Day 22. Clearly some optimizing could be done, but it works.

inputs/day22.txt

@@ -0,0 +1,53 @@
+Player 1:
+43
+36
+13
+11
+20
+25
+37
+38
+4
+18
+1
+8
+27
+23
+7
+22
+10
+5
+50
+40
+45
+26
+15
+32
+33
+
+Player 2:
+21
+29
+12
+28
+46
+9
+44
+6
+16
+39
+19
+24
+17
+14
+47
+48
+42
+34
+31
+3
+41
+35
+2
+30
+49

inputs/day22_test.txt

@@ -0,0 +1,13 @@
+Player 1:
+9
+2
+6
+3
+1
+
+Player 2:
+5
+8
+4
+7
+10

inputs/day22_test_halt.txt

@@ -0,0 +1,8 @@
+Player 1:
+43
+19
+
+Player 2:
+2
+29
+14

src/bin/crab_combat.rs

@@ -0,0 +1,212 @@
+use advent2020::errors::TopLevelError;
+use std::env;
+use std::fmt;
+use std::fs;
+use std::iter::FromIterator;
+use std::str::FromStr;
+use std::{
+    collections::{HashMap, VecDeque},
+    sync::atomic::{AtomicUsize, Ordering},
+};
+
+#[derive(Clone, Eq, Hash, PartialEq)]
+struct Deck {
+    player: usize,
+    cards: VecDeque<usize>,
+}
+
+impl Deck {
+    fn read<'a, I: Iterator<Item = &'a str>>(lines: &mut I) -> Result<Deck, TopLevelError> {
+        match lines.next() {
+            None => Err(TopLevelError::NoInputFound),
+            Some("") => Deck::read(lines),
+            Some(x) if x.starts_with("Player ") => {
+                let numeric_string = x.trim_start_matches("Player ").trim_end_matches(':');
+                let player = usize::from_str(numeric_string)?;
+                let mut cards = VecDeque::new();
+
+                loop {
+                    match lines.next() {
+                        None => break,
+                        Some("") => break,
+                        Some(x) => cards.push_back(usize::from_str(x)?),
+                    }
+                }
+
+                Ok(Deck { player, cards })
+            }
+            Some(_) => Err(TopLevelError::UnknownError),
+        }
+    }
+
+    fn top(&mut self) -> Option<usize> {
+        self.cards.pop_front()
+    }
+
+    fn add_cards(&mut self, winning_card: usize, cards: &mut Vec<usize>) {
+        cards.sort_unstable();
+        cards.reverse();
+        self.cards.push_back(winning_card);
+        for card in cards.drain(..).filter(|x| *x != winning_card) {
+            self.cards.push_back(card);
+        }
+    }
+
+    fn size(&self) -> usize {
+        self.cards.len()
+    }
+
+    fn resize(&mut self, new_size: usize) {
+        self.cards.truncate(new_size);
+    }
+}
+
+impl fmt::Display for Deck {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Player {}'s deck: {:?}", self.player, self.cards)
+    }
+}
+
+struct Game {
+    id: usize,
+    round: usize,
+    history: Vec<HashMap<usize, Deck>>,
+    decks: HashMap<usize, Deck>,
+}
+
+static NEXT_GAME_NUMBER: AtomicUsize = AtomicUsize::new(1);
+
+impl Game {
+    fn new(decks: &[Deck]) -> Game {
+        Game {
+            id: NEXT_GAME_NUMBER.fetch_add(1, Ordering::SeqCst),
+            history: Vec::new(),
+            round: 0,
+            decks: HashMap::from_iter(decks.iter().map(|x| (x.player, x.clone()))),
+        }
+    }
+
+    fn winner(&self) -> Option<&Deck> {
+        let mut result = None;
+
+        for deck in self.decks.values() {
+            if deck.size() > 0 && result.is_none() {
+                result = Some(deck);
+            } else if deck.size() > 0 {
+                return None;
+            }
+        }
+
+        result
+    }
+
+    fn play(&mut self, recursive: bool) -> Result<Deck, TopLevelError> {
+        loop {
+            // first, see if we're done
+            if let Some(winner) = self.winner() {
+                return Ok(winner.clone());
+            }
+
+            // OK, let's print out some state information
+            self.round += 1;
+            println!(
+                "-- Game {}, Round {} {}--",
+                self.id,
+                self.round,
+                if recursive { "[recursive]" } else { "" }
+            );
+            for deck in self.decks.values() {
+                println!("{}", deck);
+            }
+
+            // if this is the recursive version of the game and we've been here before,
+            // just stop, and player 1 won.
+            if recursive && self.history.contains(&self.decks) {
+                return self
+                    .decks
+                    .get(&1)
+                    .cloned()
+                    .ok_or(TopLevelError::UnknownError);
+            }
+
+            if recursive {
+                self.history.push(self.decks.clone());
+            }
+
+            // otherwise, grab the first card off each of the decks
+            let mut top_card_info = Vec::new();
+
+            for (player, deck) in self.decks.iter_mut() {
+                if let Some(top_card) = deck.top() {
+                    let recurse_check_value = deck.size() >= top_card;
+                    println!(
+                        "Top card for Player {}: {} [recurse check: {}]",
+                        player, top_card, recurse_check_value
+                    );
+                    top_card_info.push((deck, top_card, recurse_check_value));
+                }
+            }
+
+            let mut top_cards = top_card_info.iter().map(|(_, x, _)| *x).collect();
+
+            // if we're in a recursive game, and we meet the length conditions, recurse
+            if recursive && top_card_info.iter().all(|(_, _, x)| *x) {
+                let new_decks: Vec<Deck> = top_card_info
+                    .iter()
+                    .map(|(deck, newlen, _)| {
+                        let mut new_deck = (&**deck).clone();
+                        new_deck.resize(*newlen);
+                        new_deck
+                    })
+                    .collect();
+                println!("Creating recursive game!");
+                let mut subgame = Game::new(&new_decks);
+                let subgame_result = subgame.play(true)?;
+
+                for (deck, top, _) in top_card_info.drain(..) {
+                    if deck.player == subgame_result.player {
+                        deck.add_cards(top, &mut top_cards);
+                    }
+                }
+            } else {
+                // this is just a normal case
+                let winning_card = top_card_info
+                    .iter()
+                    .map(|(_, x, _)| *x)
+                    .max()
+                    .ok_or(TopLevelError::UnknownError)?;
+                println!("The winning card is {}", winning_card);
+                let (winner, _, _) = top_card_info
+                    .drain(..)
+                    .filter(|(_, x, _)| *x == winning_card)
+                    .next()
+                    .ok_or(TopLevelError::UnknownError)?;
+                winner.add_cards(winning_card, &mut top_cards);
+            }
+
+            println!();
+        }
+    }
+}
+
+fn main() -> Result<(), TopLevelError> {
+    let filename = env::args().nth(1).expect("No file argument given.");
+    let contents = fs::read_to_string(filename)?;
+    let mut lines = contents.lines();
+    let deck1 = Deck::read(&mut lines)?;
+    let deck2 = Deck::read(&mut lines)?;
+    let decks = [deck1, deck2];
+    let mut game1 = Game::new(&decks);
+
+    let result = game1.play(true)?;
+    let mut sum = 0;
+
+    for (num, card) in result.cards.iter().rev().enumerate() {
+        sum += (num + 1) * card;
+    }
+
+    println!("Winning deck: {}", result);
+    println!("Final puzzle result: {}", sum);
+
+    Ok(())
+}