#include <ctime>
#include <iostream>
#include <string>

using namespace std::literals;

template <typename T>
T read(const std::string& request)
{
    auto except = std::cin.exceptions();
    try {
        std::cin.exceptions(std::ios::eofbit | std::ios::badbit);
        std::cout << request;
        T value;
        bool repeat = true;
        while (repeat) {
            repeat = false;
            std::cin >> value;
            if(!std::cin) {
                std::cout << "Invalid input\n";
                std::cin.clear();
                repeat = true;
                while(!isspace(std::cin.get()) && std::cin)
                    ;
            }
        }
        std::cin.exceptions(except);
        return value;
    } catch(...) {
        std::cin.exceptions(except);
        throw;
    }
}

bool ask(const std::string& question)
{
    std::cout << question << '\n';
    char c;
    do {
        c = static_cast<char>(tolower(read<char>("")));
        if(c != 'y' && c != 'n')
            std::cout << "Invalid answer. Only Y, y, N and n are allowed\n";
    } while(c != 'y' && c != 'n');
    return c == 'y';
}

namespace date
{
//                            ↓ January                           December ↓
constexpr int month_days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
constexpr int year_days = 365;

struct date_diff
{
    int year;
    int mon;
    int day;
};

date_diff operator-(const std::tm& lhs, const std::tm& rhs)
{
    date_diff result {lhs.tm_year - rhs.tm_year,
                      lhs.tm_mon  - rhs.tm_mon ,
                      lhs.tm_mday - rhs.tm_mday };
    if(result.day < 0) {
        result.day += month_days[rhs.tm_mon];
        --result.mon;
    }
    if(result.mon < 0) {
        result.mon += 12;
        --result.year;
    }
    return result;
}

std::tm get_current_date()
{
    std::time_t now = std::time(nullptr);
    return *std::localtime(&now);
}

std::tm get_date()
{
    std::tm result {0, 0, 0 ,0, 0, 0, 0, 0, 0};
    result.tm_year = read<int>("Enter year: " ) - 1900; //Epoch start
    result.tm_mon  = read<int>("Enter month: ") - 1; //Internal representation fix
    result.tm_mday = read<int>("Enter day: "  );
    std::mktime(&result);
    return result;
}

std::string date_to_string(const std::tm& date)
{
    constexpr std::size_t buff_size = 128;
    char buff[buff_size];
    if(strftime(buff, buff_size - 1, "%d/%m/%Y", &date) == 0)
        return "";
    return buff;
}

void agecalculator()
{
    std::tm now = get_current_date();
    std::cout << "Your current year is: " << date_to_string(now) << "\n\n";

    std::cout << "What date were you born?\n";
    std::tm birthday = get_date();
    std::cout << "The date you were born is: " << date_to_string(birthday) << '\n';

    auto diff = now - birthday;
    birthday.tm_year = now.tm_year + 1;
    auto bdiff = birthday - now;

    std::cout << "You are " <<
        (diff.year != 0 ? std::to_string(diff.year) + " years, " : ""s) <<
        (diff.mon  != 0 ? std::to_string(diff.mon ) + " month "  : ""s) << ((diff.year || diff.mon) && diff.day ? "and " : "") <<
        (diff.day  != 0 ? std::to_string(diff.day ) + " days "   : ""s) << "old\n\n";
    std::cout << "There is " <<
        (bdiff.mon != 0 ? std::to_string(bdiff.mon) + " month " : ""s) <<  (bdiff.mon && bdiff.day ? "and " : ""s) <<
        (bdiff.day != 0 ? std::to_string(bdiff.day) + " days "  : ""s) <<
        "before your next birthday where you will be " << diff.year + 1 << " years old\n";
}

void currentyearcalculator()
{
    int birthyear  = read<int>(  "What year were you born?\n");
    int currentage = read<int>("\nWhat is your current age?\n");
    bool birthday  = ask("\nHave you had your birthday this year? Y/N\n");
    int year = currentage + birthyear + (birthday ? 0 : 1);
    std::cout << "\n\nThe current year is " << year << '\n';
}

void birthdatecalculator()
{
    std::tm now = get_current_date();
    int myageyear  = read<int>("What is your current age in years?\n");
    int myagemonth = read<int>("\nHow many months into your age?\n");
    int myageday   = read<int>("\nHow many days into your age, don't write months?\n");

    int resultday   = now.tm_mday - myageday;
    int resultmonth = now.tm_mon + 1 - myagemonth;
    int resultyear  = now.tm_year + 1900 - myageyear;
    if (resultday < 0) {
        resultmonth = resultmonth + 1;
        resultday = resultday + 30;
    }
    if (resultmonth < 0) {
        resultyear = resultyear + 1;
        resultmonth = resultmonth + 12;
    }
    std::cout << "This is the date you were born aprox: " << resultyear << '/' << resultmonth << '/' << resultday << '\n';
}

}

void menue()
{
    int menu = read<int>("1 for agecalculator. \n2 for current year calculator. \n3 for birthdat calculator.\nEnter another number to exit.\n\nEnter your choice here: ");
    if (menu == 1)
        date::agecalculator();
    if (menu == 2)
        date::currentyearcalculator();
    if (menu == 3)
        date::birthdatecalculator();
}

int main()
{
    menue();
}