2020: day22: part 2
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@ -1,5 +1,5 @@
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use std::cmp::Ordering;
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use std::collections::VecDeque;
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use std::collections::{HashSet, VecDeque};
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use std::fmt::Write;
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use anyhow::{Context, Result};
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@ -10,11 +10,12 @@ pub fn run() -> Result<String> {
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let mut res = String::with_capacity(128);
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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writeln!(res, "part 2: {}", part2(INPUT)?)?;
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Ok(res)
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}
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fn play_game<'a>(deck_a: &'a mut Deck, deck_b: &'a mut Deck) -> &'a Deck {
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fn play_game(mut deck_a: Deck, mut deck_b: Deck) -> Deck {
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while !(deck_a.0.is_empty() || deck_b.0.is_empty()) {
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let card_a = deck_a.0.pop_front().unwrap();
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let card_b = deck_b.0.pop_front().unwrap();
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@ -39,23 +40,104 @@ fn play_game<'a>(deck_a: &'a mut Deck, deck_b: &'a mut Deck) -> &'a Deck {
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}
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}
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fn part1(input: &str) -> Result<u64> {
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let mut decks = input.split("\n\n");
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fn play_recursive_game(mut deck_a: Deck, mut deck_b: Deck) -> (Deck, bool) {
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let mut seen: HashSet<(Deck, Deck)> = HashSet::new();
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let mut deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
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let mut deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
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while !(deck_a.0.is_empty() || deck_b.0.is_empty()) {
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// Before either player deals a card, if there was a previous round in this game that had
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// exactly the same cards in the same order in the same players' decks, the game instantly
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// ends in a win for player 1. Previous rounds from other games are not considered. (This
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// prevents infinite games of Recursive Combat, which everyone agrees is a bad idea.)
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if seen.contains(&(deck_a.clone(), deck_b.clone())) {
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return (deck_a, true);
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} else {
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seen.insert((deck_a.clone(), deck_b.clone()));
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}
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let winning_deck = play_game(&mut deck_a, &mut deck_b);
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// Otherwise, this round's cards must be in a new configuration; the players begin the round
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// by each drawing the top card of their deck as normal.
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let card_a = deck_a.0.pop_front().unwrap();
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let card_b = deck_b.0.pop_front().unwrap();
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Ok(winning_deck
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.0
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// true if first player won, false otherwise
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let winner: bool;
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if deck_a.0.len() >= card_a as usize && deck_b.0.len() >= card_b as usize {
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// If both players have at least as many cards remaining in their deck as the value of
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// the card they just drew, the winner of the round is determined by playing a new game
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// of Recursive Combat (see below).
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// To play a sub-game of Recursive Combat, each player creates a new deck by making a
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// copy of the next cards in their deck (the quantity of cards copied is equal to the
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// number on the card they drew to trigger the sub-game). During this sub-game, the game
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// that triggered it is on hold and completely unaffected; no cards are removed from
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// players' decks to form the sub-game. (For example, if player 1 drew the 3 card, their
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// deck in the sub-game would be copies of the next three cards in their deck.)
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let mut new_deck_a = deck_a.clone();
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let mut new_deck_b = deck_b.clone();
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new_deck_a.0.truncate(card_a as usize);
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new_deck_b.0.truncate(card_b as usize);
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let (_, deck_a_won) = play_recursive_game(new_deck_a, new_deck_b);
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winner = deck_a_won;
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} else {
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// Otherwise, at least one player must not have enough cards left in their deck to
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// recurse; the winner of the round is the player with the higher-value card.
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match card_a.cmp(&card_b) {
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Ordering::Greater => winner = true,
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Ordering::Less => winner = false,
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Ordering::Equal => unreachable!(),
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}
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}
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if winner {
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deck_a.0.push_back(card_a);
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deck_a.0.push_back(card_b);
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} else {
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deck_b.0.push_back(card_b);
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deck_b.0.push_back(card_a);
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}
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}
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if deck_a.0.is_empty() {
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(deck_b, false)
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} else {
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(deck_a, true)
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}
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}
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fn deck_score(deck: &Deck) -> u64 {
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deck.0
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.iter()
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.rev()
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.enumerate()
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.map(|(i, card)| card * (i as u64 + 1))
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.sum())
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.sum()
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}
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fn part1(input: &str) -> Result<u64> {
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let mut decks = input.split("\n\n");
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let deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
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let deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
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let winning_deck = play_game(deck_a, deck_b);
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Ok(deck_score(&winning_deck))
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}
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fn part2(input: &str) -> Result<u64> {
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let mut decks = input.split("\n\n");
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let deck_a: Deck = decks.next().context("couldn't get first deck")?.parse()?;
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let deck_b: Deck = decks.next().context("couldn't get second deck")?.parse()?;
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let (winning_deck, _) = play_recursive_game(deck_a, deck_b);
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Ok(deck_score(&winning_deck))
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}
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#[derive(Debug, Clone, Hash, PartialEq, Eq)]
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struct Deck(VecDeque<u64>);
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impl Deck {}
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@ -92,4 +174,15 @@ mod tests {
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fn part1_real() {
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assert_eq!(part1(INPUT).unwrap(), 30780);
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}
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#[test]
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fn part2_provided() {
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assert_eq!(part2(PROVIDED).unwrap(), 291);
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}
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#[test]
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#[ignore]
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 36621);
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}
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}
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