Hash function
Deterministic function mapping arbitrary input to a fixed-length output. Two main categories: fast hashing (for data structures) and cryptographic (for security).
Fast hashing: FNV, MurmurHash, xxHash, CityHash. Used in hash tables. Cryptographic: SHA-256 (Bitcoin), SHA-3, BLAKE3 (modern choice), Argon2 (passwords). Properties you want: avalanche effect, collision resistance, preimage resistance. MD5 + SHA-1 are formally broken (2017+). Universal hashing lets you pick a function from a family ⇒ protects against adversarial input.