use bigint;

{ 
  my @lookup = (1);    # 2, 4, 8, ...

  sub int_log2 {
    my $n = shift;
    return if $n < 1;
    my $exp = 0;
    # this still could be optimized by starting at
    # $exp = min( $#lookup, $some_educated_guess_using_floating_point_approximation )
    while ( $lookup[ $exp++ ] <= $n ) {
      $lookup[$exp] = 2 * $lookup[ $exp - 1 ] if not exists $lookup[$exp];
    }
    return $exp - 2;
  }
}

for ( 1 .. 2**10 ) {
  print int_log2(2**$_-1), ' ', int_log2(2**$_), ' ', int_log2(2**$_+1), "\n";
}