Function lut_dists_4b

cpp/src/quantize/multi_codebook.hpp:351–383

// luts must be of size [16][16]
template<int NBytes, class dist_t>
inline void lut_dists_4b(const uint8_t* codes, const dist_t* luts,
    dist_t* dists_out, int64_t N)
{
    static_assert(NBytes > 0, "Code length <= 0 is not valid");

    for (int64_t i = 0; i < N; i++) {
        dists_out[i] = 0;
        auto lut_ptr = luts;
        for (uint8_t j = 0; j < NBytes; j++) {
            uint8_t code_high = static_cast<uint8_t>(codes[j] >> 4);
            uint8_t code_low = static_cast<uint8_t>(codes[j] & mask_low4b);

            // // TODO rm after debug
            // dists_out[i] += popcount(code_left ^ j);
            // dists_out[i] += popcount(code_right ^ j);

            auto lut_low = lut_ptr;
            auto lut_high = lut_low + 16;
            dists_out[i] += lut_low[code_low];
            dists_out[i] += lut_high[code_high];
            // dists_out[i] += 42;
            // auto dist = lut_low[code_low] + lut_high[code_high];
            // std::cout << "---- " << (int)j << "\n";
            // std::cout << "code: " << (int)codes[j] << "\n";
            // std::cout << "code_low: " << (int)code_low << "\n";
            // std::cout << "code_high: " << (int)code_high << "\n";
            // std::cout << "dist: " << (int)dist << "\n";
            // std::cout <i< "dist: " << (int)dist << "\n";
            lut_ptr += 32;
        }
        codes += NBytes;
    }
}

// non-vectorized NBytes-byte codes, where each byte is one codeword
template<int NBytes, class dist_t>