Finding Good Coffee in Paris

Finding a good cup of coffee in Paris is difficult even among its world-renowned cafés, at least according to author David Downie (2011). We propose a solution that would allow tourists to create a map of the Paris Métro system from scratch that shows the locations of the cafés with the good coffee, while addressing the problem of the tourists losing interest in the process once they have found good coffee. We map the problem to the black hole search problem in the subway model introduced by Flocchini et al. at Fun with Algorithms 2010. We provide a solution that allows the tourists to start anywhere and at any time, communicate using whiteboards on the subway trains, rely on much less information than is normally available to subway passengers, and work independently but collectively to map the subway network. Our solution is the first to deal with scattered agents searching for black holes in a dynamic network and is optimal both in terms of the team size and the number of carrier moves required to complete the map.