Bit-exact port of fastText's string hash function.

fastText uses a Fowler–Noll–Vo (FNV-1a) variant with one quirk: each input byte is reinterpreted as a signed 8-bit integer before being widened to unsigned 32-bit. Bytes with the high bit set therefore contribute their sign-extended value to the hash mix step, not their unsigned value. This is documented in src/dictionary.cc (Dictionary::hash) of the fastText source as a deliberate compatibility decision so that all already-released models hash identically.

Translating the C++ literally:

uint32_t h = 2166136261;
for (size_t i = 0; i < str.size(); i++) {
  h = h ^ uint32_t(int8_t(str[i]));
  h = h * 16777619;
}

The two constants are the canonical FNV offset basis and FNV prime for 32-bit FNV-1a.

Any deviation from the reference here will silently produce wrong subword indices and wreck the model's predictions for non-ASCII scripts. The hash is exercised by golden tests against fastText's own get_subwords/1 output for a large corpus of words.