BBHash Limasset et al. (2017) uses multiple layers to create a minimal perfect hashing functions (MPHF), that hashes some input set into \([n]\). Simply said, it maps the \(n\) elements into \([\gamma \cdot n]\) using hashing function \(h_0\). I have no idea how the analysis works out though – maybe the recursive strategy works better on a larger scale and when hashing only so few items there isn’t much benefit to the BBhash approach in the first place.