1 files changed, 62 insertions, 0 deletions
diff --git a/d18/src/main.rs b/d18/src/main.rs
new file mode 100644
index 0000000..a9410f2
--- /dev/null
+++ b/d18/src/main.rs
@@ -0,0 +1,62 @@
+use std::fs;
+
+fn solve_puzzle(first: &str, num_rows: usize) -> usize {
+    let mut total = first.matches('.').count();
+
+    let mut tiles: Vec<char> = first.chars().collect();
+    for _i in 1..num_rows {
+        let mut next = Vec::new();
+
+        for j in 0..tiles.len() {
+            let is_trap;
+
+            // At left wall
+            if j == 0 {
+                is_trap = tiles[j + 1] == '^'; // rule 2 + 4
+            // At right wall
+            } else if j == tiles.len() - 1 {
+                is_trap = tiles[j - 1] == '^'; // rule 1 + 3
+            // Away from the walls
+            } else {
+                is_trap = tiles[j - 1] == '^' && tiles[j + 1] == '.'  // rule 1 + 3
+                       || tiles[j - 1] == '.' && tiles[j + 1] == '^'; // rule 2 + 4
+            }
+
+            next.push(if is_trap { '^' } else { '.' });
+        }
+
+        tiles = next;
+        total += tiles.iter().filter(|&c| *c == '.').count();
+    }
+
+    total
+}
+
+fn main() {
+    // Read my personal puzzle input
+    let input = fs::read_to_string("input.txt").unwrap();
+    let first = input.trim_end();
+
+    // Part 1 gives 1939 for me
+    println!("Part 1 solution: {}", solve_puzzle(first, 40));
+
+    // Part 2 gives 19999535 for me
+    println!("Part 1 solution: {}", solve_puzzle(first, 400000));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let input = "..^^.";
+        assert_eq!(solve_puzzle(input, 3), 6);
+    }
+
+    #[test]
+    fn part1_example2() {
+        let input = ".^^.^.^^^^";
+        assert_eq!(solve_puzzle(input, 10), 38);
+    }
+}