hash_word.h

namespace detail {
    constexpr uint64_t sHashLo1 = 0x124b34c732f4a485ull;
    constexpr uint64_t sHashLo2 = 0x6fafc7de1f728990ull;
    constexpr uint64_t sHashLo3 = 0x9299b5768ee5e36dull;
    constexpr uint64_t sHashHi1 = 0x4d8271fe7646891full;
    constexpr uint64_t sHashHi2 = 0x6d48b8a30939437cull;
    constexpr uint64_t sHashHi3 = 0x122f44ec3b08f41cull;
}

uint64_t hash_word(uint64_t hash, uint64_t w)
{
    uint64_t whi = w >> 32;
    uint64_t wlo = w & 0xFFFFFFFF;

    uint64_t hashlo = (detail::sHashLo1 * whi + detail::sHashLo2 * wlo + detail::sHashLo3 * hash) >> 32;
    uint64_t hashhi = (detail::sHashHi1 * whi + detail::sHashHi2 * wlo + detail::sHashHi3 * hash) >> 32;
    return hashlo | (hashhi << 32);
}