From d8780f97a770eb14d263d3a6849a0992995ae7b4 Mon Sep 17 00:00:00 2001
From: SebastianStork <git@sstork.dev>
Date: Thu, 4 Dec 2025 14:43:03 +0100
Subject: [PATCH] Solve 2025 day 4 in rust

---
 2025/rust/day-04/Cargo.lock  |   7 ++
 2025/rust/day-04/Cargo.toml  |   6 ++
 2025/rust/day-04/src/main.rs | 166 +++++++++++++++++++++++++++++++++++
 flake.lock                   |   6 +-
 4 files changed, 182 insertions(+), 3 deletions(-)
 create mode 100644 2025/rust/day-04/Cargo.lock
 create mode 100644 2025/rust/day-04/Cargo.toml
 create mode 100644 2025/rust/day-04/src/main.rs

diff --git a/2025/rust/day-04/Cargo.lock b/2025/rust/day-04/Cargo.lock
new file mode 100644
index 0000000..bc5d859
--- /dev/null
+++ b/2025/rust/day-04/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day-04"
+version = "0.1.0"
diff --git a/2025/rust/day-04/Cargo.toml b/2025/rust/day-04/Cargo.toml
new file mode 100644
index 0000000..ca7eca6
--- /dev/null
+++ b/2025/rust/day-04/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day-04"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/2025/rust/day-04/src/main.rs b/2025/rust/day-04/src/main.rs
new file mode 100644
index 0000000..9b5a3a8
--- /dev/null
+++ b/2025/rust/day-04/src/main.rs
@@ -0,0 +1,166 @@
+use std::{cell::RefCell, fs, rc::Rc};
+
+struct Roll {
+    neighbours: Vec<Rc<RefCell<Roll>>>,
+    deleted: bool,
+}
+
+impl Roll {
+    fn new() -> Rc<RefCell<Roll>> {
+        Rc::new(RefCell::new(Roll {
+            neighbours: Vec::new(),
+            deleted: false,
+        }))
+    }
+
+    fn add_neighbour(this: &Rc<RefCell<Roll>>, neighbour: &Rc<RefCell<Roll>>) {
+        this.borrow_mut().neighbours.push(Rc::clone(neighbour));
+    }
+
+    fn is_accessible(&self) -> bool {
+        self.neighbours
+            .iter()
+            .filter(|neighbour| !neighbour.borrow().deleted)
+            .count()
+            < 4
+    }
+
+    fn delete(this: &Rc<RefCell<Roll>>) {
+        if this.borrow().deleted {
+            return;
+        }
+
+        this.borrow_mut().deleted = true;
+
+        this.borrow()
+            .neighbours
+            .iter()
+            .filter(|neighbour| !neighbour.borrow().deleted)
+            .filter(|neighbour| neighbour.borrow().is_accessible())
+            .for_each(Roll::delete);
+    }
+}
+
+fn main() {
+    let grid = parse_input(&fs::read_to_string("../../inputs/04.txt").unwrap());
+    let rolls = build_graph(grid);
+
+    println!(
+        "Number of accessible rolls: {}",
+        number_of_accessible_rolls(&rolls)
+    );
+    println!(
+        "Number of removeable rolls: {}",
+        number_of_removeable_rolls(&rolls)
+    );
+}
+
+fn parse_input(contents: &str) -> Vec<Vec<Option<Rc<RefCell<Roll>>>>> {
+    contents
+        .trim()
+        .lines()
+        .map(|line: &str| {
+            line.bytes()
+                .map(|b| if b == b'@' { Some(Roll::new()) } else { None })
+                .collect()
+        })
+        .collect()
+}
+
+fn build_graph(grid: Vec<Vec<Option<Rc<RefCell<Roll>>>>>) -> Vec<Rc<RefCell<Roll>>> {
+    for (row_index, row) in grid.iter().enumerate() {
+        for (col_index, cell) in row.iter().enumerate() {
+            if let Some(roll) = cell {
+                for neighbour in get_neighbours(&grid, row_index, col_index) {
+                    Roll::add_neighbour(roll, neighbour);
+                }
+            }
+        }
+    }
+
+    grid.into_iter().flatten().flatten().collect()
+}
+
+fn get_neighbours(
+    grid: &[Vec<Option<Rc<RefCell<Roll>>>>],
+    row: usize,
+    col: usize,
+) -> Vec<&Rc<RefCell<Roll>>> {
+    let offsets = [
+        (-1, -1),
+        (-1, 0),
+        (-1, 1),
+        (0, -1),
+        (0, 1),
+        (1, -1),
+        (1, 0),
+        (1, 1),
+    ];
+
+    offsets
+        .iter()
+        .filter_map(|(di, dj)| {
+            let (ni, nj) = (row as i32 + di, col as i32 + dj);
+            grid.get(ni as usize)?.get(nj as usize)?.as_ref()
+        })
+        .collect()
+}
+
+fn number_of_accessible_rolls(rolls: &[Rc<RefCell<Roll>>]) -> usize {
+    rolls
+        .iter()
+        .filter(|roll| roll.borrow().is_accessible())
+        .count()
+}
+
+fn number_of_removeable_rolls(rolls: &[Rc<RefCell<Roll>>]) -> usize {
+    loop {
+        let deleted_any = rolls
+            .iter()
+            .filter(|roll| !roll.borrow().deleted)
+            .filter(|roll| roll.borrow().is_accessible())
+            .inspect(|roll| Roll::delete(roll))
+            .count()
+            > 0;
+
+        if !deleted_any {
+            break;
+        }
+    }
+
+    rolls.iter().filter(|roll| roll.borrow().deleted).count()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = "
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
+";
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(
+            number_of_accessible_rolls(&build_graph(parse_input(TEST_INPUT))),
+            13
+        );
+    }
+
+    #[test]
+    fn test_part2() {
+        assert_eq!(
+            number_of_removeable_rolls(&build_graph(parse_input(TEST_INPUT))),
+            43
+        );
+    }
+}
diff --git a/flake.lock b/flake.lock
index 90110f9..ec6ecaf 100644
--- a/flake.lock
+++ b/flake.lock
@@ -3,10 +3,10 @@
     "inputs": {
       "flake": false,
       "locked": {
-        "lastModified": 1764767558,
-        "narHash": "sha256-AqEynDX4eu61liwLdGPzk0b6J9sE+dKWSpxfO+mZpBc=",
+        "lastModified": 1764840758,
+        "narHash": "sha256-GTN8SBnhvv6t2oDS1H85xcYAhafy8mpUQWSrCErhtgw=",
         "ref": "refs/heads/main",
-        "rev": "a1a4c48fcc3eddac81d35ce2030c887ac38f4002",
+        "rev": "415a59297c64d0448d22975ca5ad3d8ef064c9f0",
         "shallow": true,
         "type": "git",
         "url": "ssh://git@github.com/SebastianStork/advent-of-code-inputs.git"