foldhash

Kinogaki's own foldhash, published under the crates.io name so the frozen hashbrown resolves to it through [patch.crates-io] for its default hasher. It is not upstream foldhash: it is our own fast, seeded, non-cryptographic hash with the fast::RandomState hashbrown builds.

Each 64-bit word is mixed with a folded multiply (the 128-bit product of two words, high half xor low half), keyed by the builder's random seeds; the result's high bits, which hashbrown's control bytes use, depend on every input bit. Seeds are random per process and differ per builder, as upstream's, so map iteration order is not stable between runs.