Day13.java

package com.sbaars.adventofcode.year20.days;

import com.sbaars.adventofcode.year20.Day2020;

import static java.lang.Integer.parseInt;
import static java.lang.Long.parseLong;
import static java.util.Arrays.stream;
import static java.util.stream.IntStream.range;

public class Day13 extends Day2020 {
  public Day13() {
    super(13);
  }

  public static void main(String[] args) {
    new Day13().printParts();
  }

  @Override
  public Object part1() {
    String[] day = dayStrings();
    int timestamp = parseInt(day[0]);
    int[] times = stream(day[1].replace(",x", "").split(","))
        .mapToInt(Integer::parseInt).toArray();
    for (int i = timestamp; true; i++) {
      for (int j : times) {
        if (i % j == 0) {
          return j * (i - timestamp);
        }
      }
    }
  }

  @Override
  public Object part2() {
    String[] s = dayStrings()[1].split(",");
    long[][] nums = range(0, s.length).filter(i -> !s[i].equals("x"))
        .mapToObj(i -> new long[]{parseLong(s[i]), i})
        .toArray(long[][]::new);
    long product = stream(nums).mapToLong(a -> a[0]).reduce((a, b) -> a * b).getAsLong();
    long sum = stream(nums).mapToLong(a -> a[1] * (product / a[0]) * inverseModulo(product / a[0], a[0])).sum();
    return product - sum % product;
  }

  long inverseModulo(long x, long y) {
    if (x != 0) {
      long modulo = y % x;
      return modulo == 0 ? 1 : y - inverseModulo(modulo, x) * y / x;
    }
    return 0;
  }
}