It's really quite simple. Perfect hashes exists and are hashes. Perfect hashes do not, as a matter of definition, compress. They typically aren't reversible, be

It's really quite simple. Perfect hashes exists and are hashes. Perfect hashes do not, as a matter of definition, compress. They typically aren't reversible, because it's not an useful feature for a hash, but they could be - and if nothing else, they can always be deterministically brute forced (as they have no collisions), which is a (very bad) form of reversibility. It's not meaningful to study hashes as compression, since compression is by definition reversible.