fnv.h

#include <array>
#include <type_traits>

template <typename T>
constexpr auto array_from_object(const T& obj) {
    std::array<unsigned char, sizeof(T)> ret;
    for (std::size_t i = 0; i < sizeof(T); ++i) {
        ret[i] = reinterpret_cast<const unsigned char*>(&obj)[i];
    }
    return ret;
}

// iterator-aware hasher
template <
    typename it,
    typename = std::enable_if_t<
        std::is_pointer<decltype(&*std::declval<it>())>::value
        // &&
        // std::has_unique_object_representations<decltype(*std::declval<it>())>::value
        >>
constexpr uint32_t FNV32a(it begin, it end) {
    const uint32_t FNV_32_PRIME = 16777619;
    uint32_t hval = 2166136261;
    for (it p = begin; p != end; ++p) {
        // get *p as a reference to array of unsigned char
        auto repr = array_from_object(*p);
        for (auto ch : repr) {
            hval ^= std::uint32_t{ch};
            hval *= FNV_32_PRIME;
        }
    }
    return hval;
}