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.