#!/usr/bin/perl

# clockwise( $x0, $y0, $x1, $y1, $x2, $y2 )
#    Return positive if one must turn clockwise (right) when moving
#    from p0 (x0, y0) to p1 to p2, negative if counterclockwise (left).
#    It returns zero if the three points lie on the same line --
#    but beware of floating point errors.
#
sub clockwise {
    my ( $x0, $y0, $x1, $y1, $x2, $y2 ) = @_;
    return ( $x2 - $x0 ) * ( $y1 - $y0 ) - ( $x1 - $x0 ) * ( $y2 - $y0 );
}

print clockwise( 1, 1,   4, 3,   4, 4 ), "\n";
print clockwise( 1, 1,   4, 3,   7, 5 ), "\n";
print clockwise( 1, 1,   4, 3,   7, 4 ), "\n";