#include <algorithm>
#include <experimental/filesystem>
#include <fstream>
#include <iostream>
#include <optional>
#include <string>
#include <string_view>
#include <sstream>
#include <utility>
#include <vector>

using namespace std;
namespace fs = std::experimental::filesystem;

[[nodiscard]] auto split_string(const string_view& input, char delimiter)
{
    vector<string_view> result;

    for (size_t start = 0, i = 0; i < input.size(); ++i) {
        if (input[i] == delimiter) {
            result.push_back({input.data() + start, i - start});
            start = i + 1;
        }
    }

    return result;
}

[[nodiscard]] optional<int> get_target_column(ifstream& input,
                                              const string_view& label,
                                              const char delimiter)
{
    string first_line;

    if (getline(input, first_line); // Init-statement for if/switch
        first_line.size() == 0) { throw runtime_error("Input file missing"); }
    input.seekg(0);

    auto tokens = split_string(first_line, delimiter);

    if (auto it = find(begin(tokens), end(tokens), label); // Init-statement for if/switch
        it == tokens.end()) {
            return {}; //return empty optional
    }
    else {
        return distance(begin(tokens), it);
    }
}

[[nodiscard]] auto get_file_handlers(const string_view& input, const string_view& output)
{
    ifstream in_file {input.data(), ios::in};
    if (!in_file.is_open()) { throw runtime_error("Unable to open input file"); }

    ofstream out_file {output.data(), ios::out | ios::trunc};
    if (!out_file.is_open()) { throw runtime_error("Unable to open output file");}

    return pair(move(in_file), move(out_file)); //Template argument deduction for class templates (no make_pair)
}

void do_work(ifstream& input,
             ofstream& output,
             int target_index,
             const string_view& new_value,
             const char delimiter)
{
    string buffer;

    getline(input, buffer); // for the header line
    output << buffer << endl;

    while (getline(input, buffer)) {
        auto tokens = split_string(buffer, delimiter);
        tokens[target_index] = new_value;

        bool first = true;
        for (const auto &token : tokens) {
            if (!first) {
                output << delimiter;
            }
            output << token;
            first = false;
        }
        output << "\n";
    }
}

[[noreturn]] void usage_terminate(const string_view& progname) noexcept
{
    cout << "Usage: " << progname << " [IN_FILE] [COLUMN] [NEW_VALUE] [OUT_FILE]" << endl;
    abort();
}

int main(int argc, char* argv[])
{
    try
    {
        if (argc != 5) { throw runtime_error("Bad arguments"); }

        auto [in_file, out_file] = get_file_handlers(argv[1], argv[4]);

        string_view new_value = argv[3];
        auto target_index = get_target_column(in_file, argv[2], ',');
        if (target_index) {
            do_work(in_file, out_file, *target_index, new_value, ',');
        }
        else {
            throw runtime_error("Column name doesn’t exist in the input file");
        }
    }
    catch (runtime_error& e)
    {
        cout << e.what() << endl;
        usage_terminate(argv[0]);
    }

    return 0;
}

//clang++ -std=c++17 main.cc -O3