Day23.java

package com.sbaars.adventofcode.year20.days;

import com.sbaars.adventofcode.common.CircularLinkedList;
import com.sbaars.adventofcode.common.CircularLinkedList.Node;
import com.sbaars.adventofcode.year20.Day2020;

import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import static java.lang.Long.parseLong;
import static java.util.Arrays.stream;

public class Day23 extends Day2020 {
  public Day23() {
    super(23);
  }

  public static void main(String[] args) {
    new Day23().printParts();
  }

  @Override
  public Object part1() {
    return getSolution(true);
  }

  @Override
  public Object part2() {
    return getSolution(false);
  }

  private long getSolution(boolean part1) {
    IntStream input = day().chars().map(Character::getNumericValue);
    CircularLinkedList cups = new CircularLinkedList(Stream.concat(input.boxed(), part1 ? Stream.empty() : IntStream.rangeClosed(10, 1000000).boxed()).mapToInt(e -> e).toArray());
    for (int i = 0; i < (part1 ? 100 : 10000000); i++) {
      int current = cups.current();
      int j;
      Node next = cups.currentNode().next;
      Node last = next.next.next;
      for (j = current - 2 + cups.size(); j > 0; j--) {
        int n = j % cups.size() + 1;
        if (next.value != n && next.next.value != n && last.value != n) {
          break;
        }
      }
      int d = j % cups.size() + 1;
      cups.insertAfter(next, last, d);
      cups.next();
    }
    cups.setCurrent(1);
    if (part1) return parseLong(stream(cups.next(8)).mapToObj(Integer::toString).collect(Collectors.joining()));
    int[] next = cups.next(2);
    return (long) next[0] * next[1];
  }
}