1 files changed, 96 insertions, 0 deletions
diff --git a/src/bin/day18.rs b/src/bin/day18.rs
new file mode 100644
index 0000000..36850eb
--- /dev/null
+++ b/src/bin/day18.rs
@@ -0,0 +1,96 @@
+use std::collections::HashMap;
+
+fn main() {
+    let input = advent::read_lines(18);
+    println!("18a: {}", count_lights(&input, 100, false));
+    println!("18b: {}", count_lights(&input, 100, true));
+}
+
+fn count_neighbors(grid: &HashMap<(isize, isize), bool>, pos: (isize, isize)) -> usize {
+    let mut count = 0;
+    for x in pos.0 - 1 ..= pos.0 + 1 {
+        for y in pos.1 - 1 ..= pos.1 + 1 {
+            if (x, y) == pos {
+                continue;
+            }
+            if let Some(true) = grid.get(&(x, y)) {
+                count += 1;
+            }
+        }
+    }
+    count
+}
+
+fn stuck_grid(grid: &mut HashMap<(isize, isize), bool>, dim: isize) {
+    grid.insert((0, 0), true);
+    grid.insert((dim-1, dim-1), true);
+    grid.insert((0, dim-1), true);
+    grid.insert((dim-1, 0), true);
+}
+
+fn step(grid: HashMap<(isize, isize), bool>, dim: usize, stuck: bool) -> HashMap<(isize, isize), bool> {
+    let mut next_grid = HashMap::new();
+
+    for y in 0 .. dim {
+        for x in 0 .. dim {
+            let pos = (x as isize, y as isize);
+            let val = *grid.get(&pos).unwrap();
+            let neighbors = count_neighbors(&grid, pos);
+
+            let new_val = match val {
+                true => [2, 3].contains(&neighbors),
+                false => neighbors == 3,
+            };
+            next_grid.insert(pos, new_val);
+        }
+    }
+    if stuck {
+        stuck_grid(&mut next_grid, dim as isize);
+    }
+
+    next_grid
+}
+
+fn count_lights(input: &[String], steps: u16, stuck: bool) -> usize {
+    let dim = input.len();
+
+    let mut grid = HashMap::new();
+
+    for (y, line) in input.iter().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            let pos = (x as isize, y as isize);
+            match c {
+                '#' => grid.insert(pos, true),
+                '.' => grid.insert(pos, false),
+                _ => unimplemented!(),
+            };
+        }
+    }
+    if stuck {
+        stuck_grid(&mut grid, dim as isize);
+    }
+
+    for _ in 0 .. steps {
+        grid = step(grid, dim, stuck);
+    }
+    grid.values().filter(|&val| *val).count()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            ".#.#.#".to_string(),
+            "...##.".to_string(),
+            "#....#".to_string(),
+            "..#...".to_string(),
+            "#.#..#".to_string(),
+            "####..".to_string(),
+        ];
+        assert_eq!(count_lights(&input, 4, false), 4);
+        assert_eq!(count_lights(&input, 5, true), 17);
+    }
+}