# `UltraLogLog.Hash`
[🔗](https://github.com/thatsme/ultra_log_log/blob/v0.1.0/lib/ultra_log_log/hash.ex#L1)

64-bit hash function for UltraLogLog.

v0.1 ships with `:erlang.phash2/2` upgraded to 64 bits via concatenation.
This is **fine for correctness testing but not statistically ideal** for
production sketches at high precision — `phash2` is not a high-quality
hash for cardinality estimation.

### v0.2 plan

  - Pure-Elixir xxhash3 (slow but no NIF) as fallback.
  - Optional Rustler NIF for xxhash3 / komihash5 (matches Hash4j reference).
  - Pluggable behaviour so callers can pass pre-computed hashes from
    whatever they already use.

## Why 64 bits

UltraLogLog needs 64 bits because:
  - top `p` bits select the register (up to p=26)
  - remaining bits contribute to the geometric leading-zero variable
  - at exa-scale (~10^18 distinct items) 64 bits are exactly what's needed
    to keep collision probability negligible (Heule++ argument)

# `hash64`

```elixir
@spec hash64(term()) :: non_neg_integer()
```

Hash a term to a 64-bit unsigned integer.

Placeholder using two phash2 calls with different seeds. Replace with a
real 64-bit hash before publishing v0.1 final.

---

*Consult [api-reference.md](api-reference.md) for complete listing*