2020: day22: part 1

aoc2020/src/day22.rs

@@ -0,0 +1,95 @@
+use std::cmp::Ordering;
+use std::collections::VecDeque;
+use std::fmt::Write;
+
+use anyhow::{Context, Result};
+
+const INPUT: &str = include_str!("../input/day22.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn play_game<'a>(deck_a: &'a mut Deck, deck_b: &'a mut Deck) -> &'a Deck {
+    while !(deck_a.0.is_empty() || deck_b.0.is_empty()) {
+        let card_a = deck_a.0.pop_front().unwrap();
+        let card_b = deck_b.0.pop_front().unwrap();
+
+        match card_a.cmp(&card_b) {
+            Ordering::Greater => {
+                deck_a.0.push_back(card_a);
+                deck_a.0.push_back(card_b);
+            }
+            Ordering::Less => {
+                deck_b.0.push_back(card_b);
+                deck_b.0.push_back(card_a);
+            }
+            Ordering::Equal => unreachable!(),
+        }
+    }
+
+    if deck_a.0.is_empty() {
+        deck_b
+    } else {
+        deck_a
+    }
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let mut decks = input.split("\n\n");
+
+    let mut deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
+    let mut deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
+
+    let winning_deck = play_game(&mut deck_a, &mut deck_b);
+
+    Ok(winning_deck
+        .0
+        .iter()
+        .rev()
+        .enumerate()
+        .map(|(i, card)| card * (i as u64 + 1))
+        .sum())
+}
+
+struct Deck(VecDeque<u64>);
+
+impl Deck {}
+
+impl std::str::FromStr for Deck {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let mut lines = s.lines();
+
+        // `Player N:`
+        lines.next().context("couldn't skip first line")?;
+
+        let deck = lines
+            .map(|line| line.parse().map_err(anyhow::Error::new))
+            .collect::<Result<_>>()?;
+
+        Ok(Self(deck))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day22_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 306);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 30780);
+    }
+}

aoc2020/src/lib.rs

@@ -20,3 +20,4 @@ pub mod day17;
 pub mod day18;
 pub mod day19;
 pub mod day21;
+pub mod day22;

aoc2020/src/main.rs

@@ -22,6 +22,7 @@ use aoc2020::day17;
 use aoc2020::day18;
 use aoc2020::day19;
 use aoc2020::day21;
+use aoc2020::day22;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -45,6 +46,7 @@ fn main() -> Result<()> {
         day18::run,
         day19::run,
         day21::run,
+        day22::run,
     ];
 
     aoc::run(days)