CityHash

CityHash is a family of non-cryptographic hash functions developed by Google for fast hashing of strings and other data on modern processors. The primary variants are CityHash32, CityHash64, and CityHash128, producing 32-, 64-, and 128-bit hash values respectively. CityHash is designed for speed in non-security contexts such as hash tables, data structures, checksums, and data deduplication, rather than for cryptographic purposes.

Overview of design: CityHash targets high performance on both 32-bit and 64-bit architectures. It uses routines that adapt to input length, with short inputs hashed by simple mixing and longer inputs processed in larger blocks using a sequence of bit operations, rotations, and XORs to spread input bits. The implementation emphasizes branchless code and cache-friendly access patterns to maximize throughput on common workloads.

Algorithm characteristics: CityHash employs a collection of mixing functions with constants, using 64-bit and 128-bit arithmetic in different paths to produce well-distributed outputs. It is non-cryptographic, meaning it is not designed to resist deliberate collision or preimage attacks, and should not be used for security-sensitive hashing or cryptographic applications.

Variants and ecosystem: CityHash32 targets 32-bit inputs, CityHash64 is tuned for 64-bit platforms, and CityHash128 provides 128-bit results. Google released CityHash as open-source software, and FarmHash, a later family from Google, is intended to replace CityHash with broader portability and performance improvements.

Usage and licensing: CityHash code is released under a permissive open-source license accompanying Google projects. It is widely used in environments requiring fast in-memory hashing, including language runtimes, databases, and distributed systems, while avoiding cryptographic use cases.