Euler09Test.java

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

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

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

public class Euler09Test {

    /**
     * <strong>Problem 9: Special Pythagorean triplet</strong>
     * <p>
     * A Pythagorean triplet is a set of three natural numbers, a < b < c, for which,
     * a<sup>2</sup> + b<sup>2</sup> = c<sup>2</sup>
     * <p>
     * For example, 3<sup>2</sup> + 4<sup>2</sup> = 9 + 16 = 25 = 5<sup>2</sup>.
     * <p>
     * There exists exactly one Pythagorean triplet for which a + b + c = 1000.
     * Find the product abc.
     * <p>
     * See also <a href="https://projecteuler.net/problem=9">projecteuler.net problem 9</a>.
     */
    @Test
    public void shouldSolveProblem9() {
        assertThat(abc(1_000)).isEqualTo(31_875_000);
    }

    public int abc(int sum) {
        return List.rangeClosed(1, sum)
                .crossProduct()
                .filter(t -> t._1 + t._2 < sum)
                .map(t -> Tuple.of(t._1, t._2, sum - t._1 - t._2))
                .filter(t -> t._1 * t._1 + t._2 * t._2 == t._3 * t._3)
                .map(t -> t._1 * t._2 * t._3)
                .head();
    }
}