FNV-1a hashing a wide-string with multiple of 4 length in chunks of 64-bits

hash_64bitwise.hpp

constexpr auto hash_64bitwise(std::wstring_view str) noexcept
{
  using SizeType = decltype(str)::size_type;
  constexpr auto offset_basis = 0xcbf29ce484222325ull;
  constexpr auto prime = 0x00000100000001B3ull;

  auto value = offset_basis;
  auto size = str.length();
  if (size % 4)
    return 0ull;

  for (SizeType i{}; i < size / 4; ++i) {
    std::uint64_t chunk = 0;
    if (std::is_constant_evaluated()) {
      chunk = []<std::size_t... Is>(wchar_t const *ptr, std::index_sequence<Is...>) constexpr {
        return ((static_cast<std::uint64_t>(ptr[Is]) << (16u * Is)) | ...);
      }(&str[(i * 4)], std::make_index_sequence<4>{});
    } else {
      std::memcpy(&chunk, &str[i * 4], 8);
    }
    value = (value ^ chunk) * prime;
  }

  return value;
}