package ProjectEuler;

/**
 * If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
 * The sum of these multiples is 23.
 *
 * <p>Find the sum of all the multiples of 3 or 5 below 1000.
 *
 * <p>Link: https://projecteuler.net/problem=1
 */
public class Problem01 {
 public static void main(String[] args) {
 int[][] testNumber = {
 {3, 0},
 {4, 3},
 {10, 23},
 {1000, 233168},
 {-1, 0}
 };

 for (int[] ints : testNumber) {
 assert solution1(ints[0]) == ints[1];
 assert solution2(ints[0]) == ints[1];
 }
 }

 private static int solution1(int n) {
 int sum = 0;
 for (int i = 3; i < n; ++i) {
 if (i % 3 == 0 || i % 5 == 0) {
 sum += i;
 }
 }
 return sum;
 }

 private static int solution2(int n) {
 int sum = 0;

 int terms = (n - 1) / 3;
 sum += terms * (6 + (terms - 1) * 3) / 2;

 terms = (n - 1) / 5;
 sum += terms * (10 + (terms - 1) * 5) / 2;

 terms = (n - 1) / 15;
 sum -= terms * (30 + (terms - 1) * 15) / 2;

 return sum;
 }
}