Solve 11

src/bin/11.rs

@@ -0,0 +1,76 @@
+use aoc::*;
+use cached::proc_macro::cached;
+
+const INPUT: &str = include_str!("../../input/11");
+
+fn main() {
+    assert_example!(part1, "11-test", 55312);
+    println!("Part 1: {}", part1(INPUT));
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    parse_ws_separated(input)
+        .map(|stone| blink(stone, 25))
+        .sum()
+}
+
+fn part2(input: &str) -> usize {
+    parse_ws_separated(input)
+        .map(|stone| blink(stone, 75))
+        .sum()
+}
+
+#[cached]
+fn blink(stone: usize, times: usize) -> usize {
+    if times == 0 {
+        return 1;
+    }
+
+    if stone == 0 {
+        blink(1, times - 1)
+    } else if even_digits(stone) {
+        let (left, right) = split(stone);
+        blink(left, times - 1) + blink(right, times - 1)
+    } else {
+        blink(stone * 2024, times - 1)
+    }
+}
+
+fn even_digits(n: usize) -> bool {
+    digits(n) % 2 == 0
+}
+
+fn digits(n: usize) -> usize {
+    if n == 0 {
+        1
+    } else {
+        n.ilog10() as usize + 1
+    }
+}
+
+#[test]
+fn test_digits() {
+    assert_eq!(digits(0), 1);
+    assert_eq!(digits(1), 1);
+    assert_eq!(digits(9), 1);
+    assert_eq!(digits(10), 2);
+    assert_eq!(digits(99), 2);
+    assert_eq!(digits(100), 3);
+    assert_eq!(digits(999), 3);
+}
+
+fn split(n: usize) -> (usize, usize) {
+    let half = digits(n) / 2;
+    let split = 10usize.pow(half as u32);
+    let left = n / split;
+    let right = n % split;
+    (left, right)
+}
+
+#[test]
+fn test_split() {
+    assert_eq!(split(10), (1, 0));
+    assert_eq!(split(99), (9, 9));
+    assert_eq!(split(253000), (253, 0));
+}