[Ether]

Originally Posted by Strants

Let U be the raw angle given by the encoder, and Q be that same angle put in the range [0, 2pi].

If sin(U) >= 0, then Q = acos(cos(U))
Otherwise, Q = 2pi - acos(cos(U))

Yikes! use this:

U = U - 2pi*floor(U/2pi)