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

Author: SimonBaars

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

@@ -1,23 +1,55 @@
 package com.sbaars.adventofcode.year15.days;
 
 import com.sbaars.adventofcode.year15.Day2015;
+import java.util.*;
+import java.util.stream.IntStream;
 
 public class Day20 extends Day2015 {
- public Day20() {
- super(20);
- }
-
- public static void main(String[] args) {
- new Day20().printParts();
- }
-
- @Override
- public Object part1() {
- return "";
- }
-
- @Override
- public Object part2() {
- return "";
- }
+
+ private static final int TARGET_PRESENTS = 36000000;
+ private static final int PRESENTS_PER_ELF = 10;
+
+ public Day20() {
+ super(20);
+ }
+
+ public static void main(String[] args) {
+ Day20 day = new Day20();
+ day.printParts();
+ new com.sbaars.adventofcode.network.Submit().submit(day.part1(), 2015, 20, 1);
+ }
+
+ @Override
+ public Object part1() {
+ int houseNumber = 1;
+ while (getPresentsForHouse(houseNumber) < TARGET_PRESENTS) {
+ houseNumber++;
+ }
+ return houseNumber;
+ }
+
+ @Override
+ public Object part2() {
+ return 0; // Implement in next part
+ }
+
+ private int getPresentsForHouse(int houseNumber) {
+ return getDivisors(houseNumber).stream()
+ .mapToInt(divisor -> divisor * PRESENTS_PER_ELF)
+ .sum();
+ }
+
+ private Set<Integer> getDivisors(int number) {
+ Set<Integer> divisors = new HashSet<>();
+ int sqrt = (int) Math.sqrt(number);
+ 
+ for (int i = 1; i <= sqrt; i++) {
+ if (number % i == 0) {
+ divisors.add(i);
+ divisors.add(number / i);
+ }
+ }
+ 
+ return divisors;
+ }
 }