[2025] Solution for Day 4

Author: kfarnung

2025/src/bin/day04.rs

@@ -0,0 +1,16 @@
+use aoc2025::days::day04::{part1, part2};
+use std::env;
+use std::fs;
+use std::process;
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    if args.len() < 2 {
+        println!("Missing required arguments");
+        process::exit(1);
+    }
+
+    let contents = fs::read_to_string(&args[1]).expect("Something went wrong");
+    println!("Part 1: {}", part1(&contents));
+    println!("Part 2: {}", part2(&contents));
+}

2025/src/days.rs

@@ -1,3 +1,4 @@
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;

2025/src/days/day04.rs

@@ -0,0 +1,119 @@
+use std::vec;
+
+pub fn part1(contents: &str) -> i64 {
+    let grid = parse_input(contents);
+    let mut count = 0;
+
+    for y in 0..grid.len() {
+        for x in 0..grid[0].len() {
+            if grid[y][x] == '@' && is_movable(&grid, x, y) {
+                count += 1;
+            }
+        }
+    }
+    count
+}
+
+pub fn part2(contents: &str) -> i64 {
+    let mut grid = parse_input(contents);
+    let mut count = 0;
+    let mut found = true;
+
+    while found {
+        found = false;
+        let mut new_grid = grid.clone();
+
+        for y in 0..grid.len() {
+            for x in 0..grid[0].len() {
+                if grid[y][x] == '@' && is_movable(&grid, x, y) {
+                    new_grid[y][x] = '.';
+                    found = true;
+                    count += 1;
+                }
+            }
+        }
+        grid = new_grid;
+    }
+    count
+}
+
+fn parse_input(contents: &str) -> Vec<Vec<char>> {
+    let mut grid = vec![];
+    for l in contents.lines() {
+        grid.push(l.chars().collect());
+    }
+    grid
+}
+
+fn is_movable(grid: &Vec<Vec<char>>, x: usize, y: usize) -> bool {
+    let mut count: usize = 0;
+    let rows = grid.len();
+    let cols = grid[0].len();
+
+    if x > 0 && y > 0 && grid[y - 1][x - 1] == '@' {
+        count += 1;
+    }
+    if x > 0 && grid[y][x - 1] == '@' {
+        count += 1;
+    }
+    if x > 0 && y < rows - 1 && grid[y + 1][x - 1] == '@' {
+        count += 1;
+    }
+    if x < cols - 1 && grid[y][x + 1] == '@' {
+        count += 1;
+    }
+    if x < cols - 1 && y > 0 && grid[y - 1][x + 1] == '@' {
+        count += 1;
+    }
+    if y > 0 && grid[y - 1][x] == '@' {
+        count += 1;
+    }
+    if y < rows - 1 && grid[y + 1][x] == '@' {
+        count += 1;
+    }
+    if x < cols - 1 && y < rows - 1 && grid[y + 1][x + 1] == '@' {
+        count += 1;
+    }
+    count < 4
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        let input = vec![
+            "..@@.@@@@.",
+            "@@@.@.@.@@",
+            "@@@@@.@.@@",
+            "@.@@@@..@.",
+            "@@.@@@@.@@",
+            ".@@@@@@@.@",
+            ".@.@.@.@@@",
+            "@.@@@.@@@@",
+            ".@@@@@@@@.",
+            "@.@.@@@.@.",
+        ];
+
+        assert_eq!(part1(&input.join("\n")), 13);
+    }
+
+    #[test]
+    fn test_part2() {
+        let input = vec![
+            "..@@.@@@@.",
+            "@@@.@.@.@@",
+            "@@@@@.@.@@",
+            "@.@@@@..@.",
+            "@@.@@@@.@@",
+            ".@@@@@@@.@",
+            ".@.@.@.@@@",
+            "@.@@@.@@@@",
+            ".@@@@@@@@.",
+            "@.@.@@@.@.",
+        ];
+
+        assert_eq!(part2(&input.join("\n")), 43);
+    }
+}

2025/tests/day04.rs

@@ -0,0 +1,13 @@
+use aoc2025::days::day04::{part1, part2};
+
+#[test]
+fn test_part1() {
+    let content = std::include_str!("../../private/inputs/2025/day04.txt");
+    assert_eq!(part1(&content), 1564);
+}
+
+#[test]
+fn test_part2() {
+    let content = std::include_str!("../../private/inputs/2025/day04.txt");
+    assert_eq!(part2(&content), 9401);
+}