From d49157dd7982dee0fef34633f22c513415382761 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@gotroot.ca>
Date: Thu, 4 Dec 2025 17:29:12 -0800
Subject: [PATCH] day4: add removal queue implementation for part2

---
 src/day4.rs | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/src/day4.rs b/src/day4.rs
index 41f64f2..d781cd6 100644
--- a/src/day4.rs
+++ b/src/day4.rs
@@ -1,12 +1,13 @@
 use aoc_runner_derive::{aoc, aoc_generator};
 use grid::Grid;
+use itertools::Itertools;
 
 #[aoc_generator(day4)]
 fn parse(input: &str) -> Grid<u8> {
     input.parse().unwrap()
 }
 
-#[aoc(day4, part1)]
+#[aoc(day4, part1, Brute)]
 fn part1(input: &Grid<u8>) -> u64 {
     (0..input.height() * input.width())
         .filter(|i| *input.get(&input.coord(*i as i64).unwrap()).unwrap() == b'@')
@@ -14,7 +15,8 @@ fn part1(input: &Grid<u8>) -> u64 {
         .filter(|n| *n < 4)
         .count() as u64
 }
-#[aoc(day4, part2)]
+
+#[aoc(day4, part2, Brute)]
 fn part2(input: &Grid<u8>) -> u64 {
     let mut grid = input.clone();
     let mut removed = 0;
@@ -38,6 +40,30 @@ fn part2(input: &Grid<u8>) -> u64 {
     }
 }
 
+#[aoc(day4, part2, RollList)]
+fn part2_list(input: &Grid<u8>) -> u64 {
+    let mut grid = input.clone();
+    let mut to_check = grid.find_all(&b'@').collect_vec();
+    let mut removed = 0;
+
+    while let Some(roll) = to_check.pop() {
+        if grid.get(&roll).is_none_or(|c| *c == b'.') {
+            continue;
+        }
+        let adjacent_rolls = grid
+            .adjacent_iter(&roll)
+            .filter_map(|i| if *i.value == b'@' { Some(i.pos) } else { None })
+            .collect_vec();
+        if adjacent_rolls.len() < 4 {
+            grid.set(&roll, b'.');
+            removed += 1;
+            to_check.extend_from_slice(&adjacent_rolls);
+        }
+    }
+
+    removed
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -62,4 +88,9 @@ mod tests {
     fn part2_example() {
         assert_eq!(part2(&parse(EXAMPLE)), 43);
     }
+
+    #[test]
+    fn part2_list_example() {
+        assert_eq!(part2_list(&parse(EXAMPLE)), 43);
+    }
 }