GRHayL: an open-source, modular, extensible library for GRMHD simulations

Most current general relativistic magnetohydrodynamic codes exist as monolithic blocks of code with limited code separation and modularity. This makes improvements and extensions to these codes difficult and lengthens the new user/contributor pipeline. These monoliths also work against the native benefits of open-source programming, as research groups have far less capability to use and interoperate with other groups' work. These problems are exacerbated by the underlying differences of different code infrastructures, which further limit the ability to share code between groups. Our new open-source General Relativistic Hydrodynamic Library (GRHayL) aims to resolve this by providing a modular, extensible, infrastructure-agnostic library of functions with standardized interfaces. This code is the successor to the IllinoisGRMHD code, inheriting and extending that code while compartmentalizing it into self-contained modules ("gems"). I discuss the motivations, design philosophies, and general structure of the GRHayL library. I also describe the methodology of unit and integration testing used for continuous code validation.