Computing Microwave Background Polarization Power Spectra from Cosmic
  Birefringence
  We present a new publicly available code, $\texttt{class_rot}$, which
modifies $\texttt{class}$ to enable fast non-perturbative calculation of cosmic
microwave background polarization power spectra due to both isotropic and
anisotropic polarization rotation from cosmic birefringence. Cosmic
birefringence can arise from new parity-violating physics such as axion dark
matter with a Chern-Simons coupling to photons or Faraday rotation due to a
primordial magnetic field. Constraints on these effects can be obtained by
comparing measurements to precise numerical calculations of the polarization
power spectra. We describe the implementation of $\texttt{class_rot}$ in terms
of both mathematical formalism and coding architecture. We also provide usage
examples and demonstrate the accuracy of the code by comparing with
simulations.
