diff --git a/2018/inputs/18.txt b/2018/inputs/18.txt
new file mode 100644
index 0000000..161ad17
--- /dev/null
+++ b/2018/inputs/18.txt
@@ -0,0 +1,50 @@
+..|#..#......#|.#..#|#.|##|.|.##.||..|.||....###|.
+||.|....|#.#|.#...#.|...|#.#.#.#....#......#....#.
+...#...|.....#.#..|#...||..#.|.|#....#|#|..##...|.
+.....#....|#.|..|.....#|##...#.#.#||....||||....#.
+..###...#.||.|||.##.#.|....|.##.#....|.#.|..|.....
+....|..#.#.|...|..|.....#....#.#.......|.||...|...
+#..|.....##|..#..#...#..|.##.....#......||##|.|#..
+##........#...|#|.##.#|#..#|...#...#.##|||.#||..||
+......#..|..|.#...|#||.#....#.#.|#.||......|.....|
+.|...|.##....#.||...|..|..|.|........#.#..|||..|##
+..|....|...#####..|#|..|...#...#.|...|..|.|..|.#|#
+....|...#.#.#.......#......#.#|......#|.##..##.#|.
+.....|.#..|#...||..#......|..#.|#.#|...#|.|..#||..
+#....#.......|#.|..|...#...|..|.##|#.|#.#|.....|..
+....#.||#.....#..#...|....##.#.......#.|.|||.|....
+|.|..||##....|#..#..|..|.|.|..|||.##..#.|......##|
+###..|.#|##|#.|||.#|.#..|#|..#..|.#|....#.#.#..||.
+.|.....|#.#.|#||..#.....#.|.||.#.|.....|#..|...#..
+...##.........|...#.#|....##..#.|.|.......#..|...|
+..#.#.|.|.....||#..||...##||.#|..|.....#|...|...#|
+....#...#||..|...|.|..|#.#.........|#...#.|||...#.
+.#..|.##.|.|.#...#.....#.#.......|#.|.#||#.#.....#
+|...#|..#....#...|.##.####....|#.##|#.#.|.....||..
+....|.#.|#||..|#.|.|.#|...|.#....||.#...#|.#...|.#
+.|..#.#..|#|..##..|.##..||...#...||....#||..#.|...
+##......#.|...|.||.#.||....|.......#......##|#..|.
+|#....#||....##...........#.|....|....|#|#.|..#...
+..#...#|....|.|..|...#.......#.##.#.....#.||......
+...|....#|#..#|...|...#|....#.#|.......|.......|#.
+.#||..##||.|..|.|#..|.|....|.#|.|.|.....#.#.|..#.#
+.....|..|...|...|......||...##.....##.......|.#..|
+.....#..#|.#...#..#...||.|##..|#..##.|#.....##....
+|...|.#||.........#..#..#||||....|...|..|..#...##.
+#.#..|.|.......||..|#|..|....|.|#|#|.|..|.|...#.#.
+#.|..||#.||||..###.|......|.#|||.##........||...||
+.....#|..#.#.....|..|#.....|....|.#|||#.|.....#...
+...||#..#...#...||#.||......|#..#..#.|#.|#|...|..#
+.||.....||..|#.|#||...##..###|.#....|.|..|#...#|.|
+...#.|#||..#.|||......#...||.#..|||..|#.|.##..#.|#
+.#||..#.|||.......|.|#.....|.|#.#..##..|.|....|#..
+..#.###..|.........|.....#..###|.|#..........#|.#|
+#.||.|.#.|..||#|||#..##.|#....#.#.|.#.....|.|.#.|#
+|..|..#.#..|.#.......#...#.|..|..|#..|..###|.||..|
+|..|...||...|.#.#..##|.#..#...#|#..|.#..|.#..|.||.
+..#.##..#.|..#.#..|..|#.|||.#..#..|#####.|..#.....
+.|.|#.|#...||..##...#|#.........#...#|..##.#.#..#.
+##|.|..#.#|....#..|..#....#......#||.|....||##..||
+.##|#....#..#..#.....#|.#...#..#.|#||||.##.#....|.
+..|.......#|....|#|..||..##..#.|#|..#.#|....|..#|#
+....|||.|||#..||...|||.##..#.#|##....|..|..||..#|#
diff --git a/2018/src/day18.rs b/2018/src/day18.rs
index aef91b5..71bea19 100644
--- a/2018/src/day18.rs
+++ b/2018/src/day18.rs
@@ -1,25 +1,189 @@
+use std::io::BufRead;
+use std::io::BufReader;
 use std::io::Read;
 
+use common::GroupingCount;
 use common::Solution;
+use std::mem::swap;
+use std::collections::HashMap;
+
+#[derive(Copy, Clone, Hash, Eq, PartialEq, Debug)]
+enum Tile {
+    Tree,
+    Lumber,
+    Open,
+}
+
+impl Tile {
+    pub fn next(self, counts: &[usize; 3]) -> Self {
+        match self {
+            Tile::Open => {
+                if counts[Tile::Tree as usize] >= 3 {
+                    Tile::Tree
+                } else {
+                    Tile::Open
+                }
+            }
+            Tile::Tree => {
+                if counts[Tile::Lumber as usize] >= 3 {
+                    Tile::Lumber
+                } else {
+                    Tile::Tree
+                }
+            }
+            Tile::Lumber => {
+                if counts[Tile::Tree as usize] >= 1 && counts[Tile::Lumber as usize] >= 1 {
+                    Tile::Lumber
+                } else {
+                    Tile::Open
+                }
+            }
+        }
+    }
+}
+
+impl From<char> for Tile {
+    fn from(c: char) -> Self {
+        match c {
+            '|' => Tile::Tree,
+            '#' => Tile::Lumber,
+            '.' => Tile::Open,
+            _ => panic!("Invalid tile '{}'", c)
+        }
+    }
+}
+
+impl From<Tile> for char {
+    fn from(t: Tile) -> Self {
+        match t {
+            Tile::Tree => '|',
+            Tile::Lumber => '#',
+            Tile::Open => '.',
+        }
+    }
+}
 
 #[derive(Default)]
-pub struct Day18 {}
+pub struct Day18 {
+    grid: Vec<Vec<Tile>>,
+    buf: Vec<Vec<Tile>>,
+    width: usize,
+}
 
 impl Day18 {
     pub fn new() -> Self {
         Default::default()
     }
+
+    fn read_input(&mut self, input: &mut Read) {
+        let reader = BufReader::new(input);
+        self.grid.clear();
+
+        for line in reader.lines() {
+            let line = line.unwrap();
+
+            self.grid.push(line.chars().map(From::from).collect());
+        }
+
+        self.width = self.grid.first().unwrap().len();
+        self.buf = vec![vec![Tile::Tree; self.width]; self.grid.len()];
+    }
+
+    fn simulate(&mut self) {
+        let height = self.grid.len();
+        let width = self.width;
+
+        for y in 0..height {
+            let ymin = if y > 0 { y - 1 } else { y };
+            let ymax = if y < height - 1 { y + 1 } else { y };
+
+            for x in 0..self.width {
+                let mut counts = [0; 3];
+                let xmin = if x > 0 { x - 1 } else { x };
+                let xmax = if x < width - 1 { x + 1 } else { x };
+
+                for ys in ymin..=ymax {
+                    for xs in xmin..=xmax {
+                        if ys != y || xs != x {
+                            counts[self.grid[ys][xs] as usize] += 1;
+                        }
+                    }
+                }
+
+                self.buf[y][x] = self.grid[y][x].next(&counts);
+            }
+        }
+
+        swap(&mut self.buf, &mut self.grid);
+    }
+
+    fn print(&self) -> String {
+        let mut buf = String::with_capacity(self.width * self.grid.len());
+        for row in &self.grid {
+            buf.extend(row.iter().cloned().map(Into::<char>::into));
+        }
+        buf
+    }
+
+    fn score(&self) -> String {
+        let result = self.grid.iter()
+            .flat_map(|x| x.iter())
+            .cloned().grouping_count();
+
+        format!("{}", result[&Tile::Tree] * result[&Tile::Lumber])
+    }
 }
 
 impl Solution for Day18 {
-    fn part1(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+    fn part1(&mut self, input: &mut Read) -> String {
+        self.read_input(input);
+
+        for _ in 0..10 {
+            self.simulate();
+        }
+
+        self.score()
     }
 
-    fn part2(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+    fn part2(&mut self, input: &mut Read) -> String {
+        self.read_input(input);
+        let limit = 1_000_000_000;
+
+        let mut seen = HashMap::new();
+        seen.insert(self.print(), 0);
+
+        for i in 1..=limit {
+            self.simulate();
+            let summary = self.print();
+            if let Some(first) = seen.get(&summary) {
+                let period = i - *first;
+                let remaining = (limit - *first) % period;
+
+                for _ in 0..remaining {
+                    self.simulate();
+                }
+
+                return self.score();
+            }
+
+            seen.insert(summary, i);
+        }
+
+        // I encourage everyone to hit this line of code.
+        self.score()
     }
 }
 
 #[cfg(test)]
-mod tests {}
+mod tests {
+    use common::Solution;
+    use day18::Day18;
+
+    const SAMPLE_INPUT: &[u8] = include_bytes!("samples/18.txt");
+
+    #[test]
+    fn sample_part1() {
+        let mut instance = Day18::new();
+        assert_eq!("1147", instance.part1(&mut SAMPLE_INPUT));
+    }
+}
diff --git a/2018/src/samples/18.txt b/2018/src/samples/18.txt
new file mode 100644
index 0000000..13d299d
--- /dev/null
+++ b/2018/src/samples/18.txt
@@ -0,0 +1,10 @@
+.#.#...|#.
+.....#|##|
+.|..|...#.
+..|#.....#
+#.#|||#|#|
+...#.||...
+.|....|...
+||...#|.#|
+|.||||..|.
+...#.|..|.