From 6bef9799b941c498bf0c3e8dbbcfbc6d40d9c720 Mon Sep 17 00:00:00 2001
From: Jan Doumont <jan@6a616e.net>
Date: Thu, 4 Dec 2025 23:15:17 +0100
Subject: [PATCH] day4

---
 src/day4.rs | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs  |   3 +-
 2 files changed, 122 insertions(+), 1 deletion(-)
 create mode 100644 src/day4.rs

diff --git a/src/day4.rs b/src/day4.rs
new file mode 100644
index 0000000..63a51fa
--- /dev/null
+++ b/src/day4.rs
@@ -0,0 +1,120 @@
+#[aoc(day4, part1)]
+pub fn part1(input: &str) -> u32 {
+    let array = make_array(&input);
+    get_accessible(&array)
+        .iter()
+        .flatten()
+        .fold(0, |acc, x| acc + (*x as u32))
+}
+
+#[aoc(day4, part2)]
+pub fn part2(input: &str) -> u32 {
+    let mut array = make_array(&input);
+    remove_accessible(&mut array)
+}
+
+fn make_array(input: &str) -> Vec<Vec<u8>> {
+    let n = input.split("\n").next().unwrap().len();
+    let m = input.split("\n").count();
+
+    // Top row padded with 0s for convenience and performance
+    let mut array = vec![vec![0_u8; m + 2]; n + 2];
+
+    for (i, line) in input.split("\n").enumerate() {
+        for (j, ch) in line.chars().enumerate() {
+            if ch == '@' {
+                array[i + 1][j + 1] = 1;
+            }
+        }
+    }
+    // println!("{:?}", array);
+    array
+}
+
+fn get_accessible(array: &Vec<Vec<u8>>) -> Vec<Vec<u8>> {
+    let n = array.len();
+    let m = array[0].len();
+
+    let mut accessible = vec![vec![0_u8; m - 2]; n - 2];
+
+    for i in 1..n - 1 {
+        for j in 1..m - 1 {
+            accessible[i - 1][j - 1] = array[i][j]
+                * ((array[i - 1][j - 1]
+                    + array[i - 1][j]
+                    + array[i - 1][j + 1]
+                    + array[i][j - 1]
+                    + array[i][j + 1]
+                    + array[i + 1][j - 1]
+                    + array[i + 1][j]
+                    + array[i + 1][j + 1])
+                    < 4) as u8;
+        }
+    }
+    // println!("{:?}", accessible);
+    accessible
+}
+
+fn remove_accessible(rolls: &mut Vec<Vec<u8>>) -> u32 {
+    let n = rolls.len();
+    let m = rolls[0].len();
+
+    let mut removed: u32 = 0;
+
+    loop {
+        let mut removed_new: u32 = 0;
+        let accessible = get_accessible(rolls);
+
+        for i in 0..n - 2 {
+            for j in 0..m - 2 {
+                rolls[i + 1][j + 1] = rolls[i + 1][j + 1] - accessible[i][j];
+                removed_new = removed_new + accessible[i][j] as u32;
+            }
+        }
+        if removed_new == 0 {
+            break;
+        }
+        removed = removed + removed_new;
+    }
+    removed
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part2_1() {
+        let input = String::from(
+            "..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.",
+        );
+        assert_eq!(part2(&input), 43)
+    }
+
+    #[test]
+    fn test_part1_1() {
+        let input = String::from(
+            "..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.",
+        );
+
+        assert_eq!(part1(&input), 13)
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index dacd112..3e01eb1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,6 +5,7 @@ extern crate aoc_runner_derive;
 
 // pub mod day1;
 // pub mod day2;
-pub mod day3;
+// pub mod day3;
+pub mod day4;
 
 aoc_lib! { year = 2025 }