Array.h

/**
* This file is based on the std::array implementation of libstdc++ at
* https://gcc.gnu.org/onlinedocs/gcc-7.1.0/libstdc++/api/a01056_source.html
*
* Changes:
*  - isolate, i.e. remove dependencies on internal libstdc++ stuff
*  - use c++17 behavior even in c++11 or c++14
*  - remove std::swappable special case because that doesn't work with MSVC
*  - constexpr more things
*  - add some features like prepend/tail
*
* If using std::array at runtime, feel free to either keep using std::array or use this one - it doesn't really matter.
* For compile time computations, this one here is preferred because std::array in C++11
* misses some constexpr specifiers, forcing these methods to be called at runtime instead of compile time.
*/

// Copyright (C) 2007-2017 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

#pragma once

#include <algorithm>
#include <c10/util/C++17.h>
#include <stdexcept>
#include <string>
#include <utility>

namespace c10 { namespace guts {

namespace detail {
template<typename _Tp, std::size_t _Nm>
struct __array_traits final {
  using _Type = _Tp[_Nm];

  static constexpr _Tp& _S_ref(const _Type& __t, std::size_t __n) noexcept {
    return const_cast<_Tp&>(__t[__n]);
  }

  static constexpr _Tp* _S_ptr(const _Type& __t) noexcept {
    return const_cast<_Tp*>(__t);
  }
};

template<typename _Tp>
struct __array_traits<_Tp, 0> final {
  struct _Type final {};

  static constexpr _Tp& _S_ref(const _Type& __t, std::size_t) noexcept {
    return *_S_ptr(__t);
  }

  static constexpr _Tp* _S_ptr(const _Type&) noexcept {
    return nullptr;
  }
};

[[noreturn]] inline void __throw_out_of_range(std::string msg) {
  throw std::out_of_range(std::move(msg));
}
}

template<typename _Tp, std::size_t _Nm>
class array final {
public:
  using value_type = _Tp;
  using pointer = value_type*;
  using const_pointer = const value_type*;
  using reference = value_type&;
  using const_reference = const value_type&;
  using iterator = value_type*;
  using const_iterator = const value_type*;
  using size_type = std::size_t;
  using difference_type = std::ptrdiff_t;
  using reverse_iterator = std::reverse_iterator<iterator>;
  using const_reverse_iterator = std::reverse_iterator<const_iterator>;

private:
  using _AT_Type = detail::__array_traits<_Tp, _Nm>;
public: // needs to be public member for aggregate initialization
  typename _AT_Type::_Type _M_elems;

public:
  // No explicit construct/copy/destroy for aggregate type.

  // DR 776.
  AT_CPP14_CONSTEXPR void fill(const value_type& __u)
  { std::fill_n(begin(), size(), __u); }

  AT_CPP14_CONSTEXPR void swap(array& __other)
  { std::swap_ranges(begin(), end(), __other.begin()); }

  // Iterators.
  AT_CPP14_CONSTEXPR iterator begin() noexcept
  { return iterator(data()); }

  constexpr const_iterator begin() const noexcept
  { return const_iterator(data()); }

  AT_CPP14_CONSTEXPR iterator end() noexcept
  { return iterator(data() + _Nm); }

  constexpr const_iterator end() const noexcept
  { return const_iterator(data() + _Nm); }

  AT_CPP14_CONSTEXPR reverse_iterator rbegin() noexcept
  { return reverse_iterator(end()); }

  constexpr const_reverse_iterator rbegin() const noexcept
  { return const_reverse_iterator(end()); }

  AT_CPP14_CONSTEXPR reverse_iterator rend() noexcept
  { return reverse_iterator(begin()); }

  constexpr const_reverse_iterator rend() const noexcept
  { return const_reverse_iterator(begin()); }

  constexpr const_iterator cbegin() const noexcept
  { return const_iterator(data()); }

  constexpr const_iterator cend() const noexcept
  { return const_iterator(data() + _Nm); }

  constexpr const_reverse_iterator crbegin() const noexcept
  { return const_reverse_iterator(end()); }

  constexpr const_reverse_iterator crend() const noexcept
  { return const_reverse_iterator(begin()); }

  // Capacity.
  constexpr size_type size() const noexcept { return _Nm; }

  constexpr size_type max_size() const noexcept { return _Nm; }

  constexpr bool empty() const noexcept { return size() == 0; }

  // Element access.
  AT_CPP14_CONSTEXPR reference operator[](size_type __n) noexcept
  { return _AT_Type::_S_ref(_M_elems, __n); }

  constexpr const_reference operator[](size_type __n) const noexcept
  { return _AT_Type::_S_ref(_M_elems, __n); }

  AT_CPP14_CONSTEXPR reference at(size_type __n) {
    if (__n >= _Nm) {
      detail::__throw_out_of_range(std::string() +
          "array::at: __n (which is " + to_string(__n) + ") " +
          ">= _Nm (which is " + to_string(_Nm) + ")");
    }
    return _AT_Type::_S_ref(_M_elems, __n);
  }

  constexpr const_reference at(size_type __n) const {
    // Result of conditional expression must be an lvalue so use
    // boolean ? lvalue : (throw-expr, lvalue)
    return __n < _Nm ? _AT_Type::_S_ref(_M_elems, __n)
      : (detail::__throw_out_of_range(std::string() +
            "array::at: __n (which is " + to_string(__n) + ") " +
            ">= _Nm (which is " + to_string(_Nm) + ")"),
         _AT_Type::_S_ref(_M_elems, 0));
     }

  AT_CPP14_CONSTEXPR reference front() noexcept
  { return *begin(); }

  constexpr const_reference front() const noexcept
  { return _AT_Type::_S_ref(_M_elems, 0); }

  AT_CPP14_CONSTEXPR reference back() noexcept
  { return _Nm ? *(end() - 1) : *end(); }

  constexpr const_reference back() const noexcept
  {
    return _Nm ? _AT_Type::_S_ref(_M_elems, _Nm - 1)
            : _AT_Type::_S_ref(_M_elems, 0);
  }

  AT_CPP14_CONSTEXPR pointer data() noexcept
  { return _AT_Type::_S_ptr(_M_elems); }

  constexpr const_pointer data() const noexcept
  { return _AT_Type::_S_ptr(_M_elems); }
};

#if defined(__cpp_deduction_guides) && __cpp_deduction_guides >= 201606
  template<typename _Tp, typename... _Up>
  array(_Tp, _Up...) ->
    array<enable_if_t<(std::is_same<_Tp, _Up>::value && ...), _Tp>, 1 + sizeof...(_Up)>;
#endif

// Array comparisons.
namespace detail {
template<class T, size_t N>
constexpr inline bool array_equals_(const array<T, N>& lhs, const array<T, N>& rhs, size_t current_index) {
  return (current_index == N)
         ? true
         : (lhs.at(current_index) == rhs.at(current_index) && array_equals_(lhs, rhs, current_index + 1));
}
template<class T, size_t N>
constexpr inline bool array_less_(const array<T, N>& lhs, const array<T, N>& rhs, size_t current_index) {
  return (current_index == N)
         ? false
         : (lhs.at(current_index) < rhs.at(current_index) || array_less_(lhs, rhs, current_index + 1));
}
}
template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator==(const array<_Tp, _Nm>& __one, const array<_Tp, _Nm>& __two)
{ return detail::array_equals_(__one, __two, 0); }

template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator!=(const array<_Tp, _Nm>& __one, const array<_Tp, _Nm>& __two)
{ return !(__one == __two); }

template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator<(const array<_Tp, _Nm>& __a, const array<_Tp, _Nm>& __b)
{ return detail::array_less_(__a, __b, 0); }

template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator>(const array<_Tp, _Nm>& __one, const array<_Tp, _Nm>& __two)
{ return __two < __one; }

template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator<=(const array<_Tp, _Nm>& __one, const array<_Tp, _Nm>& __two)
{ return !(__one > __two); }

template<typename _Tp, std::size_t _Nm>
constexpr inline bool operator>=(const array<_Tp, _Nm>& __one, const array<_Tp, _Nm>& __two)
{ return !(__one < __two); }

// Specialized algorithms.
template<typename _Tp, std::size_t _Nm>
inline void swap(array<_Tp, _Nm>& __one, array<_Tp, _Nm>& __two) noexcept(noexcept(__one.swap(__two)))
{ __one.swap(__two); }

template<std::size_t _Int, typename _Tp, std::size_t _Nm>
constexpr _Tp& get(array<_Tp, _Nm>& __arr) noexcept {
   static_assert(_Int < _Nm, "array index is within bounds");
   return detail::__array_traits<_Tp, _Nm>::_S_ref(__arr._M_elems, _Int);
}

template<std::size_t _Int, typename _Tp, std::size_t _Nm>
constexpr _Tp&& get(array<_Tp, _Nm>&& __arr) noexcept
{
  static_assert(_Int < _Nm, "array index is within bounds");
  return guts::move(get<_Int>(__arr));
}

template<std::size_t _Int, typename _Tp, std::size_t _Nm>
constexpr const _Tp& get(const array<_Tp, _Nm>& __arr) noexcept
{
  static_assert(_Int < _Nm, "array index is within bounds");
  return detail::__array_traits<_Tp, _Nm>::_S_ref(__arr._M_elems, _Int);
}

/**
 * Some added features not available in std::array.
 * Only call these at compile time, they're slow if called at runtime.
 * Examples:
 *  tail({2, 3, 4}) == {3, 4}
 *  prepend(2, {3, 4}) == {2, 3, 4}
 */
namespace detail {
template<class T, size_t N, size_t... I>
constexpr inline array<T, N-1> tail_(const array<T, N>& arg, guts::index_sequence<I...>) {
  static_assert(sizeof...(I) == N-1, "invariant");
  return {{get<I+1>(arg)...}};
}
}
template<class T, size_t N>
constexpr inline array<T, N-1> tail(const array<T, N>& arg) {
  static_assert(N > 0, "Can only call tail() on an array with at least one element");
  return detail::tail_(arg, guts::make_index_sequence<N-1>());
}

namespace detail {
template<class T, size_t N, size_t... I>
constexpr inline array<T, N+1> prepend_(T&& head, const array<T, N>& tail, guts::index_sequence<I...>) {
  return {{guts::forward<T>(head), get<I>(tail)...}};
}
}
template<class T, size_t N>
constexpr inline array<T, N+1> prepend(T&& head, const array<T, N>& tail) {
  return detail::prepend_(guts::forward<T>(head), tail, guts::make_index_sequence<N>());
}

/**
 * Convert a C array into a std::array.
 * Example:
 *   int source[3] = {2, 3, 4};
 *   std::array<int, 3> target = to_std_array(source);
 */

namespace detail {
template<class T, size_t N, size_t... I>
constexpr array<T, N> to_array_(const T (&arr)[N], guts::index_sequence<I...>) {
  return {{arr[I]...}};
}
}

template<class T, size_t N>
constexpr array<T, N> to_array(const T (&arr)[N]) {
  return detail::to_array_(arr, guts::make_index_sequence<N>());
}

}}