Euler04Test.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;

public class Euler04Test {

    /**
     * <strong>Problem 4: Largest palindrome product</strong>
     * <p>
     * A palindromic number reads the same both ways. The largest palindrome made
     * from the product of two 2-digit numbers is 9009 = 91 × 99.
     * <p>
     * Find the largest palindrome made from the product of two 3-digit numbers.
     * <p>
     * See also <a href="https://projecteuler.net/problem=4">projecteuler.net problem 4</a>.
     */
    @Test
    public void shouldSolveProblem4() {
        assertThat(largestPalindromeOfProductsFromFactorsInRange(10, 99)).isEqualTo(9009);
        assertThat(largestPalindromeOfProductsFromFactorsInRange(100, 999)).isEqualTo(906609);
    }

    private static int largestPalindromeOfProductsFromFactorsInRange(final int min, final int max) {
        return List.rangeClosed(min, max)
                .crossProduct()
                .filter(t -> t._1 <= t._2)
                .map(t -> t._1 * t._2)
                .filter(Utils::isPalindrome)
                .max().get();
    }
}