Non-cryptographic

FNV-1a

The Fowler-Noll-Vo hash family (1991) is a tiny multiply-XOR mixer that fits in a few lines of code. FNV-1a, the “XOR-then-multiply” variant, distributes input bits noticeably better than the original FNV-1 and is the de facto choice for the family today.

The whole algorithm, in one fragment

hash = FNV_OFFSET_BASIS
for each byte b in input:
    hash = hash XOR b
    hash = hash * FNV_PRIME    # modulo 2^n, where n = 32 or 64

At a glance

Output	32 or 64 bits (1024-bit variants exist but rarely used)
Internal state	1 integer of output size
Throughput	~1–3 GiB/s (single-byte loops; can be SIMD-vectorized)
Year	1991 (original); FNV-1a variant ~1996
Status	Non-cryptographic; trivially collidable but fine for hash tables

Constants

Variant	Offset basis	Prime
FNV-1a 32	0x811c9dc5	0x01000193
FNV-1a 64	0xcbf29ce484222325	0x00000100000001b3

Where it shows up

Language internals , Python (CPython’s dict pre-3.4), Rust’s FxHash derivative.
Embedded firmware , sub-200-byte assembly hash for hash tables / Bloom filters.
Stub identifiers , quick uniqueness keys in build tools, code generators.
Bloom-filter back-ends , FNV is the “simplest correct choice” default in many libraries.

Limitations

Avalanche is decent but not great; deliberate collisions are easy to construct.
Pure byte-loop; harder to SIMD-vectorize than xxHash3.
No seed (unless you treat the offset basis as one), so vulnerable to hash flooding when used as a key for hash tables.

For hot paths with adversarial inputs, prefer SipHash. For sheer throughput, prefer xxHash3.

References

Quick quiz

Test yourself on fnv-1a

10 multiple-choice questions. Pick an answer for each, then submit to see explanations.

Q1.What does FNV stand for?
Q2.What year was FNV first published?
Q3.FNV-1a differs from FNV-1 by:
Q4.What is the FNV-1a 32-bit prime?
Q5.FNV-1a's internal state size:
Q6.Is FNV-1a a cryptographic hash?
Q7.Throughput of FNV-1a (byte-loop) on modern CPUs?
Q8.Where is FNV-1a still common?
Q9.Is FNV-1a seeded by default?
Q10.FNV-1a vs xxHash3, which has better SMHasher scores?