Day 04 in the morning

Author: Cadiac

README.md

@@ -89,3 +89,4 @@ This should start the server at `localhost:8080`.
 ❄️ [Day 01](aoc-solver/src/y2025/day01.rs)
 ❄️ [Day 02](aoc-solver/src/y2025/day02.rs)
 ❄️ [Day 03](aoc-solver/src/y2025/day03.rs)
+❄️ [Day 04](aoc-solver/src/y2025/day04.rs)

aoc-solver/src/y2025/day04.rs

@@ -0,0 +1,118 @@
+use std::collections::HashSet;
+
+use crate::solution::{AocError, Solution};
+
+pub type Coords = (isize, isize);
+
+#[rustfmt::skip]
+const NEIGHBOURS: [Coords; 8] = [
+    (-1, -1), (0, -1), (1, -1),
+    (-1,  0),          (1,  0),
+    (-1,  1), (0,  1), (1,  1),
+];
+
+pub struct Day04;
+
+fn parse(input: &str) -> Result<HashSet<Coords>, AocError> {
+    let mut result = HashSet::new();
+
+    for (y, row) in input.trim().lines().enumerate() {
+        for (x, tile) in row.chars().enumerate() {
+            if tile == '@' {
+                result.insert((x as isize, y as isize));
+            }
+        }
+    }
+
+    Ok(result)
+}
+
+fn count_neighbours((x, y): &Coords, occupied: &HashSet<Coords>) -> usize {
+    NEIGHBOURS
+        .iter()
+        .filter(|(dx, dy)| occupied.contains(&(x + dx, y + dy)))
+        .count()
+}
+
+impl Solution for Day04 {
+    type Part1 = usize;
+    type Part2 = usize;
+
+    fn default_input(&self) -> &'static str {
+        include_str!("../../../inputs/2025/day04.txt")
+    }
+
+    fn part_1(&self, input: &str) -> Result<usize, AocError> {
+        let occupied = parse(input)?;
+
+        let result = occupied
+            .iter()
+            .filter(|roll| count_neighbours(roll, &occupied) < 4)
+            .count();
+
+        Ok(result)
+    }
+
+    fn part_2(&self, input: &str) -> Result<usize, AocError> {
+        let mut occupied = parse(input)?;
+        let mut removed = 0;
+
+        loop {
+            let before = occupied.len();
+            let current = occupied.clone();
+            occupied.retain(|roll| count_neighbours(roll, &current) >= 4);
+            let after = occupied.len();
+
+            removed += before - after;
+
+            if before == after {
+                break;
+            }
+        }
+
+        Ok(removed)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_solves_part1_example() {
+        assert_eq!(
+            Day04.part_1(
+                "..@@.@@@@.\n\
+                 @@@.@.@.@@\n\
+                 @@@@@.@.@@\n\
+                 @.@@@@..@.\n\
+                 @@.@@@@.@@\n\
+                 .@@@@@@@.@\n\
+                 .@.@.@.@@@\n\
+                 @.@@@.@@@@\n\
+                 .@@@@@@@@.\n\
+                 @.@.@@@.@."
+            ),
+            Ok(13)
+        );
+    }
+
+    #[test]
+    fn it_solves_part2_example() {
+        assert_eq!(
+            Day04.part_2(
+                "..@@.@@@@.\n\
+                 @@@.@.@.@@\n\
+                 @@@@@.@.@@\n\
+                 @.@@@@..@.\n\
+                 @@.@@@@.@@\n\
+                 .@@@@@@@.@\n\
+                 .@.@.@.@@@\n\
+                 @.@@@.@@@@\n\
+                 .@@@@@@@@.\n\
+                 @.@.@@@.@."
+            ),
+            Ok(43)
+        );
+    }
+}

aoc-solver/src/y2025/mod.rs

@@ -3,8 +3,9 @@ use crate::solution::{Solution, Solver};
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
 
-pub const MAX_DAYS: u8 = 3;
+pub const MAX_DAYS: u8 = 4;
 
 pub struct Y2025;
 
@@ -14,6 +15,7 @@ impl Solver for Y2025 {
             1 => day01::Day01.run(input, 1, 2025),
             2 => day02::Day02.run(input, 2, 2025),
             3 => day03::Day03.run(input, 3, 2025),
+            4 => day04::Day04.run(input, 4, 2025),
             _ => vec![String::from("Solution not implemented (yet?)")],
         }
     }
@@ -34,6 +36,7 @@ impl Solver for Y2025 {
             1 => include_str!("./day01.rs"),
             2 => include_str!("./day02.rs"),
             3 => include_str!("./day03.rs"),
+            4 => include_str!("./day04.rs"),
             _ => unimplemented!(),
         }
     }

aoc-web/src/header.rs

@@ -22,6 +22,7 @@ pub fn header(props: &HeaderProps) -> Html {
                                     <NavLink route={Route::Solution { year: 2025, day: 1 }} current={props.route.clone()} text={"1"}/>
                                     <NavLink route={Route::Solution { year: 2025, day: 2 }} current={props.route.clone()} text={"2"}/>
                                     <NavLink route={Route::Solution { year: 2025, day: 3 }} current={props.route.clone()} text={"3"}/>
+                                    <NavLink route={Route::Solution { year: 2025, day: 4 }} current={props.route.clone()} text={"4"}/>
                                 </>
                             }
                         },