OpenFisca is an open source software project that aims to simulate the French tax and social benefit system. It covers both taxes and benefits, allows for efficient simulations of individuals and families, and aims to be understandable. OpenFisca uses a directed acyclic graph and vectorized computations to efficiently simulate tax outcomes. It is used to improve public understanding of the tax system, design reforms, simulate personal situations, and understand the overall social system. During a hackathon, the developers worked on efficiently traversing and computing the graph, visualizing results, simplifying the graph for common situations, and transpilng the system to other languages like JavaScript.