#!/usr/bin/perl
use warnings;
use strict;

use Test::More;

sub f {
    my $n = shift;
    $n % 4 + 4 * int((15 - $n) / 4)
}


my @in  = qw( 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15);
my @out = qw(12 13 14 15  8  9 10 11  4  5  6  7  0  1  2  3);

for my $i (0 .. $#in) {
    is(f($in[$i]), $out[$i], "_$i");
}

done_testing(0 + @in);