Day22.java

package com.sbaars.adventofcode.year20.days;

import com.sbaars.adventofcode.year20.Day2020;

import java.util.*;
import java.util.stream.LongStream;

import static java.lang.Math.toIntExact;
import static java.util.stream.Collectors.toCollection;

public class Day22 extends Day2020 {
  public Day22() {
    super(22);
  }

  public static void main(String[] args) {
    new Day22().printParts();
  }

  @Override
  public Object part1() {
    String[] input = day().split("\n\n");
    Deque<Long> p1 = getInput(0, input);
    Deque<Long> p2 = getInput(1, input);

    while (p1.size() > 0 && p2.size() > 0) {
      long l1 = p1.pop();
      long l2 = p2.pop();
      if (l1 > l2) {
        p1.add(l1);
        p1.add(l2);
      } else {
        p2.add(l2);
        p2.add(l1);
      }
    }
    Deque<Long> winner = p1.size() > 0 ? p1 : p2;
    return calcScore(winner);
  }

  private long calcScore(Deque<Long> winner) {
    return LongStream.rangeClosed(1, winner.size()).boxed().sorted(Comparator.reverseOrder()).mapToLong(l -> winner.pop() * l).sum();
  }

  @Override
  public Object part2() {
    String[] input = day().split("\n\n");
    Deque<Long> p1 = getInput(0, input);
    Deque<Long> p2 = getInput(1, input);
    return calcScore(playGame(p1, p2) == Player.P1 ? p1 : p2);
  }

  private ArrayDeque<Long> getInput(int i, String[] input) {
    return Arrays.stream(input[i].split("\n")).filter(e -> !e.startsWith("Player")).map(Long::parseLong).collect(toCollection(ArrayDeque::new));
  }

  public Player playGame(Deque<Long> p1, Deque<Long> p2) {
    Set<List<Long>> playedGames = new HashSet<>();
    while (p1.size() > 0 && p2.size() > 0) {
      if (!playedGames.add(new ArrayList<>(p1))) {
        return Player.P1;
      }

      long l1 = p1.pop();
      long l2 = p2.pop();
      if (p1.size() < l1 || p2.size() < l2) {
        if (l1 > l2) {
          p1.add(l1);
          p1.add(l2);
        } else {
          p2.add(l2);
          p2.add(l1);
        }
      } else {
        if (playGame(new ArrayDeque<>(new ArrayList<>(p1).subList(0, toIntExact(l1))), new ArrayDeque<>(new ArrayList<>(p2).subList(0, toIntExact(l2)))) == Player.P1) {
          p1.add(l1);
          p1.add(l2);
        } else {
          p2.add(l2);
          p2.add(l1);
        }
      }
    }
    return p1.size() > 0 ? Player.P1 : Player.P2;
  }

  enum Player {P1, P2}
}