2020: day22: part 2

aoc2020/src/day22.rs

@@ -1,5 +1,5 @@
 use std::cmp::Ordering;
-use std::collections::VecDeque;
+use std::collections::{HashSet, VecDeque};
 use std::fmt::Write;
 
 use anyhow::{Context, Result};
@@ -10,11 +10,12 @@ pub fn run() -> Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
 
-fn play_game<'a>(deck_a: &'a mut Deck, deck_b: &'a mut Deck) -> &'a Deck {
+fn play_game(mut deck_a: Deck, mut deck_b: Deck) -> 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();
@@ -39,23 +40,104 @@ fn play_game<'a>(deck_a: &'a mut Deck, deck_b: &'a mut Deck) -> &'a Deck {
     }
 }
 
-fn part1(input: &str) -> Result<u64> {
-    let mut decks = input.split("\n\n");
+fn play_recursive_game(mut deck_a: Deck, mut deck_b: Deck) -> (Deck, bool) {
+    let mut seen: HashSet<(Deck, Deck)> = HashSet::new();
 
-    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()?;
+    while !(deck_a.0.is_empty() || deck_b.0.is_empty()) {
+        // Before either player deals a card, if there was a previous round in this game that had
+        // exactly the same cards in the same order in the same players' decks, the game instantly
+        // ends in a win for player 1. Previous rounds from other games are not considered. (This
+        // prevents infinite games of Recursive Combat, which everyone agrees is a bad idea.)
+        if seen.contains(&(deck_a.clone(), deck_b.clone())) {
+            return (deck_a, true);
+        } else {
+            seen.insert((deck_a.clone(), deck_b.clone()));
+        }
 
-    let winning_deck = play_game(&mut deck_a, &mut deck_b);
+        // Otherwise, this round's cards must be in a new configuration; the players begin the round
+        // by each drawing the top card of their deck as normal.
+        let card_a = deck_a.0.pop_front().unwrap();
+        let card_b = deck_b.0.pop_front().unwrap();
 
-    Ok(winning_deck
-        .0
+        // true if first player won, false otherwise
+        let winner: bool;
+
+        if deck_a.0.len() >= card_a as usize && deck_b.0.len() >= card_b as usize {
+            // If both players have at least as many cards remaining in their deck as the value of
+            // the card they just drew, the winner of the round is determined by playing a new game
+            // of Recursive Combat (see below).
+
+            // To play a sub-game of Recursive Combat, each player creates a new deck by making a
+            // copy of the next cards in their deck (the quantity of cards copied is equal to the
+            // number on the card they drew to trigger the sub-game). During this sub-game, the game
+            // that triggered it is on hold and completely unaffected; no cards are removed from
+            // players' decks to form the sub-game. (For example, if player 1 drew the 3 card, their
+            // deck in the sub-game would be copies of the next three cards in their deck.)
+            let mut new_deck_a = deck_a.clone();
+            let mut new_deck_b = deck_b.clone();
+            new_deck_a.0.truncate(card_a as usize);
+            new_deck_b.0.truncate(card_b as usize);
+
+            let (_, deck_a_won) = play_recursive_game(new_deck_a, new_deck_b);
+            winner = deck_a_won;
+        } else {
+            // Otherwise, at least one player must not have enough cards left in their deck to
+            // recurse; the winner of the round is the player with the higher-value card.
+            match card_a.cmp(&card_b) {
+                Ordering::Greater => winner = true,
+                Ordering::Less => winner = false,
+                Ordering::Equal => unreachable!(),
+            }
+        }
+
+        if winner {
+            deck_a.0.push_back(card_a);
+            deck_a.0.push_back(card_b);
+        } else {
+            deck_b.0.push_back(card_b);
+            deck_b.0.push_back(card_a);
+        }
+    }
+
+    if deck_a.0.is_empty() {
+        (deck_b, false)
+    } else {
+        (deck_a, true)
+    }
+}
+
+fn deck_score(deck: &Deck) -> u64 {
+    deck.0
         .iter()
         .rev()
         .enumerate()
         .map(|(i, card)| card * (i as u64 + 1))
-        .sum())
+        .sum()
 }
 
+fn part1(input: &str) -> Result<u64> {
+    let mut decks = input.split("\n\n");
+
+    let deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
+    let deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
+
+    let winning_deck = play_game(deck_a, deck_b);
+
+    Ok(deck_score(&winning_deck))
+}
+
+fn part2(input: &str) -> Result<u64> {
+    let mut decks = input.split("\n\n");
+
+    let deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
+    let deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
+
+    let (winning_deck, _) = play_recursive_game(deck_a, deck_b);
+
+    Ok(deck_score(&winning_deck))
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
 struct Deck(VecDeque<u64>);
 
 impl Deck {}
@@ -92,4 +174,15 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 30780);
     }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 291);
+    }
+
+    #[test]
+    #[ignore]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 36621);
+    }
 }