Hilbert Curve, Java

Hilbert Curve

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[]) {
        System.out.println(xy2d(1, 1, 2)); // expected: 2
        System.out.println(xy2d(5, 6, 3)); // expected: 39
        System.out.println(xy2d(1, 5, 3)); // expected: 18
        System.out.println(Arrays.toString(d2xy(2, 2))); // expected: 2
        System.out.println(Arrays.toString(d2xy(39, 3))); // expected: 1
        System.out.println(Arrays.toString(d2xy(18, 3))); // expected: 18
    }
    
    private static int xy2d(int x, int y, int n) {
        if (n == 0) return 0;
        int count = 1 << (2 * (n - 1)); // steps to go through a quater square
        int len = 1 << (n - 1); // size of quater square
        if (x >= len && y >= len) return 2*count + xy2d(x - len, y - len, n - 1);
        else if (x < len && y >= len) return count + xy2d(x, y - len, n - 1);
        else if (x < len && y < len) return xy2d(y, x, n - 1);
        else return 3*count + xy2d(len - 1 - y, 2*len - 1 - x, n - 1);
    }
    
    private static int[] d2xy(int d, int n) {
        if (n == 0) return new int[] {0, 0};
        int count = 1 << (2 * (n - 1)); // steps to go through a quater square
        int len = 1 << (n - 1); // size of quater square
        if (d < count) {
            int[] res = d2xy(d, n - 1);
            return new int[] {res[1], res[0]};
        } else if (d < 2*count) {
            int[] res = d2xy(d - count, n - 1);
            return new int[] {res[0], res[1] + len};
        } else if (d < 3*count) {
            int[] res = d2xy(d - 2*count, n - 1);
            return new int[] {res[0] + len, res[1] + len};
        } else {
            int[] res = d2xy(d - 3*count, n - 1);
            return new int[] {-res[1] + 2*len - 1, -res[0] + len - 1};
        }
    }
}


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