This is a smart implementation of Robin Hood hashing I am not aware of. In my understanding, a standard implementation keeps the probe length of each entry. This one avoids that due to its extra constraints. I don't quite understand the following strategy, though

> To meet property (3) [if the key 0 is present, its value is not 0] ... "array index plus one" can be stored rather than "array index".

If hash code can take any value in [0,2^32), how to define a special value for empty buckets? The more common solution is to have a special key, not a special hash code, for empty slots, which is easier to achieve. In addition, as the author points out, supporting generic keys requires to store 32-bit hash values. With the extra 4 bytes per bucket, it is not clear if this implementation is better than plain linear probing (my favorite). The fastest hash table implementations like boost and abseil don't often use Robin Hood hashing.