() -> Result<()> {
     day_03::solve,
     day_04::solve,
     day_05::solve,
+    day_06::solve,
   ];
 
   for day in days {