Day 4 down

Author: BakerNet

README.md

@@ -10,7 +10,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 
 | Year | Completed |
 | :---: | :---: |
-| [2025](aoc2025) | 6/24 |
+| [2025](aoc2025) | 8/24 |
 | [2024](aoc2024) | 50/50 |
 | [2023](aoc2023) | 50/50 |
 

aoc2025/README.md

@@ -9,7 +9,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 1](https://adventofcode.com/2025/day/1) | [code](src/bin/01.rs) | ⭐ | ⭐ |
 | [Day 2](https://adventofcode.com/2025/day/2) | [code](src/bin/02.rs) | ⭐ | ⭐ |
 | [Day 3](https://adventofcode.com/2025/day/3) | [code](src/bin/03.rs) | ⭐ | ⭐ |
-| [Day 4](https://adventofcode.com/2025/day/4) | [code](src/bin/04.rs) | _ | _ |
+| [Day 4](https://adventofcode.com/2025/day/4) | [code](src/bin/04.rs) | ⭐ | ⭐ |
 | [Day 5](https://adventofcode.com/2025/day/5) | [code](src/bin/05.rs) | _ | _ |
 | [Day 6](https://adventofcode.com/2025/day/6) | [code](src/bin/06.rs) | _ | _ |
 | [Day 7](https://adventofcode.com/2025/day/7) | [code](src/bin/07.rs) | _ | _ |

aoc2025/data/examples/04.txt

@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.

aoc2025/src/bin/04.rs

@@ -0,0 +1,77 @@
+use std::collections::BTreeSet;
+
+use aoc_utils::*;
+use itertools::Itertools;
+
+advent_of_code::solution!(4);
+
+pub fn part_one(input: &str) -> Option<u64> {
+    let map = input.c_map();
+    let num = map
+        .iter()
+        .enumerate()
+        .flat_map(|(x, v)| {
+            v.iter()
+                .enumerate()
+                .filter(|(_, c)| **c == '@')
+                .map(|(y, _)| Point(x, y))
+                .collect_vec()
+        })
+        .filter(|p| {
+            DirExt::neighbors(*p, Bounds(map.len() - 1, map[0].len() - 1))
+                .iter()
+                .filter(|p2| map[p2.0][p2.1] == '@')
+                .count()
+                < 4
+        })
+        .count();
+    Some(num as u64)
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    let mut map = input.c_map();
+    let mut queue = map
+        .iter()
+        .enumerate()
+        .flat_map(|(x, v)| {
+            v.iter()
+                .enumerate()
+                .filter(|(_, c)| **c == '@')
+                .map(|(y, _)| Point(x, y))
+                .collect_vec()
+        })
+        .collect::<BTreeSet<Point>>();
+    let mut removed = 0;
+    while !queue.is_empty() {
+        let curr = queue.pop_first().unwrap();
+        let neighbors = DirExt::neighbors(curr, Bounds(map.len() - 1, map[0].len() - 1));
+        if neighbors.iter().filter(|p| map[p.0][p.1] == '@').count() < 4 {
+            removed += 1;
+            map[curr.0][curr.1] = '.';
+            neighbors
+                .into_iter()
+                .filter(|p| map[p.0][p.1] == '@')
+                .for_each(|p| {
+                    queue.insert(p);
+                });
+        }
+    }
+    Some(removed)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(13));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(43));
+    }
+}

aoc_utils/src/lib.rs

@@ -5,7 +5,7 @@ use regex::{Captures, Regex};
 use tinyvec::{ArrayVec, array_vec};
 
 /// 2D grid point: `Point(row, col)`
-#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Hash)]
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Hash, Ord)]
 pub struct Point(pub usize, pub usize);
 
 /// Find first occurrence of value in 2D grid