constexpr_wrapper.h

/* SPDX-License-Identifier: LGPL-3.0-or-later */
/* Copyright © 2023-2024 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH
 *                       Matthias Kretz <m.kretz@gsi.de>
 */

#ifndef VIR_CONSTEXPR_WRAPPER_H_
#define VIR_CONSTEXPR_WRAPPER_H_

#if defined __cpp_concepts && __cpp_concepts >= 201907 && __has_include(<concepts>)
#define VIR_HAVE_CONSTEXPR_WRAPPER 1
#include <algorithm>
#include <array>
#include <concepts>
#include <limits>
#include <type_traits>
#include <utility>

namespace vir
{
  /**
   * Implements P2781 with modifications:
   *
   * - Overload operator-> to return value.operator->() if well-formed, otherwise act like a pointer
   *   dereference to underlying value (allows calling functions on wrapped type).
   *
   * - Overload operator() to act like integral_constant::operator(), unless value() is
   *   well-formed.
   */
  template <auto Xp, typename = std::remove_cvref_t<decltype(Xp)>>
    struct constexpr_wrapper;

  // *** constexpr_value<T, U = void> ***
  // If U is given, T must be convertible to U (`constexpr_value<int> auto` is analogous to `int`).
  // If U is void (default), the type of the value is unconstrained.
  template <typename Tp, typename Up = void>
    concept constexpr_value = (std::same_as<Up, void> or std::convertible_to<Tp, Up>)
#if defined __GNUC__ and __GNUC__ < 13
                                and not std::is_member_pointer_v<decltype(&Tp::value)>
#endif
                                and requires { typename constexpr_wrapper<Tp::value>; };

  namespace detail
  {
    // exposition-only
    // Note: `not _any_constexpr_wrapper` is not equivalent to `not derived_from<T,
    // constexpr_wrapper<T::value>>` because of SFINAE (i.e. SFINAE would be reversed)
    template <typename Tp>
      concept _any_constexpr_wrapper = std::derived_from<Tp, constexpr_wrapper<Tp::value>>;

    // exposition-only
    // Concept that enables declaring a single binary operator overload per operation:
    // 1. LHS is derived from This, then RHS is either also derived from This (and there's only a
    //    single candidate) or RHS is not a constexpr_wrapper (LHS provides the only operator
    //    candidate).
    // 2. LHS is not a constexpr_wrapper, then RHS is derived from This (RHS provides the only
    //    operator candidate).
    template <typename Tp, typename This>
      concept _lhs_constexpr_wrapper
        = constexpr_value<Tp> and (std::derived_from<Tp, This> or not _any_constexpr_wrapper<Tp>);
  }

  // Prefer to use `constexpr_value<type> auto` instead of `template <typename T> void
  // f(constexpr_wrapper<T, type> ...` to constrain the type of the constant.
  template <auto Xp, typename Tp>
    struct constexpr_wrapper
    {
      using value_type = Tp;

      using type = constexpr_wrapper;

      static constexpr value_type value{ Xp };

      constexpr
      operator value_type() const
      { return Xp; }

      // overloads the following:
      //
      // unary:
      // + - ~ ! & * ++ -- ->
      //
      // binary:
      // + - * / % & | ^ && || , << >> == != < <= > >= ->* = += -= *= /= %= &= |= ^= <<= >>=
      //
      // [] and () are overloaded for constexpr_value or not constexpr_value, the latter not
      // wrapping the result in constexpr_wrapper

      // The overload of -> is inconsistent because it cannot wrap its return value in a
      // constexpr_wrapper. The compiler/standard requires a pointer type. However, -> can work if
      // it unwraps. The utility is questionable though, which it why may be interesting to
      // "dereferencing" the wrapper to its value if Tp doesn't implement operator->.
      constexpr const auto*
      operator->() const
      {
        if constexpr (requires{value.operator->();})
          return value.operator->();
        else
          return std::addressof(value);
      }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<+Yp>
        operator+() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<-Yp>
        operator-() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<~Yp>
        operator~() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<!Yp>
        operator!() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<&Yp>
        operator&() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<*Yp>
        operator*() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<++Yp>
        operator++() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<Yp++>
        operator++(int) const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<--Yp>
        operator--() const
        { return {}; }

      template <auto Yp = Xp>
        constexpr constexpr_wrapper<Yp-->
        operator--(int) const
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value + Bp::value>
        operator+(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value - Bp::value>
        operator-(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value * Bp::value>
        operator*(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value / Bp::value>
        operator/(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value % Bp::value>
        operator%(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value & Bp::value>
        operator&(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value | Bp::value>
        operator|(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value ^ Bp::value>
        operator^(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value && Bp::value>
        operator&&(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<Ap::value || Bp::value>
        operator||(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value , Bp::value)>
        operator,(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value << Bp::value)>
        operator<<(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value >> Bp::value)>
        operator>>(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value == Bp::value)>
        operator==(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value != Bp::value)>
        operator!=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value < Bp::value)>
        operator<(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value <= Bp::value)>
        operator<=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value > Bp::value)>
        operator>(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value >= Bp::value)>
        operator>=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value <=> Bp::value)>
        operator<=>(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value ->* Bp::value)>
        operator->*(Ap, Bp)
        { return {}; }

      template <constexpr_value Ap>
        requires requires { typename constexpr_wrapper<(Xp = Ap::value)>; }
        constexpr auto
        operator=(Ap) const
        { return constexpr_wrapper<(Xp = Ap::value)>{}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value += Bp::value)>
        operator+=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value -= Bp::value)>
        operator-=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value *= Bp::value)>
        operator*=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value /= Bp::value)>
        operator/=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value %= Bp::value)>
        operator%=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value &= Bp::value)>
        operator&=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value |= Bp::value)>
        operator|=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value ^= Bp::value)>
        operator^=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value <<= Bp::value)>
        operator<<=(Ap, Bp)
        { return {}; }

      template <detail::_lhs_constexpr_wrapper<constexpr_wrapper> Ap, constexpr_value Bp>
        friend constexpr constexpr_wrapper<(Ap::value >>= Bp::value)>
        operator>>=(Ap, Bp)
        { return {}; }

#if defined __cpp_static_call_operator && __cplusplus >= 202302L
#define _static static
#define _const
#else
#define _static
#define _const const
#endif

      // overload operator() for constexpr_value and non-constexpr_value
      template <constexpr_value... Args>
        requires requires { typename constexpr_wrapper<value(Args::value...)>; }
        _static constexpr auto
        operator()(Args...) _const
        { return constexpr_wrapper<value(Args::value...)>{}; }

      template <typename... Args>
        requires (not constexpr_value<std::remove_cvref_t<Args>> || ...)
        _static constexpr decltype(value(std::declval<Args>()...))
        operator()(Args&&... _args) _const
        { return value(std::forward<Args>(_args)...); }

      _static constexpr Tp
      operator()() _const
      requires (not requires { value(); })
        { return value; }

      // overload operator[] for constexpr_value and non-constexpr_value
#if __cpp_multidimensional_subscript
      template <constexpr_value... Args>
        requires std::is_compound_v<value_type>
        _static constexpr constexpr_wrapper<value[Args::value...]>
        operator[](Args...) _const
        { return {}; }

      template <typename... Args>
        requires (not constexpr_value<std::remove_cvref_t<Args>> || ...)
          && std::is_compound_v<value_type>
        _static constexpr decltype(value[std::declval<Args>()...])
        operator[](Args&&... _args) _const
        { return value[std::forward<Args>(_args)...]; }
#else
      template <constexpr_value Arg>
        requires std::is_compound_v<value_type>
        _static constexpr constexpr_wrapper<value[Arg::value]>
        operator[](Arg) _const
        { return {}; }

      template <typename Arg>
        requires (not constexpr_value<std::remove_cvref_t<Arg>>)
          && std::is_compound_v<value_type>
        _static constexpr decltype(value[std::declval<Arg>()])
        operator[](Arg&& _arg) _const
        { return value[std::forward<Arg>(_arg)]; }
#endif

#undef _static
#undef _const
    };

  // constexpr_wrapper constant (cw)
  template <auto Xp>
    inline constexpr constexpr_wrapper<Xp> cw{};

  namespace detail
  {
    template <char... Chars>
      consteval auto
      cw_prepare_array()
      {
	constexpr auto not_digit_sep = [](char c) { return c != '\''; };
	constexpr char arr0[sizeof...(Chars)] = {Chars...};
	constexpr auto size
	  = std::count_if(std::begin(arr0), std::end(arr0), not_digit_sep);
	std::array<char, size> tmp = {};
	std::copy_if(std::begin(arr0), std::end(arr0), tmp.begin(), not_digit_sep);
	return tmp;
      }

    template <char... Chars>
      consteval auto
      cw_parse()
      {
        constexpr std::array arr = cw_prepare_array<Chars...>();
        constexpr int base = arr[0] == '0' and 2 < arr.size()
                               ? arr[1] == 'x' or arr[1] == 'X' ? 16
                                                                : arr[1] == 'b' ? 2 : 8
                               : 10;
        constexpr int offset = base == 10 ? 0 : base == 8 ? 1 : 2;
        constexpr bool valid_chars = std::all_of(arr.begin() + offset, arr.end(), [=](char c) {
                                       if constexpr (base == 16)
                                         return (c >= 'a' and c <= 'f') or (c >= 'A' and c <= 'F')
                                                  or (c >= '0' and c <= '9');
                                       else
                                         return c >= '0' and c < char('0' + base);
                                     });
        static_assert(valid_chars, "invalid characters in constexpr_wrapper literal");

        // common values, freeing values for error conditions
        if constexpr (arr.size() == 1 and arr[0] =='0')
          return static_cast<signed char>(0);
        else if constexpr (arr.size() == 1 and arr[0] =='1')
          return static_cast<signed char>(1);
        else if constexpr (arr.size() == 1 and arr[0] =='2')
          return static_cast<signed char>(2);
        else
          {
            constexpr unsigned long long x = [&]() {
              unsigned long long x = {};
              constexpr auto max = std::numeric_limits<unsigned long long>::max();
              auto it = arr.begin() + offset;
              for (; it != arr.end(); ++it)
                {
                  unsigned nextdigit = *it - '0';
                  if constexpr (base == 16)
                    {
                      if (*it >= 'a')
                        nextdigit = *it - 'a' + 10;
                      else if (*it >= 'A')
                        nextdigit = *it - 'A' + 10;
                    }
                  if (x > max / base)
                    return 0ull;
                  x *= base;
                  if (x > max - nextdigit)
                    return 0ull;
                  x += nextdigit;
                }
              return x;
            }();
            static_assert(x != 0, "constexpr_wrapper literal value out of range");
            if constexpr (x <= std::numeric_limits<signed char>::max())
              return static_cast<signed char>(x);
            else if constexpr (x <= std::numeric_limits<signed short>::max())
              return static_cast<signed short>(x);
            else if constexpr (x <= std::numeric_limits<signed int>::max())
              return static_cast<signed int>(x);
            else if constexpr (x <= std::numeric_limits<signed long>::max())
              return static_cast<signed long>(x);
            else if constexpr (x <= std::numeric_limits<signed long long>::max())
              return static_cast<signed long long>(x);
            else
              return x;
          }
      }
  }

  namespace literals
  {
    template <char... Chars>
      constexpr auto operator"" _cw()
      { return vir::cw<vir::detail::cw_parse<Chars...>()>; }
  }

} // namespace vir

#endif // has concepts
#endif // VIR_CONSTEXPR_WRAPPER_H_
// vim: et tw=100 ts=8 sw=2 cc=101