csf is the Rust library (by Piotr Beling) of compressed static functions (maps) that use perfect hashing and value compression.

The compressed static functions contained in csf represent immutable maps from a set of (hashable) keys K into a set of values V. Since they do not explicitly store keys and compress values, their size usually slightly exceeds the entropy of the values alone. They can quickly (usually in O(1) time) return the value assigned to a given key k. However, they are not always able to detect that k is not in K, and may for such k return an arbitrary value from V.