CityHash64

CityHash64 is a non-cryptographic hash function developed by Google for use in hashing strings. It is designed to be fast and efficient for general-purpose hashing, particularly in scenarios such as hash tables and data deduplication. The algorithm produces a 64-bit hash value from an input string of arbitrary length.

CityHash64 belongs to the CityHash family of hash functions, which are known for their speed on modern CPUs. It achieves its performance through a combination of techniques, including the use of SIMD instructions where applicable, and carefully chosen arithmetic operations. The function's design prioritizes speed over cryptographic security, meaning it is not suitable for applications where collision resistance against malicious attackers is paramount.

The core of CityHash64 involves processing the input string in chunks, mixing the bits using a series of multiplication and rotation operations. It maintains internal state that is updated as each chunk is processed. The final hash value is derived from this internal state after all input has been consumed. Variants of CityHash64 exist, such as CityHash64WithSeed, which allows for a seed value to be incorporated into the hashing process, useful for applications requiring different hash outputs for the same input.

In practice, CityHash64 is often used in systems where quick hashing of large amounts of data is required, and the potential for deliberate hash collisions is low. Its widespread adoption in various software projects attests to its effectiveness for its intended use cases.