The Jenkins hash functions are a collection of (non-cryptographic

Cryptographic hash function

A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the hash value, such that an accidental or intentional change to the data will change the hash value...

) hash functions for multi-byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

 keys designed by Bob Jenkins

Robert John Jenkins Junior

Robert John Jenkins Junior, also known as Bob Jenkins, is a computer professional and author of several fast pseudorandom number generators such as ISAAC and hash functions Bob Jenkins was born in Akron, Ohio, in 1966.-References:...

. They can be used also as checksum

Checksum

A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...

s to detect accidental data corruption or detect identical records in a database

Database

A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

. The first one was formally published in 1997.

one-at-a-time

Jenkins's one-at-a-time hash is adapted here from a WWW page by Bob Jenkins, which is an expanded version of his Dr. Dobbs article.


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

Avalanche effect

In cryptography, the avalanche effect refers to a desirable property of cryptographic algorithms, typically block ciphers and cryptographic hash functions. The avalanche effect is evident if, when an input is changed slightly the output changes significantly...

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.

lookup3

The lookup3 function consumes input in 12 byte (96 bit) chunks. 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.