my ($dx1, $dy1) = ($x2 - $x1, $y2 - $y1);
my ($dx2, $dy2) = ($x3 - $x2, $y3 - $y2);
my $cross = $dx1*$dy2 - $dy1*$dx2;
print $cross < 0
      ? "clockwise"
      :   $cross == 0
          ? "collinear"
          : "counterclockwise";