TREVR: A general N log2N radiative transfer algorithm

We present Tree-based REVerse Ray Tracing (TREVR), a general algorithm for computing the radiation field, including absorption, in astrophysical simulations. TREVR is designed to handle large numbers of sources and absorbers; it is based on a tree data structure and is thus suited to codes that use trees for their gravity or hydrodynamics solvers (e.g. adaptive mesh refinement). It achieves computational speed while maintaining a specified accuracy via controlled lowering of the resolution of both sources and rays from each source. TREVR computes the radiation field in order N log N_source time without absorption and order Nlog N_sourcelog N time with absorption. These scalings arise from merging sources of radiation according to an opening angle criterion and walking the tree structure to trace a ray to a depth that gives the chosen accuracy for absorption. The absorption-depth refinement criterion is unique to TREVR. We provide a suite of tests demonstrating the algorithm's ability to accurately compute fluxes, ionization fronts, and shadows.