2D Euclidean vector rotations

you should remove the vars from the function:

x = x * cs - y * sn; // now x is something different than original vector x
y = x * sn + y * cs;

create new coordinates becomes, to avoid calculation of x before it reaches the second line:

px = x * cs - y * sn; 
py = x * sn + y * cs;

Rotate by 90 degress around 0,0:

x' = -y
y' = x

Rotate by 90 degress around px,py:

x' = -(y - py) + px
y' = (x - px) + py

Rotating a vector 90 degrees is particularily simple.

(x, y) rotated 90 degrees around (0, 0) is (-y, x).

If you want to rotate clockwise, you simply do it the other way around, getting (y, -x).