The Jenkins hash functions are a collection of (non-cryptographic) hash functions for multi-byte keys designed by Bob Jenkins. They can be used also as checksums to detect accidental data corruption or detect identical records in a database. The first one was formally published in 1997.
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Jenkins's one-at-a-time hash is adapted here from a WWW page by Bob Jenkins,[1] which is an expanded version of his Dr. Dobbs article.[2]
uint32_t jenkins_one_at_a_time_hash(char *key, size_t len) { uint32_t hash, i; for(hash = i = 0; i < len; ++i) { hash += key[i]; hash += (hash << 10); hash ^= (hash >> 6); } hash += (hash << 3); hash ^= (hash >> 11); hash += (hash << 15); return hash; }
The avalanche behavior of this hash is shown on the right.
Each of the 24 rows corresponds to a single bit in the 3-byte input key, and each of the 32 columns corresponds to a bit in the output hash. Colors are chosen by how well the input key bit affects the given output hash bit: a green square indicates good mixing behavior, a yellow square weak mixing behavior, and red would indicate no mixing. Only a few bits in the last byte of the input key are weakly mixed to a minority of bits in the output hash.
The lookup2 function was an interim successor to one-at-a-time.
The lookup3 function consumes input in 12 byte (96 bit) chunks.[3] It may be appropriate when speed is more important than simplicity. Note, though, that any speed improvement from the use of this hash is only likely to be useful for large keys, and that the increased complexity may also have speed consequences such as preventing an optimizing compiler from inlining the hash function.