libstdc++-v3/include/std/span

// Components for manipulating non-owning sequences of objects -*- C++ -*-

// Copyright (C) 2019 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/>.

/** @file span
 *  This is a Standard C++ Library header.
 */

//
// P0122 span library
// Contributed by ThePhD
//

#ifndef _GLIBCXX_SPAN
#define _GLIBCXX_SPAN 1

#pragma GCC system_header

#if __cplusplus > 201703L

#include <type_traits>
#include <tuple>
#include <utility>
#include <array>
#include <bits/stl_iterator.h>
#include <bits/range_access.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#define __cpp_lib_span 201902L

  inline constexpr size_t dynamic_extent = static_cast<size_t>(-1);

  template<typename _Type, size_t _Extent>
    class span;

  namespace __detail
  {
    template<typename _Tp>
      struct __is_std_span : false_type { };

    template<typename _Tp, size_t _Num>
      struct __is_std_span<span<_Tp, _Num>> : true_type { };

    template<typename _Tp>
      struct __is_std_array : false_type { };

    template<typename _Tp, size_t _Num>
      struct __is_std_array<_GLIBCXX_STD_C::array<_Tp, _Num>> : true_type { };

#ifdef _GLIBCXX_DEBUG
    template<typename _Tp, size_t _Num>
      struct __is_std_array<__debug::array<_Tp, _Num>> : true_type { };
#endif

    template<size_t _Extent>
      class __extent_storage
      {
      public:
	constexpr
	__extent_storage(size_t) noexcept
	{ }

	static constexpr size_t
	_M_extent() noexcept
	{ return _Extent; }
      };

    template<>
      class __extent_storage<dynamic_extent>
      {
      public:
	constexpr
	__extent_storage(size_t __extent) noexcept
	: _M_extent_value(__extent)
	{ }

	constexpr size_t
	_M_extent() const noexcept
	{ return this->_M_extent_value; }

      private:
	size_t _M_extent_value;
      };

  } // namespace __detail

  template<typename _Type, size_t _Extent = dynamic_extent>
    class span
    {
      template<size_t _Offset, size_t _Count>
	static constexpr size_t
	_S_subspan_extent()
	{
	  if constexpr (_Count != dynamic_extent)
	    return _Count;
	  else if constexpr (extent != dynamic_extent)
	    return _Extent - _Offset;
	  else
	    return dynamic_extent;
	}

      template<typename _Tp>
	using __is_compatible = is_convertible<_Tp(*)[], _Type(*)[]>;

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3255. span's array constructor is too strict
      template<typename _Tp, size_t _ArrayExtent,
	       typename = enable_if_t<_Extent == dynamic_extent
				      || _ArrayExtent == _Extent>>
	using __is_compatible_array  = __is_compatible<_Tp>;

    public:
      // member types
      using value_type             = remove_cv_t<_Type>;
      using element_type           = _Type;
      using index_type             = size_t;
      using reference              = element_type&;
      using const_reference        = const element_type&;
      using pointer                = _Type*;
      using const_pointer          = const _Type*;
      using iterator
	= __gnu_cxx::__normal_iterator<pointer, span>;
      using const_iterator
	= __gnu_cxx::__normal_iterator<const_pointer, span>;
      using reverse_iterator       = std::reverse_iterator<iterator>;
      using const_reverse_iterator = std::reverse_iterator<const_iterator>;
      using difference_type        = ptrdiff_t;
      // Official wording has no size_type -- why??
      // using size_type = size_t;

      // member constants
      static inline constexpr size_t extent = _Extent;

      // constructors

      template<bool _DefaultConstructible = (_Extent + 1u) <= 1u,
	       enable_if_t<_DefaultConstructible>* = nullptr>
	constexpr
	span() noexcept : _M_extent(0), _M_ptr(nullptr)
	{ }

      constexpr
      span(const span&) noexcept = default;

      template<typename _Tp, size_t _ArrayExtent,
	       typename = _Require<__is_compatible_array<_Tp, _ArrayExtent>>>
	constexpr
	span(_Tp (&__arr)[_ArrayExtent]) noexcept
	: span(static_cast<pointer>(__arr), _ArrayExtent)
	{ }

      template<typename _Tp, size_t _ArrayExtent,
	       typename = _Require<__is_compatible_array<_Tp, _ArrayExtent>>>
	constexpr
	span(array<_Tp, _ArrayExtent>& __arr) noexcept
	: span(static_cast<pointer>(__arr.data()), _ArrayExtent)
	{ }

      template<typename _Tp, size_t _ArrayExtent,
	       typename = _Require<__is_compatible_array<_Tp, _ArrayExtent>>>
	constexpr
	span(const array<_Tp, _ArrayExtent>& __arr) noexcept
	: span(static_cast<pointer>(__arr.data()), _ArrayExtent)
	{ }

      // NOTE: when the time comes, and P1394 -
      // range constructors for std::span - ships in
      // the standard, delete the #else block and remove
      // the conditional
      // if the paper fails, delete #if block
      // and keep the crappy #else block
      // and then cry that NB comments failed C++20...
      // but maybe for C++23?
#ifdef _GLIBCXX_P1394
    private:
      // FIXME: use std::iter_reference_t
      template<typename _Iterator>
	using iter_reference_t = decltype(*std::declval<_Iterator&>());
      // FIXME: use std::ranges::iterator_t
      // N.B. constraint is needed to prevent a cycle when __adl_begin finds
      // begin(span) which does overload resolution on span(Range&&).
      template<typename _Rng,
	       typename _Rng2 = remove_cvref_t<_Rng>,
	       typename = enable_if_t<!__detail::__is_std_span<_Rng2>::value>>
	using iterator_t = decltype(std::__adl_begin(std::declval<_Rng&>()));
      // FIXME: use std::iter_value_t
      template<typename _Iter>
	using iter_value_t = typename iterator_traits<_Iter>::value_type;
      // FIXME: use std::derived_from concept
      template<typename _Derived, typename _Base>
	using derived_from
	  = __and_<is_base_of<_Base, _Derived>,
	      is_convertible<const volatile _Derived*, const volatile _Base*>>;
      // FIXME: require contiguous_iterator<_Iterator>
      template<typename _Iter,
	       typename _Ref = iter_reference_t<_Iter>,
	       typename _Traits = iterator_traits<_Iter>,
	       typename _Tag = typename _Traits::iterator_category>
	using __is_compatible_iterator
	  = __and_<derived_from<_Tag, random_access_iterator_tag>,
		   is_lvalue_reference<_Ref>,
		   is_same<iter_value_t<_Iter>, remove_cvref_t<_Ref>>,
		   __is_compatible<remove_reference_t<_Ref>>>;

      template<typename _Range>
	using __is_compatible_range
	  = __is_compatible_iterator<iterator_t<_Range>>;

    public:
      template<typename _Range, typename = _Require<
	  bool_constant<_Extent == dynamic_extent>,
	  __not_<__detail::__is_std_span<remove_cvref_t<_Range>>>,
	  __not_<__detail::__is_std_array<remove_cvref_t<_Range>>>,
	  __not_<is_array<remove_reference_t<_Range>>>,
	  __is_compatible_range<_Range>>,
	  typename = decltype(std::__adl_data(std::declval<_Range&>()))>
	constexpr
	span(_Range&& __range)
	noexcept(noexcept(::std::__adl_data(__range))
		  && noexcept(::std::__adl_size(__range)))
	: span(::std::__adl_data(__range), ::std::__adl_size(__range))
	{ }

      template<typename _ContiguousIterator, typename _Sentinel, typename
		= _Require<__not_<is_convertible<_Sentinel, index_type>>,
			   __is_compatible_iterator<_ContiguousIterator>>>
	constexpr
	span(_ContiguousIterator __first, _Sentinel __last)
	: _M_extent(static_cast<index_type>(__last - __first)),
	  _M_ptr(std::to_address(__first))
	{
	  if (_Extent != dynamic_extent)
	    __glibcxx_assert((__last - __first) == _Extent);
	}

      template<typename _ContiguousIterator, typename
		= _Require<__is_compatible_iterator<_ContiguousIterator>>>
	constexpr
	span(_ContiguousIterator __first, index_type __count)
	noexcept(noexcept(std::to_address(__first)))
	: _M_extent(__count), _M_ptr(std::to_address(__first))
	{ __glibcxx_assert(_Extent == dynamic_extent || __count == _Extent); }
#else
    private:
      template<typename _Container,
	  typename _DataT = decltype(std::data(std::declval<_Container&>())),
	  typename _SizeT = decltype(std::size(std::declval<_Container&>()))>
	using __is_compatible_container
	  = __is_compatible<remove_pointer_t<_DataT>>;

    public:
      template<typename _Container, typename = _Require<
		bool_constant<_Extent == dynamic_extent>,
		__not_<__detail::__is_std_span<remove_cv_t<_Container>>>,
		__not_<__detail::__is_std_array<remove_cv_t<_Container>>>,
		__not_<is_array<_Container>>,
		__is_compatible_container<_Container>>>
	constexpr
	span(_Container& __cont)
	noexcept(noexcept(std::data(__cont)) && noexcept(std::size(__cont)))
	: _M_extent(std::size(__cont)), _M_ptr(std::data(__cont))
	{ }

      template<typename _Container, typename = _Require<
		bool_constant<_Extent == dynamic_extent>,
		__not_<__detail::__is_std_span<remove_cv_t<_Container>>>,
		__not_<__detail::__is_std_array<remove_cv_t<_Container>>>,
		__not_<is_array<_Container>>,
		__is_compatible_container<const _Container>>>
	constexpr
	span(const _Container& __cont)
	noexcept(noexcept(std::data(__cont)) && noexcept(std::size(__cont)))
	: _M_extent(std::size(__cont)), _M_ptr(std::data(__cont))
	{ }

      constexpr
      span(pointer __first, index_type __count) noexcept
      : _M_extent(__count), _M_ptr(__first)
      { __glibcxx_assert(_Extent == dynamic_extent || __count == _Extent); }

      constexpr
      span(pointer __first, pointer __last) noexcept
      : span(__first, static_cast<index_type>(__last - __first))
      { }
#endif // P1394

      template<typename _OType, size_t _OExtent, typename = _Require<
	  __bool_constant<_Extent == dynamic_extent || _Extent == _OExtent>,
	  is_convertible<_OType(*)[], _Type(*)[]>>>
	constexpr
	span(const span<_OType, _OExtent>& __s) noexcept
	: _M_extent(__s.size()), _M_ptr(__s.data())
	{ }

      // assignment

      constexpr span&
      operator=(const span&) noexcept = default;

      // observers

      constexpr index_type
      size() const noexcept
      { return this->_M_extent._M_extent(); }

      constexpr index_type
      size_bytes() const noexcept
      { return this->_M_extent._M_extent() * sizeof(element_type); }

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

      // element access

      constexpr reference
      front() const noexcept
      {
	static_assert(extent != 0);
	__glibcxx_assert(!empty());
	return *this->_M_ptr;
      }

      constexpr reference
      back() const noexcept
      {
	static_assert(extent != 0);
	__glibcxx_assert(!empty());
	return *(this->_M_ptr + (size() - 1));
      }

      constexpr reference
      operator[](index_type __idx) const noexcept
      {
	static_assert(extent != 0);
	__glibcxx_assert(__idx < size());
	return *(this->_M_ptr + __idx);
      }

      constexpr pointer
      data() const noexcept
      { return this->_M_ptr; }

      // iterator support

      constexpr iterator
      begin() const noexcept
      { return iterator(this->_M_ptr); }

      constexpr const_iterator
      cbegin() const noexcept
      { return const_iterator(this->_M_ptr); }

      constexpr iterator
      end() const noexcept
      { return iterator(this->_M_ptr + this->size()); }

      constexpr const_iterator
      cend() const noexcept
      { return const_iterator(this->_M_ptr + this->size()); }

      constexpr reverse_iterator
      rbegin() const noexcept
      { return reverse_iterator(this->end()); }

      constexpr const_reverse_iterator
      crbegin() const noexcept
      { return const_reverse_iterator(this->cend()); }

      constexpr reverse_iterator
      rend() const noexcept
      { return reverse_iterator(this->begin()); }

      constexpr const_reverse_iterator
      crend() const noexcept
      { return const_reverse_iterator(this->cbegin()); }

      // subviews

      template<size_t _Count>
	constexpr span<element_type, _Count>
	first() const noexcept
	{
	  if constexpr (_Extent == dynamic_extent)
	    __glibcxx_assert(_Count <= size());
	  else
	    static_assert(_Count <= extent);
	  return { this->data(), _Count };
	}

      constexpr span<element_type, dynamic_extent>
      first(index_type __count) const noexcept
      {
	__glibcxx_assert(__count <= size());
	return { this->data(), __count };
      }

      template<size_t _Count>
	constexpr span<element_type, _Count>
	last() const noexcept
	{
	  if constexpr (_Extent == dynamic_extent)
	    __glibcxx_assert(_Count <= size());
	  else
	    static_assert(_Count <= extent);
	  return { this->data() + (this->size() - _Count), _Count };
	}

      constexpr span<element_type, dynamic_extent>
      last(index_type __count) const noexcept
      {
	__glibcxx_assert(__count <= size());
	return { this->data() + (this->size() - __count), __count };
      }

      template<size_t _Offset, size_t _Count = dynamic_extent>
	constexpr auto
	subspan() const noexcept
	-> span<element_type, _S_subspan_extent<_Offset, _Count>()>
	{
	  if constexpr (_Extent == dynamic_extent)
	    __glibcxx_assert(_Offset <= size());
	  else
	    static_assert(_Offset <= extent);

	  if constexpr (_Count == dynamic_extent)
	    return { this->data() + _Offset, this->size() - _Offset };
	  else
	    {
	      if constexpr (_Extent == dynamic_extent)
		{
		  __glibcxx_assert(_Count <= size());
		  __glibcxx_assert(_Count <= (size() - _Offset));
		}
	      else
		{
		  static_assert(_Count <= extent);
		  static_assert(_Count <= (extent - _Offset));
		}
	      return { this->data() + _Offset, _Count };
	    }
	}

      constexpr span<element_type, dynamic_extent>
      subspan(index_type __offset, index_type __count = dynamic_extent) const
      noexcept
      {
	__glibcxx_assert(__offset <= size());
	if (__count == dynamic_extent)
	  __count = this->size() - __offset;
	else
	  {
	    __glibcxx_assert(__count <= size());
	    __glibcxx_assert(__offset + __count <= size());
	  }
	return {this->data() + __offset, __count};
      }

      // observers: range helpers

      friend constexpr iterator
      begin(span __sp) noexcept
      { return __sp.begin(); }

      friend constexpr iterator
      end(span __sp) noexcept
      { return __sp.end(); }

    private:
      [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
      pointer _M_ptr;
    };

  // deduction guides
  template<typename _Type, size_t _ArrayExtent>
    span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;

  template<typename _Type, size_t _ArrayExtent>
    span(array<_Type, _ArrayExtent>&) -> span<_Type, _ArrayExtent>;

  template<typename _Type, size_t _ArrayExtent>
    span(const array<_Type, _ArrayExtent>&)
      -> span<const _Type, _ArrayExtent>;

#ifdef _GLIBCXX_P1394

  template<typename _ContiguousIterator, typename _Sentinel>
    span(_ContiguousIterator, _Sentinel)
      -> span<remove_reference_t<
	typename iterator_traits<_ContiguousIterator>::reference>>;

  template<typename _Range>
    span(_Range &&)
      -> span<remove_reference_t<typename iterator_traits<
	  decltype(std::__adl_begin(::std::declval<_Range&>()))>::reference>>;

#else

  template<typename _Container>
    span(_Container&) -> span<typename _Container::value_type>;

  template<typename _Container>
    span(const _Container&) -> span<const typename _Container::value_type>;

#endif // P1394

  template<typename _Type, size_t _Extent>
    inline
    span<const byte, _Extent == dynamic_extent
	? dynamic_extent : _Extent * sizeof(_Type)>
    as_bytes(span<_Type, _Extent> __sp) noexcept
    {
      return {reinterpret_cast<const byte*>(__sp.data()), __sp.size_bytes()};
    }

  template<typename _Type, size_t _Extent>
    inline
    span<byte, _Extent == dynamic_extent
       ? dynamic_extent : _Extent * sizeof(_Type)>
    as_writable_bytes(span<_Type, _Extent> __sp) noexcept
    {
      return {reinterpret_cast<byte*>(__sp.data()), __sp.size_bytes()};
    }

  // tuple helpers
  template<size_t _Index, typename _Type, size_t _Extent>
    constexpr _Type&
    get(span<_Type, _Extent> __sp) noexcept
    {
      static_assert(_Extent != dynamic_extent && _Index < _Extent,
	"get<I> can only be used with a span of non-dynamic (fixed) extent");
      return __sp[_Index];
    }

  template<typename _Type, size_t _Extent>
    struct tuple_size<span<_Type, _Extent>>
    : public integral_constant<size_t, _Extent>
    {
      static_assert(_Extent != dynamic_extent, "tuple_size can only "
	"be used with a span of non-dynamic (fixed) extent");
    };

  template<size_t _Index, typename _Type, size_t _Extent>
    struct tuple_element<_Index, span<_Type, _Extent>>
    {
      static_assert(_Extent != dynamic_extent, "tuple_element can only "
	"be used with a span of non-dynamic (fixed) extent");
      static_assert(_Index < _Extent, "Index is less than Extent");
      using type = _Type;
    };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++20
#endif // _GLIBCXX_SPAN