key_type key(const array_type & df, const std::size_t index) const
    {
        if (df.shape().second == 0)
        {
            return {0, 0};
        }
        else
        {
            const int prod_id = df[df.row(index)][0]; // TODO, hardcoded
            const char segment = df[df.row(index)][8]; // TODO, hardcoded

            return {prod_id, segment};
        }
    }
std::vector<std::string>
ElectronicPartsClassification::classifyParts(
    std::vector<std::string> & i_training,
    std::vector<std::string> & i_testing) const
{

    const std::vector<std::string> raw_colnames{"PRODUCT_NUMBER", "CUSTOMER_NUMBER", "TRANSACTION_DATE",
        "PRODUCT_PRICE", "GROSS_SALES", "REGION", "WAREHOUSE", "CUSTOMER_ZIP", "CUSTOMER_SEGMENT1",
        "CUSTOMER_SEGMENT2", "CUSTOMER_TYPE1", "CUSTOMER_TYPE2", "CUSTOMER_MANAGED_LEVEL",
        "CUSTOMER_ACCOUNT_TYPE", "CUSTOMER_FIRST_ORDER_DATE", "PRODUCT_CLASS_ID1",
        "PRODUCT_CLASS_ID2", "PRODUCT_CLASS_ID3", "PRODUCT_CLASS_ID4","BRAND",
        "PRODUCT_ATTRIBUTE_X", "PRODUCT_SALES_UNIT", "SHIPPING_WEIGHT", "TOTAL_BOXES_SOLD",
        "PRODUCT_COST1", "PRODUCT_UNIT_OF_MEASURE", "ORDER_SOURCE", "PRICE_METHOD", "SPECIAL_PART"};


    const auto time0 = timestamp();

    const num::loadtxtCfg<real_type>::converters_type converters_train =
        {
            {colidx(raw_colnames, "TRANSACTION_DATE"), date_xlt},
            {colidx(raw_colnames, "CUSTOMER_SEGMENT1"), [](const char * str){return from_list_xlt({"A", "B"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_TYPE2"), [](const char * str){return from_list_xlt({"A", "B", "C"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_MANAGED_LEVEL"), [](const char * str){return from_list_xlt({"N", "L"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_ACCOUNT_TYPE"), [](const char * str){return from_list_xlt({"ST", "DM"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_FIRST_ORDER_DATE"), date_xlt},
            {colidx(raw_colnames, "BRAND"), [](const char * str){return from_list_xlt({"IN_HOUSE", "NOT_IN_HOUSE"}, str);}},
            {colidx(raw_colnames, "PRODUCT_SALES_UNIT"), [](const char * str){return from_list_xlt({"Y", "N"}, str);}},
            {colidx(raw_colnames, "PRODUCT_UNIT_OF_MEASURE"), [](const char * str){return from_list_xlt({"B", "LB", "EA"}, str);}},
            {colidx(raw_colnames, "ORDER_SOURCE"), [](const char * str){return from_list_xlt({"A", "B"}, str);}},
            {colidx(raw_colnames, "SPECIAL_PART"), [](const char * str){return from_list_xlt({"No", "Maybe", "Yes"}, str);}},
        };
    const num::loadtxtCfg<real_type>::converters_type converters_test =
        {
            {colidx(raw_colnames, "TRANSACTION_DATE"), date_xlt},
            {colidx(raw_colnames, "CUSTOMER_SEGMENT1"), [](const char * str){return from_list_xlt({"A", "B"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_TYPE2"), [](const char * str){return from_list_xlt({"A", "B", "C"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_MANAGED_LEVEL"), [](const char * str){return from_list_xlt({"N", "L"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_ACCOUNT_TYPE"), [](const char * str){return from_list_xlt({"ST", "DM"}, str);}},
            {colidx(raw_colnames, "CUSTOMER_FIRST_ORDER_DATE"), date_xlt},
            {colidx(raw_colnames, "BRAND"), [](const char * str){return from_list_xlt({"IN_HOUSE", "NOT_IN_HOUSE"}, str);}},
            {colidx(raw_colnames, "PRODUCT_SALES_UNIT"), [](const char * str){return from_list_xlt({"Y", "N"}, str);}},
            {colidx(raw_colnames, "PRODUCT_UNIT_OF_MEASURE"), [](const char * str){return from_list_xlt({"B", "LB", "EA"}, str);}},
            {colidx(raw_colnames, "ORDER_SOURCE"), [](const char * str){return from_list_xlt({"A", "B"}, str);}},
        };

    ////////////////////////////////////////////////////////////////////////////

    const array_type i_train_data =
        num::loadtxt(
            std::move(i_training),
            std::move(
                num::loadtxtCfg<real_type>()
                .delimiter(',')
                .converters(num::loadtxtCfg<real_type>::converters_type{converters_train})
            )
        );

    const array_type i_test_data =
        num::loadtxt(
            std::move(i_testing),
            std::move(
                num::loadtxtCfg<real_type>()
                .delimiter(',')
                .converters(num::loadtxtCfg<real_type>::converters_type{converters_test})
            )
        );

    std::vector<std::string> colnames;
    array_type train_data;
    array_type test_data;

    std::tie(colnames, train_data, test_data) = gen_features(raw_colnames, i_train_data, i_test_data);

//    std::cerr << train_data.shape() << test_data.shape() << std::endl;
//    std::copy(colnames.cbegin(), colnames.cend(), std::ostream_iterator<std::string>(std::cerr, "\n"));

    assert(train_data.shape().second == test_data.shape().second + 1);

    const array_type::varray_type train_y_valarr = train_data[train_data.column(colidx(colnames, "SPECIAL_PART"))];
    const std::vector<float> train_y(std::begin(train_y_valarr), std::end(train_y_valarr));

    std::cerr << "train_y size: " << train_y.size() << std::endl;

    train_data = num::del_column(train_data, colidx(colnames, "SPECIAL_PART"));
    colnames.erase(std::find(colnames.begin(), colnames.end(), "SPECIAL_PART"));
    assert(colnames.size() == train_data.shape().second);

    std::cerr << "train_data shape: " << train_data.shape() << std::endl;
    std::cerr << "test_data shape: " << test_data.shape() << std::endl;


    const std::map<const std::string, const std::string> * PARAMS_SET__no[] = {&params::no::prov47};
    std::vector<float> train_y__no;
    std::transform(train_y.cbegin(), train_y.cend(), std::back_inserter(train_y__no),
        [](const float what)
        {
            // quantize train y vector into {0,1}
            return what >= 0.5 ? 1. : 0.;
        }
    );

    const auto y_hat_no = run_binary_estimators(
        std::begin(PARAMS_SET__no), std::end(PARAMS_SET__no),
        time0, train_data, train_y__no, test_data);


    ////////////////////////////////////////////////////////////////////////////

    std::vector<std::size_t> y_hat(y_hat_no);

    const std::string yes_no_maybe[] = {"No", "Maybe", "Yes"};
    std::map<int, std::pair<std::string, std::string>> responses;
    GroupBy gb_test(i_test_data);
    int ix{0};
    for (auto group = gb_test.yield(); group.size() != 0; group = gb_test.yield())
    {
        const std::valarray<real_type> row = i_test_data[i_test_data.row(group.front())];
        const int prod_id = row[colidx(raw_colnames, "PRODUCT_NUMBER")];
        const char segment = row[colidx(raw_colnames, "CUSTOMER_SEGMENT1")];
        const auto response = y_hat[ix++];

        if (responses.count(prod_id))
        {
            if (segment == 0)
            {
                responses[prod_id].first = yes_no_maybe[response];
            }
            else
            {
                responses[prod_id].second = yes_no_maybe[response];
            }
        }
        else
        {
            if (segment == 0)
            {
                responses[prod_id] = {yes_no_maybe[response], "NA"};
            }
            else
            {
                responses[prod_id] = {"NA", yes_no_maybe[response]};
            }
        }
    }

    std::vector<std::string> str_y_hat;
    std::transform(responses.cbegin(), responses.cend(), std::back_inserter(str_y_hat),
        [](const std::pair<int, std::pair<std::string, std::string>> & kv)
        {
            return std::to_string(kv.first) + ',' + kv.second.first + ',' + kv.second.second;
        }
    );

    return str_y_hat;
}