Euler36Test.java

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

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

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

/**
 * <strong>Problem 36: Double-base palindromes</strong>
 * <p>The decimal number, 585 = 1001001001<sub>2</sub> (binary), is palindromic in both bases.</p>
 * <p>Find the sum of all numbers, less than one million, which are palindromic in base 10 and base 2.</p>
 * <p class="info">(Please note that the palindromic number, in either base, may not include leading zeros.)</p>
 * See also <a href="https://projecteuler.net/problem=36">projecteuler.net problem 36</a>.
 */
public class Euler36Test {

    @Test
    public void shouldSolveProblem36() {
        assertThat(solve(1000000)).isEqualTo(872187);
    }

    private static int solve(int n) {
        return Stream.range(1, n)
                .filter(Euler36Test::isDoubleBasePalindrome)
                .sum().intValue();
    }

    private static boolean isPalindrome(CharSeq seq) {
        return seq.dropWhile(c -> c == '0').equals(seq.reverse().dropWhile(c -> c == '0'));
    }

    private static boolean isDoubleBasePalindrome(int x) {
        final CharSeq seq = CharSeq.of(Integer.toString(x));
        final CharSeq rev = CharSeq.of(Integer.toBinaryString(x));
        return isPalindrome(seq) && isPalindrome(rev);
    }

}