Special 2D vectors
Converting between vectors and angles
Compute the direction of a vector, measured counterclockwise from the positive x-axis as a fraction of a full turn. The zero vector is arbitrarily assigned the direction 0.
Convert an angle into a unit vector pointing in that direction.
2D vector utilities
perp v is perpendicular to and has the same magnitude as
perp v == rotateBy (1/4) v.