solution: day 4

src/bin/04.rs

@@ -0,0 +1,79 @@
+use std::str::FromStr;
+
+use aoc::parsers::to_vec;
+
+struct ParseCardError;
+
+struct Card {
+    winning: Vec<u32>,
+    numbers: Vec<u32>,
+}
+
+impl FromStr for Card {
+    type Err = ParseCardError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (_, rest) = s.split_once(": ").unwrap();
+        let (win, my) = rest.split_once(" | ").unwrap();
+
+        let winning: Vec<u32> = to_vec(win, ' ');
+        let numbers: Vec<u32> = to_vec(my, ' ');
+
+        Ok(Card { winning, numbers })
+    }
+}
+
+impl Card {
+    fn n_matches(&self) -> usize {
+        self.numbers
+            .iter()
+            .filter(|n| self.winning.contains(n))
+            .count()
+    }
+    fn score(&self) -> u32 {
+        let c = self.n_matches();
+
+        if c == 0 {
+            return 0;
+        }
+
+        2_u32.pow((self.n_matches() - 1) as u32)
+    }
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    Some(
+        input
+            .lines()
+            .filter_map(|l| l.parse::<Card>().ok())
+            .map(|g| g.score())
+            .sum(),
+    )
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let cards: Vec<Card> = to_vec(input, '\n');
+    let mut multiples = vec![1; cards.len()];
+
+    for (i, card) in cards.iter().enumerate() {
+        for j in 0..card.n_matches() {
+            multiples[i + j + 1] += multiples[i];
+        }
+    }
+
+    Some(multiples.iter().sum::<u32>())
+}
+
+aoc::solution!(4);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(part_one(&aoc::template::read_file("examples", 4)), Some(13));
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(part_two(&aoc::template::read_file("examples", 4)), Some(30));
+    }
+}