2 files changed, 88 insertions, 0 deletions
diff --git a/input/day18.txt b/input/day18.txt
new file mode 100644
index 0000000..f8462d2
--- /dev/null
+++ b/input/day18.txt
@@ -0,0 +1 @@
+^^.^..^.....^..^..^^...^^.^....^^^.^.^^....^.^^^...^^^^.^^^^.^..^^^^.^^.^.^.^.^.^^...^^..^^^..^.^^^^
\ No newline at end of file
diff --git a/src/bin/day18.rs b/src/bin/day18.rs
new file mode 100644
index 0000000..658949e
--- /dev/null
+++ b/src/bin/day18.rs
@@ -0,0 +1,87 @@
+use std::io::{self, Read};
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum Tile {
+    Safe,
+    Trap,
+}
+
+impl From<char> for Tile {
+    fn from(ch: char) -> Tile {
+        match ch {
+            '.' => Tile::Safe,
+            '^' => Tile::Trap,
+            _ => panic!("invalid tile char {}", ch),
+        }
+    }
+}
+
+struct Map {
+    rows: Vec<Box<[Tile]>>,
+}
+
+impl Map {
+    fn generate_row(&mut self) {
+        let mut next = Vec::new();
+
+        {
+            let previous = self.rows.last().unwrap();
+
+            for (i, &center) in self.rows.last().unwrap().iter().enumerate() {
+                let left = previous.get(i.wrapping_sub(1)).cloned().unwrap_or(Tile::Safe);
+                let right = previous.get(i + 1).cloned().unwrap_or(Tile::Safe);
+
+                let tile = match (left, center, right) {
+                    (Tile::Trap, Tile::Trap, Tile::Safe) => Tile::Trap,
+                    (Tile::Safe, Tile::Trap, Tile::Trap) => Tile::Trap,
+                    (Tile::Trap, Tile::Safe, Tile::Safe) => Tile::Trap,
+                    (Tile::Safe, Tile::Safe, Tile::Trap) => Tile::Trap,
+                    _ => Tile::Safe,
+                };
+                next.push(tile);
+            }
+        }
+
+        self.rows.push(next.into_boxed_slice());
+    }
+
+    fn count_safe(&self) -> usize {
+        self.rows.iter()
+            .map(|row| {
+                row.iter()
+                    .filter(|&&t| t == Tile::Safe)
+                    .count()
+            })
+            .sum()
+    }
+}
+
+impl<'a> From<&'a str> for Map {
+    fn from(s: &'a str) -> Map {
+        let rows = s.lines()
+            .map(|l| l.chars().map(Tile::from).collect::<Vec<_>>().into_boxed_slice())
+            .collect();
+        Map { rows: rows }
+    }
+}
+
+fn solve(rows: u32, input: &str) -> usize {
+    let mut map = Map::from(input);
+    for _ in 1..rows {
+        map.generate_row();
+    }
+    map.count_safe()
+}
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    println!("Part 1: {}", solve(40, &input));
+}
+
+#[test]
+fn part1() {
+    assert_eq!(6, solve(3, "..^^."));
+    assert_eq!(38, solve(10, ".^^.^.^^^^"));
+}