From 57b0474d09acd15c479403e3d9908bc2307d3b64 Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Thu, 4 Dec 2025 08:17:31 +0100
Subject: [PATCH] Solve 04

---
 src/bin/04.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 src/bin/04.rs

diff --git a/src/bin/04.rs b/src/bin/04.rs
new file mode 100644
index 0000000..3f0ce5a
--- /dev/null
+++ b/src/bin/04.rs
@@ -0,0 +1,75 @@
+use aoc::*;
+use glam::IVec2;
+use std::collections::HashSet;
+
+const INPUT: &str = include_str!("../../input/04");
+
+fn main() {
+    println!("Part 1: {}", part1(INPUT));
+    println!("Part 2: {}", part2(INPUT));
+}
+
+#[test]
+fn example() {
+    assert_example!(part1, "04-test", 13);
+    assert_example!(part2, "04-test", 43);
+}
+
+fn part1(input: &str) -> usize {
+    let mut rolls = parse_input(input);
+    remove_accessible(&mut rolls)
+}
+
+fn part2(input: &str) -> usize {
+    let mut rolls = parse_input(input);
+    let mut sum = 0;
+
+    loop {
+        let removed = remove_accessible(&mut rolls);
+        sum += removed;
+        if removed == 0 {
+            break;
+        }
+    }
+
+    sum
+}
+
+fn remove_accessible(rolls: &mut HashSet<IVec2>) -> usize {
+    let mut accessible = Vec::new();
+
+    for roll in rolls.iter().copied() {
+        let mut neighbors = 0;
+        for dir in DIRECTIONS8 {
+            let neighbor = roll + dir;
+            if rolls.contains(&neighbor) {
+                neighbors += 1;
+            }
+        }
+        if neighbors < 4 {
+            accessible.push(roll);
+        }
+    }
+
+    let result = accessible.len();
+
+    for accessible in accessible {
+        rolls.remove(&accessible);
+    }
+
+    result
+}
+
+fn parse_input(input: &str) -> HashSet<IVec2> {
+    let mut result = HashSet::new();
+
+    for (y, line) in input.lines().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            if c == '@' {
+                result.insert(IVec2::new(x as i32, y as i32));
+            }
+        }
+    }
+
+    result
+}