244.md

Info

• Did you know about compiler predefined macros assosicated with the compilation date/time?

  • https://gcc.gnu.org/onlinedocs/cpp/Standard-Predefined-Macros.html

Example

static_assert(std::string_view{"07:36::22"} == std::string_view{__TIME__}); // will only pass at certin time
static_assert(std::string_view{"Sep 19 2021"} == std::string_view{__DATE__}); // will only pass at certin date

https://godbolt.org/z/KWvxdhzd8

Puzzle

• Can you implement consteval function year which will use DATE macro to return current year as integer?

[[nodiscard]] consteval auto year() -> int; // TODO
static_assert(2021 == year());

https://godbolt.org/z/ohqdon8zW

Solutions

constexpr auto from_chars(const char* first, const char* last, int& value) {
  value = 0;
  while (first != last) {
    value *= 10;
    value += *first++ - '0';
  }
}

[[nodiscard]] consteval auto year() -> int {
  constexpr auto today = __DATE__;
  int year{};
  from_chars(&today[7], &today[11], year);
  return year;
}

https://godbolt.org/z/zPv7Kbcj6

template<auto N>
[[nodiscard]] consteval auto stoi(std::string_view str) {
  return [&str]<auto... Ns>(std::index_sequence<Ns...>) {
    return (((str[Ns] - '0') * std::pow(10, N - Ns - 1)) + ...);
  }(std::make_index_sequence<N>{});
}

[[nodiscard]] consteval auto year() -> int {
  return stoi<4>(std::string_view{__DATE__}.substr(7));
}

https://godbolt.org/z/jP3cs3vbz

struct integer
{
    static constexpr auto rule = lexy::dsl::integer<int>(lexy::dsl::digits<>);
    static constexpr auto value = lexy::forward<int>;
};


[[nodiscard]] consteval auto year() -> int {
    constexpr auto date = std::string_view{__DATE__};

    constexpr auto input = lexy::string_input(date.substr(7));
    constexpr auto result = lexy::parse<integer>(input, lexy::noop);
    static_assert(result);

    return result.value();
}

https://godbolt.org/z/G4MradvsM

constexpr int char_to_digit(char x){
  return x-'0';
};

consteval auto year_string() {
    const auto date = std::string_view{__DATE__};
    return date.substr(date.size() - 4);
};

constexpr int stringview_to_int(auto str, int index, int accum){
  return index == 4
    ? accum
    : stringview_to_int(str, index+1, accum*10 + char_to_digit(str[index]));
}

consteval auto year() {
  return stringview_to_int(year_string(), 0, 0 );
};

https://godbolt.org/z/Mjs5ss9ro

constexpr int
ipow10(int b){ return b ? (10*ipow10(b-1)) : 1; }

constexpr int char_to_digit(char x){ return x-'0'; };

consteval auto year_string() {
    const auto date = std::string_view{__DATE__};
    return date.substr(date.size() - 4);
};

template<auto ... indices>
constexpr int
stringview_to_int(std::index_sequence< indices ...>, auto str){
  return ((char_to_digit(str[indices])*ipow10(3-indices)) + ...);
}

consteval auto year() {
  return stringview_to_int(std::make_index_sequence<4>(), year_string());
};

https://godbolt.org/z/G64ndYbee

[[nodiscard]] consteval auto year() -> int {
    int yr = 0;
    for(int i=7; i<11; i++)
        yr = 10*yr + __DATE__[i] - '0';
    return yr;
}

http://godbolt.org/z/j3zc8bz79

[[nodiscard]] consteval auto year() -> int
{
    auto dateStr = std::string_view{__DATE__};
    int y = 0;
    for ( int i = dateStr.size() - 1, j = 1; j <= 1000 ; i--, j*=10)
        y += int(dateStr[i] - '0') * j ;
    return y;
}

https://godbolt.org/z/8oEoTcnjq

template<class T>
consteval auto strparse( T arg) -> int
{
    return (arg[0]- '0') *1000 + (arg[1] - '0') * 100 + (arg[2] - '0') * 10 + (arg[3] - '0');
}

[[nodiscard]] consteval auto year() -> int
{
    constexpr auto date = std::string_view(__DATE__);
    constexpr auto year = std::string_view(date.data() + 7);
    return strparse(year);

}

https://godbolt.org/z/ajWKa5dcG

[[nodiscard]] consteval auto year() -> int {
    return [] (auto acc, auto size, auto... is) {
        return (..., (acc *= 10, acc += __DATE__[size - is] - '0'));
    }(0, sizeof __DATE__, 5, 4, 3, 2);
}

https://godbolt.org/z/eWWvdd74n