Euler05Test.java

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

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

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

public class Euler05Test {

    /**
     * <strong>Problem 5: Smallest multiple</strong>
     * <p>
     * 2520 is the smallest number that can be divided by each of the numbers from 1
     * to 10 without any remainder.
     * <p>
     * What is the smallest positive number that is evenly divisible by all of the
     * numbers from 1 to 20?
     * <p>
     * See also <a href="https://projecteuler.net/problem=5">projecteuler.net problem 5</a>.
     */
    @Test
    public void shouldSolveProblem5() {
        assertThat(smallestPositiveNumberEvenlyDivisibleByAllNumbersFrom1To(10)).isEqualTo(2_520L);
        assertThat(smallestPositiveNumberEvenlyDivisibleByAllNumbersFrom1To(20)).isEqualTo(232_792_560L);
    }

    private static long smallestPositiveNumberEvenlyDivisibleByAllNumbersFrom1To(int max) {
        return Stream.rangeClosed(2, max)
                .map(PrimeNumbers::factorization)
                .reduce((m1, m2) -> m1.merge(m2, Math::max))
                .foldLeft(1L, (xs, x) -> xs * pow(x._1, x._2));
    }

    private static long pow(long a, long p) {
        return Stream.rangeClosed(1, p).fold(1L, (xs, x) -> xs * a);
    }
}