llvm-project/libcxx/include/__split_buffer

// -*- C++ -*-
#ifndef _LIBCPP_SPLIT_BUFFER
#define _LIBCPP_SPLIT_BUFFER

#include <__config>
#include <type_traits>
#include <algorithm>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>


_LIBCPP_BEGIN_NAMESPACE_STD

template <bool>
class __split_buffer_common
{
protected:
    _LIBCPP_NORETURN void __throw_length_error() const;
    _LIBCPP_NORETURN void __throw_out_of_range() const;
};

template <class _Tp, class _Allocator = allocator<_Tp> >
struct __split_buffer
    : private __split_buffer_common<true>
{
private:
    __split_buffer(const __split_buffer&);
    __split_buffer& operator=(const __split_buffer&);
public:
    typedef _Tp                                             value_type;
    typedef _Allocator                                      allocator_type;
    typedef typename remove_reference<allocator_type>::type __alloc_rr;
    typedef allocator_traits<__alloc_rr>                    __alloc_traits;
    typedef value_type&                                     reference;
    typedef const value_type&                               const_reference;
    typedef typename __alloc_traits::size_type              size_type;
    typedef typename __alloc_traits::difference_type        difference_type;
    typedef typename __alloc_traits::pointer                pointer;
    typedef typename __alloc_traits::const_pointer          const_pointer;
    typedef pointer                                         iterator;
    typedef const_pointer                                   const_iterator;

    pointer                                         __first_;
    pointer                                         __begin_;
    pointer                                         __end_;
    __compressed_pair<pointer, allocator_type> __end_cap_;

    typedef typename add_lvalue_reference<allocator_type>::type __alloc_ref;
    typedef typename add_lvalue_reference<allocator_type>::type __alloc_const_ref;

    _LIBCPP_INLINE_VISIBILITY __alloc_rr&           __alloc() _NOEXCEPT         {return __end_cap_.second();}
    _LIBCPP_INLINE_VISIBILITY const __alloc_rr&     __alloc() const _NOEXCEPT   {return __end_cap_.second();}
    _LIBCPP_INLINE_VISIBILITY pointer&              __end_cap() _NOEXCEPT       {return __end_cap_.first();}
    _LIBCPP_INLINE_VISIBILITY const pointer&        __end_cap() const _NOEXCEPT {return __end_cap_.first();}

    _LIBCPP_INLINE_VISIBILITY
    __split_buffer()
        _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value);
    _LIBCPP_INLINE_VISIBILITY
    explicit __split_buffer(__alloc_rr& __a);
    _LIBCPP_INLINE_VISIBILITY
    explicit __split_buffer(const __alloc_rr& __a);
    __split_buffer(size_type __cap, size_type __start, __alloc_rr& __a);
    ~__split_buffer();

    __split_buffer(__split_buffer&& __c)
        _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value);
    __split_buffer(__split_buffer&& __c, const __alloc_rr& __a);
    __split_buffer& operator=(__split_buffer&& __c)
        _NOEXCEPT_((__alloc_traits::propagate_on_container_move_assignment::value &&
                is_nothrow_move_assignable<allocator_type>::value) ||
               !__alloc_traits::propagate_on_container_move_assignment::value);

    _LIBCPP_INLINE_VISIBILITY       iterator begin() _NOEXCEPT       {return __begin_;}
    _LIBCPP_INLINE_VISIBILITY const_iterator begin() const _NOEXCEPT {return __begin_;}
    _LIBCPP_INLINE_VISIBILITY       iterator end() _NOEXCEPT         {return __end_;}
    _LIBCPP_INLINE_VISIBILITY const_iterator end() const _NOEXCEPT   {return __end_;}

    _LIBCPP_INLINE_VISIBILITY
    void clear() _NOEXCEPT
        {__destruct_at_end(__begin_);}
    _LIBCPP_INLINE_VISIBILITY size_type size() const {return static_cast<size_type>(__end_ - __begin_);}
    _LIBCPP_INLINE_VISIBILITY bool empty()     const {return __end_ == __begin_;}
    _LIBCPP_INLINE_VISIBILITY size_type capacity() const {return static_cast<size_type>(__end_cap() - __first_);}
    _LIBCPP_INLINE_VISIBILITY size_type __front_spare() const {return static_cast<size_type>(__begin_ - __first_);}
    _LIBCPP_INLINE_VISIBILITY size_type __back_spare() const {return static_cast<size_type>(__end_cap() - __end_);}

    _LIBCPP_INLINE_VISIBILITY       reference front()       {return *__begin_;}
    _LIBCPP_INLINE_VISIBILITY const_reference front() const {return *__begin_;}
    _LIBCPP_INLINE_VISIBILITY       reference back()        {return *(__end_ - 1);}
    _LIBCPP_INLINE_VISIBILITY const_reference back() const  {return *(__end_ - 1);}

    void reserve(size_type __n);
    void shrink_to_fit() _NOEXCEPT;
    void push_front(const_reference __x);
    _LIBCPP_INLINE_VISIBILITY void push_back(const_reference __x);
    void push_front(value_type&& __x);
    void push_back(value_type&& __x);
    template <class... _Args>
        void emplace_back(_Args&&... __args);

    _LIBCPP_INLINE_VISIBILITY void pop_front() {__destruct_at_begin(__begin_+1);}
    _LIBCPP_INLINE_VISIBILITY void pop_back() {__destruct_at_end(__end_-1);}

    void __construct_at_end(size_type __n);
    void __construct_at_end(size_type __n, const_reference __x);
    template <class _InputIter>
        typename enable_if
        <
            __is_cpp17_input_iterator<_InputIter>::value &&
           !__is_cpp17_forward_iterator<_InputIter>::value,
            void
        >::type
        __construct_at_end(_InputIter __first, _InputIter __last);
    template <class _ForwardIterator>
        typename enable_if
        <
            __is_cpp17_forward_iterator<_ForwardIterator>::value,
            void
        >::type
        __construct_at_end(_ForwardIterator __first, _ForwardIterator __last);

    _LIBCPP_INLINE_VISIBILITY void __destruct_at_begin(pointer __new_begin)
        {__destruct_at_begin(__new_begin, is_trivially_destructible<value_type>());}
        _LIBCPP_INLINE_VISIBILITY
        void __destruct_at_begin(pointer __new_begin, false_type);
        _LIBCPP_INLINE_VISIBILITY
        void __destruct_at_begin(pointer __new_begin, true_type);

    _LIBCPP_INLINE_VISIBILITY
    void __destruct_at_end(pointer __new_last) _NOEXCEPT
        {__destruct_at_end(__new_last, false_type());}
    _LIBCPP_INLINE_VISIBILITY
        void __destruct_at_end(pointer __new_last, false_type) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY
        void __destruct_at_end(pointer __new_last, true_type) _NOEXCEPT;

    void swap(__split_buffer& __x)
        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value||
                   __is_nothrow_swappable<__alloc_rr>::value);

    bool __invariants() const;

private:
    _LIBCPP_INLINE_VISIBILITY
    void __move_assign_alloc(__split_buffer& __c, true_type)
        _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
        {
            __alloc() = _VSTD::move(__c.__alloc());
        }

    _LIBCPP_INLINE_VISIBILITY
    void __move_assign_alloc(__split_buffer&, false_type) _NOEXCEPT
        {}

    struct _ConstructTransaction {
      explicit _ConstructTransaction(pointer* __p, size_type __n) _NOEXCEPT
      : __pos_(*__p), __end_(*__p + __n), __dest_(__p) {
      }
      ~_ConstructTransaction() {
        *__dest_ = __pos_;
      }
      pointer __pos_;
     const pointer __end_;
    private:
     pointer *__dest_;
    };
};

template <class _Tp, class _Allocator>
bool
__split_buffer<_Tp, _Allocator>::__invariants() const
{
    if (__first_ == nullptr)
    {
        if (__begin_ != nullptr)
            return false;
        if (__end_ != nullptr)
            return false;
        if (__end_cap() != nullptr)
            return false;
    }
    else
    {
        if (__begin_ < __first_)
            return false;
        if (__end_ < __begin_)
            return false;
        if (__end_cap() < __end_)
            return false;
    }
    return true;
}

//  Default constructs __n objects starting at __end_
//  throws if construction throws
//  Precondition:  __n > 0
//  Precondition:  size() + __n <= capacity()
//  Postcondition:  size() == size() + __n
template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::__construct_at_end(size_type __n)
{
    _ConstructTransaction __tx(&this->__end_, __n);
    for (; __tx.__pos_ != __tx.__end_; ++__tx.__pos_) {
        __alloc_traits::construct(this->__alloc(), _VSTD::__to_address(__tx.__pos_));
    }
}

//  Copy constructs __n objects starting at __end_ from __x
//  throws if construction throws
//  Precondition:  __n > 0
//  Precondition:  size() + __n <= capacity()
//  Postcondition:  size() == old size() + __n
//  Postcondition:  [i] == __x for all i in [size() - __n, __n)
template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::__construct_at_end(size_type __n, const_reference __x)
{
    _ConstructTransaction __tx(&this->__end_, __n);
    for (; __tx.__pos_ != __tx.__end_; ++__tx.__pos_) {
        __alloc_traits::construct(this->__alloc(),
            _VSTD::__to_address(__tx.__pos_), __x);
    }
}

template <class _Tp, class _Allocator>
template <class _InputIter>
typename enable_if
<
     __is_cpp17_input_iterator<_InputIter>::value &&
    !__is_cpp17_forward_iterator<_InputIter>::value,
    void
>::type
__split_buffer<_Tp, _Allocator>::__construct_at_end(_InputIter __first, _InputIter __last)
{
    __alloc_rr& __a = this->__alloc();
    for (; __first != __last; ++__first)
    {
        if (__end_ == __end_cap())
        {
            size_type __old_cap = __end_cap() - __first_;
            size_type __new_cap = _VSTD::max<size_type>(2 * __old_cap, 8);
            __split_buffer __buf(__new_cap, 0, __a);
            for (pointer __p = __begin_; __p != __end_; ++__p, ++__buf.__end_)
                __alloc_traits::construct(__buf.__alloc(),
                        _VSTD::__to_address(__buf.__end_), _VSTD::move(*__p));
            swap(__buf);
        }
        __alloc_traits::construct(__a, _VSTD::__to_address(this->__end_), *__first);
        ++this->__end_;
    }
}

template <class _Tp, class _Allocator>
template <class _ForwardIterator>
typename enable_if
<
    __is_cpp17_forward_iterator<_ForwardIterator>::value,
    void
>::type
__split_buffer<_Tp, _Allocator>::__construct_at_end(_ForwardIterator __first, _ForwardIterator __last)
{
    _ConstructTransaction __tx(&this->__end_, _VSTD::distance(__first, __last));
    for (; __tx.__pos_ != __tx.__end_; ++__tx.__pos_, ++__first) {
        __alloc_traits::construct(this->__alloc(),
            _VSTD::__to_address(__tx.__pos_), *__first);
    }
}

template <class _Tp, class _Allocator>
inline
void
__split_buffer<_Tp, _Allocator>::__destruct_at_begin(pointer __new_begin, false_type)
{
    while (__begin_ != __new_begin)
        __alloc_traits::destroy(__alloc(), _VSTD::__to_address(__begin_++));
}

template <class _Tp, class _Allocator>
inline
void
__split_buffer<_Tp, _Allocator>::__destruct_at_begin(pointer __new_begin, true_type)
{
    __begin_ = __new_begin;
}

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
void
__split_buffer<_Tp, _Allocator>::__destruct_at_end(pointer __new_last, false_type) _NOEXCEPT
{
    while (__new_last != __end_)
        __alloc_traits::destroy(__alloc(), _VSTD::__to_address(--__end_));
}

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
void
__split_buffer<_Tp, _Allocator>::__destruct_at_end(pointer __new_last, true_type) _NOEXCEPT
{
    __end_ = __new_last;
}

template <class _Tp, class _Allocator>
__split_buffer<_Tp, _Allocator>::__split_buffer(size_type __cap, size_type __start, __alloc_rr& __a)
    : __end_cap_(nullptr, __a)
{
    __first_ = __cap != 0 ? __alloc_traits::allocate(__alloc(), __cap) : nullptr;
    __begin_ = __end_ = __first_ + __start;
    __end_cap() = __first_ + __cap;
}

template <class _Tp, class _Allocator>
inline
__split_buffer<_Tp, _Allocator>::__split_buffer()
    _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
    : __first_(nullptr), __begin_(nullptr), __end_(nullptr), __end_cap_(nullptr, __default_init_tag())
{
}

template <class _Tp, class _Allocator>
inline
__split_buffer<_Tp, _Allocator>::__split_buffer(__alloc_rr& __a)
    : __first_(nullptr), __begin_(nullptr), __end_(nullptr), __end_cap_(nullptr, __a)
{
}

template <class _Tp, class _Allocator>
inline
__split_buffer<_Tp, _Allocator>::__split_buffer(const __alloc_rr& __a)
    : __first_(nullptr), __begin_(nullptr), __end_(nullptr), __end_cap_(nullptr, __a)
{
}

template <class _Tp, class _Allocator>
__split_buffer<_Tp, _Allocator>::~__split_buffer()
{
    clear();
    if (__first_)
        __alloc_traits::deallocate(__alloc(), __first_, capacity());
}

template <class _Tp, class _Allocator>
__split_buffer<_Tp, _Allocator>::__split_buffer(__split_buffer&& __c)
    _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value)
    : __first_(_VSTD::move(__c.__first_)),
      __begin_(_VSTD::move(__c.__begin_)),
      __end_(_VSTD::move(__c.__end_)),
      __end_cap_(_VSTD::move(__c.__end_cap_))
{
    __c.__first_ = nullptr;
    __c.__begin_ = nullptr;
    __c.__end_ = nullptr;
    __c.__end_cap() = nullptr;
}

template <class _Tp, class _Allocator>
__split_buffer<_Tp, _Allocator>::__split_buffer(__split_buffer&& __c, const __alloc_rr& __a)
    : __end_cap_(nullptr, __a)
{
    if (__a == __c.__alloc())
    {
        __first_ = __c.__first_;
        __begin_ = __c.__begin_;
        __end_ = __c.__end_;
        __end_cap() = __c.__end_cap();
        __c.__first_ = nullptr;
        __c.__begin_ = nullptr;
        __c.__end_ = nullptr;
        __c.__end_cap() = nullptr;
    }
    else
    {
        size_type __cap = __c.size();
        __first_ = __alloc_traits::allocate(__alloc(), __cap);
        __begin_ = __end_ = __first_;
        __end_cap() = __first_ + __cap;
        typedef move_iterator<iterator> _Ip;
        __construct_at_end(_Ip(__c.begin()), _Ip(__c.end()));
    }
}

template <class _Tp, class _Allocator>
__split_buffer<_Tp, _Allocator>&
__split_buffer<_Tp, _Allocator>::operator=(__split_buffer&& __c)
    _NOEXCEPT_((__alloc_traits::propagate_on_container_move_assignment::value &&
                is_nothrow_move_assignable<allocator_type>::value) ||
               !__alloc_traits::propagate_on_container_move_assignment::value)
{
    clear();
    shrink_to_fit();
    __first_ = __c.__first_;
    __begin_ = __c.__begin_;
    __end_ = __c.__end_;
    __end_cap() = __c.__end_cap();
    __move_assign_alloc(__c,
        integral_constant<bool,
                          __alloc_traits::propagate_on_container_move_assignment::value>());
    __c.__first_ = __c.__begin_ = __c.__end_ = __c.__end_cap() = nullptr;
    return *this;
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::swap(__split_buffer& __x)
        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value||
                   __is_nothrow_swappable<__alloc_rr>::value)
{
    _VSTD::swap(__first_, __x.__first_);
    _VSTD::swap(__begin_, __x.__begin_);
    _VSTD::swap(__end_, __x.__end_);
    _VSTD::swap(__end_cap(), __x.__end_cap());
    _VSTD::__swap_allocator(__alloc(), __x.__alloc());
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::reserve(size_type __n)
{
    if (__n < capacity())
    {
        __split_buffer<value_type, __alloc_rr&> __t(__n, 0, __alloc());
        __t.__construct_at_end(move_iterator<pointer>(__begin_),
                               move_iterator<pointer>(__end_));
        _VSTD::swap(__first_, __t.__first_);
        _VSTD::swap(__begin_, __t.__begin_);
        _VSTD::swap(__end_, __t.__end_);
        _VSTD::swap(__end_cap(), __t.__end_cap());
    }
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::shrink_to_fit() _NOEXCEPT
{
    if (capacity() > size())
    {
#ifndef _LIBCPP_NO_EXCEPTIONS
        try
        {
#endif // _LIBCPP_NO_EXCEPTIONS
            __split_buffer<value_type, __alloc_rr&> __t(size(), 0, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            __t.__end_ = __t.__begin_ + (__end_ - __begin_);
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
#ifndef _LIBCPP_NO_EXCEPTIONS
        }
        catch (...)
        {
        }
#endif // _LIBCPP_NO_EXCEPTIONS
    }
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::push_front(const_reference __x)
{
    if (__begin_ == __first_)
    {
        if (__end_ < __end_cap())
        {
            difference_type __d = __end_cap() - __end_;
            __d = (__d + 1) / 2;
            __begin_ = _VSTD::move_backward(__begin_, __end_, __end_ + __d);
            __end_ += __d;
        }
        else
        {
            size_type __c = max<size_type>(2 * static_cast<size_t>(__end_cap() - __first_), 1);
            __split_buffer<value_type, __alloc_rr&> __t(__c, (__c + 3) / 4, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
        }
    }
    __alloc_traits::construct(__alloc(), _VSTD::__to_address(__begin_-1), __x);
    --__begin_;
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::push_front(value_type&& __x)
{
    if (__begin_ == __first_)
    {
        if (__end_ < __end_cap())
        {
            difference_type __d = __end_cap() - __end_;
            __d = (__d + 1) / 2;
            __begin_ = _VSTD::move_backward(__begin_, __end_, __end_ + __d);
            __end_ += __d;
        }
        else
        {
            size_type __c = max<size_type>(2 * static_cast<size_t>(__end_cap() - __first_), 1);
            __split_buffer<value_type, __alloc_rr&> __t(__c, (__c + 3) / 4, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
        }
    }
    __alloc_traits::construct(__alloc(), _VSTD::__to_address(__begin_-1),
            _VSTD::move(__x));
    --__begin_;
}

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
void
__split_buffer<_Tp, _Allocator>::push_back(const_reference __x)
{
    if (__end_ == __end_cap())
    {
        if (__begin_ > __first_)
        {
            difference_type __d = __begin_ - __first_;
            __d = (__d + 1) / 2;
            __end_ = _VSTD::move(__begin_, __end_, __begin_ - __d);
            __begin_ -= __d;
        }
        else
        {
            size_type __c = max<size_type>(2 * static_cast<size_t>(__end_cap() - __first_), 1);
            __split_buffer<value_type, __alloc_rr&> __t(__c, __c / 4, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
        }
    }
    __alloc_traits::construct(__alloc(), _VSTD::__to_address(__end_), __x);
    ++__end_;
}

template <class _Tp, class _Allocator>
void
__split_buffer<_Tp, _Allocator>::push_back(value_type&& __x)
{
    if (__end_ == __end_cap())
    {
        if (__begin_ > __first_)
        {
            difference_type __d = __begin_ - __first_;
            __d = (__d + 1) / 2;
            __end_ = _VSTD::move(__begin_, __end_, __begin_ - __d);
            __begin_ -= __d;
        }
        else
        {
            size_type __c = max<size_type>(2 * static_cast<size_t>(__end_cap() - __first_), 1);
            __split_buffer<value_type, __alloc_rr&> __t(__c, __c / 4, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
        }
    }
    __alloc_traits::construct(__alloc(), _VSTD::__to_address(__end_),
            _VSTD::move(__x));
    ++__end_;
}

template <class _Tp, class _Allocator>
template <class... _Args>
void
__split_buffer<_Tp, _Allocator>::emplace_back(_Args&&... __args)
{
    if (__end_ == __end_cap())
    {
        if (__begin_ > __first_)
        {
            difference_type __d = __begin_ - __first_;
            __d = (__d + 1) / 2;
            __end_ = _VSTD::move(__begin_, __end_, __begin_ - __d);
            __begin_ -= __d;
        }
        else
        {
            size_type __c = max<size_type>(2 * static_cast<size_t>(__end_cap() - __first_), 1);
            __split_buffer<value_type, __alloc_rr&> __t(__c, __c / 4, __alloc());
            __t.__construct_at_end(move_iterator<pointer>(__begin_),
                                   move_iterator<pointer>(__end_));
            _VSTD::swap(__first_, __t.__first_);
            _VSTD::swap(__begin_, __t.__begin_);
            _VSTD::swap(__end_, __t.__end_);
            _VSTD::swap(__end_cap(), __t.__end_cap());
        }
    }
    __alloc_traits::construct(__alloc(), _VSTD::__to_address(__end_),
                              _VSTD::forward<_Args>(__args)...);
    ++__end_;
}

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
void
swap(__split_buffer<_Tp, _Allocator>& __x, __split_buffer<_Tp, _Allocator>& __y)
        _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
{
    __x.swap(__y);
}

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP_SPLIT_BUFFER