my %map;
@map{qw(AA AC AT AG CA CC CT CG
        TA TC TT TG GA GC GT GG
        A C T G)} = (0..9, 'a'..'f', 0..3);

sub bitkey {
  my $key = shift;
  $key =~ s/(..?)/$map{$1}/g;
  return pack "H*" => $key;
}