Euler01Test.java

/*                        __    __  __  __    __  ___
 *                       \  \  /  /    \  \  /  /  __/
 *                        \  \/  /  /\  \  \/  /  /
 *                         \____/__/  \__\____/__/.ɪᴏ
 * ᶜᵒᵖʸʳᶦᵍʰᵗ ᵇʸ ᵛᵃᵛʳ ⁻ ˡᶦᶜᵉⁿˢᵉᵈ ᵘⁿᵈᵉʳ ᵗʰᵉ ᵃᵖᵃᶜʰᵉ ˡᶦᶜᵉⁿˢᵉ ᵛᵉʳˢᶦᵒⁿ ᵗʷᵒ ᵈᵒᵗ ᶻᵉʳᵒ
 */
package io.vavr.collection.euler;

import io.vavr.collection.List;
import org.junit.Test;

import static org.assertj.core.api.Assertions.assertThat;

/**
 * <strong>Problem 1: Multiples of 3 and 5</strong>
 * <p>
 * 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>
 * See also <a href="https://projecteuler.net/problem=1">projecteuler.net problem 1</a>.
 */
public class Euler01Test {

    @Test
    public void shouldSolveProblem1() {
        assertThat(sumOfMultiplesOf3and5Below(10)).isEqualTo(23);
        assertThat(sumOfMultiplesOf3and5Below(1000)).isEqualTo(233168);
    }

    private static int sumOfMultiplesOf3and5Below(int limit) {
        return List.range(1, limit).filter(Euler01Test::isMultipleOf3or5).sum().intValue();
    }

    private static boolean isMultipleOf3or5(int num) {
        return num % 3 == 0 || num % 5 == 0;
    }
}