Day 18 part 1.

src/day18.rs

@@ -0,0 +1,68 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+
+type Field = Vec<Vec<bool>>;
+
+#[aoc_generator(day18)]
+fn parse(input: &str) -> Field {
+    let w = input.lines().next().unwrap().len();
+    let h = input.lines().count();
+    let mut result = vec![vec![false; h + 2]; w + 2];
+    input.lines().enumerate().for_each(|(y, line)| {
+        line.chars()
+            .enumerate()
+            .for_each(|(x, c)| result[x + 1][y + 1] = c == '#');
+    });
+    result
+}
+
+#[aoc(day18, part1)]
+fn part1(input: &Field) -> usize {
+    let mut current = input.clone();
+    let mut next = input.clone();
+    for _ in 0..100 {
+        for x in 1..current.len() - 1 {
+            for y in 1..current.len() - 1 {
+                let now = current[x][y];
+                let neighbors_on: usize = count_neighbors(&current, x, y);
+                let new = if now {
+                    neighbors_on == 2 || neighbors_on == 3
+                } else {
+                    neighbors_on == 3
+                };
+                next[x][y] = new;
+            }
+        }
+        (current, next) = (next, current);
+    }
+    current.into_iter().flatten().filter(|x| *x).count()
+}
+
+fn count_neighbors(field: &Field, x: usize, y: usize) -> usize {
+    let mut result = 0;
+    if field[x - 1][y - 1] {
+        result += 1;
+    }
+    if field[x][y - 1] {
+        result += 1;
+    }
+    if field[x + 1][y - 1] {
+        result += 1;
+    }
+    if field[x - 1][y] {
+        result += 1;
+    }
+    if field[x + 1][y] {
+        result += 1;
+    }
+    if field[x - 1][y + 1] {
+        result += 1;
+    }
+    if field[x][y + 1] {
+        result += 1;
+    }
+    if field[x + 1][y + 1] {
+        result += 1;
+    }
+
+    result
+}