Solved Day 20 part 2 (2015) - Infinite Elves and Infinite Houses

Author: SimonBaars

src/main/java/com/sbaars/adventofcode/year15/days/Day20.java

@@ -16,7 +16,7 @@ public Day20() {
  public static void main(String[] args) {
  Day20 day = new Day20();
  day.printParts();
- new com.sbaars.adventofcode.network.Submit().submit(day.part1(), 2015, 20, 1);
+ new com.sbaars.adventofcode.network.Submit().submit(day.part2(), 2015, 20, 2);
  }
 
  @Override
@@ -30,7 +30,18 @@ public Object part1() {
 
  @Override
  public Object part2() {
- return 0; // Implement in next part
+ int houseNumber = 1;
+ while (getPresentsForHousePart2(houseNumber) < TARGET_PRESENTS) {
+ houseNumber++;
+ }
+ return houseNumber;
+ }
+
+ private int getPresentsForHousePart2(int houseNumber) {
+ return getDivisors(houseNumber).stream()
+ .filter(elf -> houseNumber / elf <= 50) // Each Elf only visits 50 houses
+ .mapToInt(elf -> elf * 11) // Each Elf delivers 11 presents
+ .sum();
  }
 
  private int getPresentsForHouse(int houseNumber) {
@@ -41,15 +52,12 @@ private int getPresentsForHouse(int houseNumber) {
 
  private Set<Integer> getDivisors(int number) {
  Set<Integer> divisors = new HashSet<>();
- int sqrt = (int) Math.sqrt(number);
- 
- for (int i = 1; i <= sqrt; i++) {
+ for (int i = 1; i <= Math.sqrt(number); i++) {
  if (number % i == 0) {
  divisors.add(i);
  divisors.add(number / i);
  }
  }
- 
  return divisors;
  }
 }