question: public static double angle ( Coordinate p0 , Coordinate p1 ) { double dx = p1 . x - p0 . x ; double dy = p1 . y - p0 . y ; return Math . atan2 ( dy , dx ) ; }
target:  Returns the angle of the vector from p0 to p1, relative to the positive X-axis. The angle is normalized to be in the range [ -Pi, Pi ].