solve: day4

Author: tom-anders

Cargo.toml

@@ -2,7 +2,7 @@
 resolver = "2"
 members = [
     "aoc_lib/aoc_derive",
-    "aoc_lib/utils", "day1", "day2", "day3",
+    "aoc_lib/utils", "day1", "day2", "day3", "day4",
 ]
 
 [workspace.dependencies]

aoc_lib

@@ -0,0 +1,18 @@
+[package]
+name = "day4"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+aoc_derive.path = '../aoc_lib/aoc_derive'
+utils.path = '../aoc_lib/utils'
+derive_more.workspace = true
+itertools.workspace = true
+lazy-regex.workspace = true
+parse-display.workspace = true
+rayon.workspace = true
+regex.workspace = true
+num.workspace = true
+
+[dev-dependencies]
+pretty_assertions.workspace = true

day4/Cargo.toml

@@ -0,0 +1,65 @@
+use aoc_derive::aoc_main;
+use derive_more::{Deref, DerefMut};
+use itertools::Itertools;
+use utils::{grid::Grid, math::Vec2D, *};
+
+#[derive(Deref, DerefMut)]
+struct Map(Grid<char>);
+
+impl Map {
+    fn accessible_positions(&self) -> impl Iterator<Item = Vec2D> {
+        self.iter().filter_map(|(pos, val)| {
+            (val == &'@'
+                && self.all_neighbor_values(&pos).filter(|&neighbor| neighbor == &'@').count() < 4)
+                .then_some(pos)
+        })
+    }
+
+    fn clear_paper(&mut self) {
+        for pos in self.accessible_positions().collect_vec() {
+            *self.get_mut(pos).unwrap() = '.'
+        }
+    }
+
+    fn clear_all(&mut self) -> usize {
+        let mut count = 0;
+        loop {
+            let to_clear = self.accessible_positions().count();
+            if to_clear == 0 {
+                return count;
+            }
+            count += to_clear;
+            self.clear_paper();
+        }
+    }
+}
+
+#[aoc_main]
+fn solve(input: Input) -> impl Into<Solution> {
+    let mut map = Map(input.char_grid());
+
+    (map.accessible_positions().count(), map.clear_all())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_examples() {
+        use utils::assert_example;
+        assert_example!(
+            r#"..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@."#,
+            13,
+            43
+        );
+    }
+}