Add solutions to day 18.

2016/day-18/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "day-18"
+version = "0.1.0"
+authors = ["Bert Peters <bert.ljpeters@gmail.com>"]
+
+[dependencies]

2016/day-18/src/main.rs

@@ -0,0 +1,44 @@
+const INPUT: &'static str = ".^^^.^.^^^^^..^^^..^..^..^^..^.^.^.^^.^^....^.^...^.^^.^^.^^..^^..^.^..^^^.^^...^...^^....^^.^^^^^^^";
+
+fn count_row(row: &[bool]) -> usize
+{
+    return row.iter().filter(|&&b| !b).count();
+}
+
+fn get_neighbours(row: &[bool], index: usize) -> [bool; 3]
+{
+    let mut output = [false; 3];
+    output[0] = if index > 0 { row[index - 1] } else { false };
+    output[1] = row[index];
+    output[2] = if index < row.len() - 1 { row[index + 1] } else { false };
+
+    return output;
+}
+
+fn solve_for(n: i32)
+{
+    let mut row: Vec<bool> = INPUT.chars().map(|c| c == '^').collect();
+    let mut count = count_row(&row);
+
+    for _ in 1..n {
+        let mut new_row = row.clone();
+
+        for j in 0..row.len() {
+            let neighbours = get_neighbours(&row, j);
+            new_row[j] = (neighbours[0] && neighbours[1] && !neighbours[2])
+                || (!neighbours[0] && neighbours[1] && neighbours[2])
+                || (neighbours[0] && !neighbours[1] && !neighbours[2])
+                || (!neighbours[0] && !neighbours[1] && neighbours[2]);
+        }
+
+        count += count_row(&new_row);
+        row = new_row;
+    }
+
+    println!("{} safe spaces in {} rows", count, n);
+}
+
+fn main() {
+    solve_for(40);
+    solve_for(400_000);
+}