Day9.java

package com.sbaars.adventofcode.year20.days;

import com.sbaars.adventofcode.year20.Day2020;

import java.util.HashSet;
import java.util.Set;

import static java.util.Arrays.copyOfRange;
import static java.util.Arrays.stream;

public class Day9 extends Day2020 {
  public Day9() {
    super(9);
  }

  public static void main(String[] args) {
    new Day9().printParts();
  }

  @Override
  public Object part1() {
    long[] input = dayNumbers();
    for (int k = 0; k < input.length - 25; k++) {
      Set<Long> sums = new HashSet<>();
      for (int i = k; i < k + 25; i++) {
        for (int j = i + 1; j < k + 25; j++) {
          sums.add(input[i] + input[j]);
        }
      }
      if (!sums.contains(input[k + 25])) {
        return input[k + 25];
      }
    }
    return 0;
  }

  @Override
  public Object part2() {
    long[] input = dayNumbers();
    long part1Solution = (long) part1();
    for (int i = 2; i < input.length; i++) {
      for (int j = 0; j <= input.length - i; j++) {
        if (stream(input, j, j + i).sum() == part1Solution) {
          long[] window = copyOfRange(input, j, j + i + 1);
          return stream(window).max().getAsLong() + stream(window).min().getAsLong();
        }
      }
    }
    return 0;
  }
}