xref: /freebsd-src/contrib/llvm-project/libcxx/include/__format/buffer.h (revision 06c3fb2749bda94cb5201f81ffdb8fa6c3161b2e)

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___FORMAT_BUFFER_H
#define _LIBCPP___FORMAT_BUFFER_H

#include <__algorithm/copy_n.h>
#include <__algorithm/fill_n.h>
#include <__algorithm/max.h>
#include <__algorithm/min.h>
#include <__algorithm/ranges_copy_n.h>
#include <__algorithm/transform.h>
#include <__algorithm/unwrap_iter.h>
#include <__concepts/same_as.h>
#include <__config>
#include <__format/concepts.h>
#include <__format/enable_insertable.h>
#include <__format/format_to_n_result.h>
#include <__iterator/back_insert_iterator.h>
#include <__iterator/concepts.h>
#include <__iterator/incrementable_traits.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/wrap_iter.h>
#include <__memory/addressof.h>
#include <__memory/allocate_at_least.h>
#include <__memory/allocator_traits.h>
#include <__memory/construct_at.h>
#include <__memory/ranges_construct_at.h>
#include <__memory/uninitialized_algorithms.h>
#include <__type_traits/add_pointer.h>
#include <__type_traits/conditional.h>
#include <__utility/exception_guard.h>
#include <__utility/move.h>
#include <cstddef>
#include <string_view>

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

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 20

namespace __format {

/// A "buffer" that handles writing to the proper iterator.
///
/// This helper is used together with the @ref back_insert_iterator to offer
/// type-erasure for the formatting functions. This reduces the number to
/// template instantiations.
template <__fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __output_buffer {
public:
  using value_type = _CharT;

  template <class _Tp>
  _LIBCPP_HIDE_FROM_ABI explicit __output_buffer(_CharT* __ptr, size_t __capacity, _Tp* __obj)
      : __ptr_(__ptr),
        __capacity_(__capacity),
        __flush_([](_CharT* __p, size_t __n, void* __o) { static_cast<_Tp*>(__o)->__flush(__p, __n); }),
        __obj_(__obj) {}

  _LIBCPP_HIDE_FROM_ABI void __reset(_CharT* __ptr, size_t __capacity) {
    __ptr_ = __ptr;
    __capacity_ = __capacity;
  }

  _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return std::back_insert_iterator{*this}; }

  // Used in std::back_insert_iterator.
  _LIBCPP_HIDE_FROM_ABI void push_back(_CharT __c) {
    __ptr_[__size_++] = __c;

    // Profiling showed flushing after adding is more efficient than flushing
    // when entering the function.
    if (__size_ == __capacity_)
      __flush();
  }

  /// Copies the input __str to the buffer.
  ///
  /// Since some of the input is generated by std::to_chars, there needs to be a
  /// conversion when _CharT is wchar_t.
  template <__fmt_char_type _InCharT>
  _LIBCPP_HIDE_FROM_ABI void __copy(basic_string_view<_InCharT> __str) {
    // When the underlying iterator is a simple iterator the __capacity_ is
    // infinite. For a string or container back_inserter it isn't. This means
    // adding a large string the the buffer can cause some overhead. In that
    // case a better approach could be:
    // - flush the buffer
    // - container.append(__str.begin(), __str.end());
    // The same holds true for the fill.
    // For transform it might be slightly harder, however the use case for
    // transform is slightly less common; it converts hexadecimal values to
    // upper case. For integral these strings are short.
    // TODO FMT Look at the improvements above.
    size_t __n = __str.size();

    __flush_on_overflow(__n);
    if (__n < __capacity_) { //  push_back requires the buffer to have room for at least one character (so use <).
      _VSTD::copy_n(__str.data(), __n, _VSTD::addressof(__ptr_[__size_]));
      __size_ += __n;
      return;
    }

    // The output doesn't fit in the internal buffer.
    // Copy the data in "__capacity_" sized chunks.
    _LIBCPP_ASSERT_UNCATEGORIZED(__size_ == 0, "the buffer should be flushed by __flush_on_overflow");
    const _InCharT* __first = __str.data();
    do {
      size_t __chunk = _VSTD::min(__n, __capacity_);
      _VSTD::copy_n(__first, __chunk, _VSTD::addressof(__ptr_[__size_]));
      __size_ = __chunk;
      __first += __chunk;
      __n -= __chunk;
      __flush();
    } while (__n);
  }

  /// A std::transform wrapper.
  ///
  /// Like @ref __copy it may need to do type conversion.
  template <__fmt_char_type _InCharT, class _UnaryOperation>
  _LIBCPP_HIDE_FROM_ABI void __transform(const _InCharT* __first, const _InCharT* __last, _UnaryOperation __operation) {
    _LIBCPP_ASSERT_UNCATEGORIZED(__first <= __last, "not a valid range");

    size_t __n = static_cast<size_t>(__last - __first);
    __flush_on_overflow(__n);
    if (__n < __capacity_) { //  push_back requires the buffer to have room for at least one character (so use <).
      _VSTD::transform(__first, __last, _VSTD::addressof(__ptr_[__size_]), _VSTD::move(__operation));
      __size_ += __n;
      return;
    }

    // The output doesn't fit in the internal buffer.
    // Transform the data in "__capacity_" sized chunks.
    _LIBCPP_ASSERT_UNCATEGORIZED(__size_ == 0, "the buffer should be flushed by __flush_on_overflow");
    do {
      size_t __chunk = _VSTD::min(__n, __capacity_);
      _VSTD::transform(__first, __first + __chunk, _VSTD::addressof(__ptr_[__size_]), __operation);
      __size_ = __chunk;
      __first += __chunk;
      __n -= __chunk;
      __flush();
    } while (__n);
  }

  /// A \c fill_n wrapper.
  _LIBCPP_HIDE_FROM_ABI void __fill(size_t __n, _CharT __value) {
    __flush_on_overflow(__n);
    if (__n < __capacity_) { //  push_back requires the buffer to have room for at least one character (so use <).
      _VSTD::fill_n(_VSTD::addressof(__ptr_[__size_]), __n, __value);
      __size_ += __n;
      return;
    }

    // The output doesn't fit in the internal buffer.
    // Fill the buffer in "__capacity_" sized chunks.
    _LIBCPP_ASSERT_UNCATEGORIZED(__size_ == 0, "the buffer should be flushed by __flush_on_overflow");
    do {
      size_t __chunk = _VSTD::min(__n, __capacity_);
      _VSTD::fill_n(_VSTD::addressof(__ptr_[__size_]), __chunk, __value);
      __size_ = __chunk;
      __n -= __chunk;
      __flush();
    } while (__n);
  }

  _LIBCPP_HIDE_FROM_ABI void __flush() {
    __flush_(__ptr_, __size_, __obj_);
    __size_ = 0;
  }

private:
  _CharT* __ptr_;
  size_t __capacity_;
  size_t __size_{0};
  void (*__flush_)(_CharT*, size_t, void*);
  void* __obj_;

  /// Flushes the buffer when the output operation would overflow the buffer.
  ///
  /// A simple approach for the overflow detection would be something along the
  /// lines:
  /// \code
  /// // The internal buffer is large enough.
  /// if (__n <= __capacity_) {
  ///   // Flush when we really would overflow.
  ///   if (__size_ + __n >= __capacity_)
  ///     __flush();
  ///   ...
  /// }
  /// \endcode
  ///
  /// This approach works for all cases but one:
  /// A __format_to_n_buffer_base where \ref __enable_direct_output is true.
  /// In that case the \ref __capacity_ of the buffer changes during the first
  /// \ref __flush. During that operation the output buffer switches from its
  /// __writer_ to its __storage_. The \ref __capacity_ of the former depends
  /// on the value of n, of the latter is a fixed size. For example:
  /// - a format_to_n call with a 10'000 char buffer,
  /// - the buffer is filled with 9'500 chars,
  /// - adding 1'000 elements would overflow the buffer so the buffer gets
  ///   changed and the \ref __capacity_ decreases from 10'000 to
  ///   __buffer_size (256 at the time of writing).
  ///
  /// This means that the \ref __flush for this class may need to copy a part of
  /// the internal buffer to the proper output. In this example there will be
  /// 500 characters that need this copy operation.
  ///
  /// Note it would be more efficient to write 500 chars directly and then swap
  /// the buffers. This would make the code more complex and \ref format_to_n is
  /// not the most common use case. Therefore the optimization isn't done.
  _LIBCPP_HIDE_FROM_ABI void __flush_on_overflow(size_t __n) {
    if (__size_ + __n >= __capacity_)
      __flush();
  }
};

/// A storage using an internal buffer.
///
/// This storage is used when writing a single element to the output iterator
/// is expensive.
template <__fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __internal_storage {
public:
  _LIBCPP_HIDE_FROM_ABI _CharT* __begin() { return __buffer_; }

  static constexpr size_t __buffer_size = 256 / sizeof(_CharT);

private:
  _CharT __buffer_[__buffer_size];
};

/// A storage writing directly to the storage.
///
/// This requires the storage to be a contiguous buffer of \a _CharT.
/// Since the output is directly written to the underlying storage this class
/// is just an empty class.
template <__fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __direct_storage {};

template <class _OutIt, class _CharT>
concept __enable_direct_output = __fmt_char_type<_CharT> &&
    (same_as<_OutIt, _CharT*>
     // TODO(hardening): the following check might not apply to hardened iterators and might need to be wrapped in an
     // `#ifdef`.
     || same_as<_OutIt, __wrap_iter<_CharT*>>
    );

/// Write policy for directly writing to the underlying output.
template <class _OutIt, __fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __writer_direct {
public:
  _LIBCPP_HIDE_FROM_ABI explicit __writer_direct(_OutIt __out_it)
      : __out_it_(__out_it) {}

  _LIBCPP_HIDE_FROM_ABI _OutIt __out_it() { return __out_it_; }

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT*, size_t __n) {
    // _OutIt can be a __wrap_iter<CharT*>. Therefore the original iterator
    // is adjusted.
    __out_it_ += __n;
  }

private:
  _OutIt __out_it_;
};

/// Write policy for copying the buffer to the output.
template <class _OutIt, __fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __writer_iterator {
public:
  _LIBCPP_HIDE_FROM_ABI explicit __writer_iterator(_OutIt __out_it)
      : __out_it_{_VSTD::move(__out_it)} {}

  _LIBCPP_HIDE_FROM_ABI _OutIt __out_it() && { return std::move(__out_it_); }

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT* __ptr, size_t __n) {
    __out_it_ = std::ranges::copy_n(__ptr, __n, std::move(__out_it_)).out;
  }

private:
  _OutIt __out_it_;
};

/// Concept to see whether a \a _Container is insertable.
///
/// The concept is used to validate whether multiple calls to a
/// \ref back_insert_iterator can be replace by a call to \c _Container::insert.
///
/// \note a \a _Container needs to opt-in to the concept by specializing
/// \ref __enable_insertable.
template <class _Container>
concept __insertable =
    __enable_insertable<_Container> && __fmt_char_type<typename _Container::value_type> &&
    requires(_Container& __t, add_pointer_t<typename _Container::value_type> __first,
             add_pointer_t<typename _Container::value_type> __last) { __t.insert(__t.end(), __first, __last); };

/// Extract the container type of a \ref back_insert_iterator.
template <class _It>
struct _LIBCPP_TEMPLATE_VIS __back_insert_iterator_container {
  using type = void;
};

template <__insertable _Container>
struct _LIBCPP_TEMPLATE_VIS __back_insert_iterator_container<back_insert_iterator<_Container>> {
  using type = _Container;
};

/// Write policy for inserting the buffer in a container.
template <class _Container>
class _LIBCPP_TEMPLATE_VIS __writer_container {
public:
  using _CharT = typename _Container::value_type;

  _LIBCPP_HIDE_FROM_ABI explicit __writer_container(back_insert_iterator<_Container> __out_it)
      : __container_{__out_it.__get_container()} {}

  _LIBCPP_HIDE_FROM_ABI auto __out_it() { return std::back_inserter(*__container_); }

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT* __ptr, size_t __n) {
    __container_->insert(__container_->end(), __ptr, __ptr + __n);
  }

private:
  _Container* __container_;
};

/// Selects the type of the writer used for the output iterator.
template <class _OutIt, class _CharT>
class _LIBCPP_TEMPLATE_VIS __writer_selector {
  using _Container = typename __back_insert_iterator_container<_OutIt>::type;

public:
  using type = conditional_t<!same_as<_Container, void>, __writer_container<_Container>,
                             conditional_t<__enable_direct_output<_OutIt, _CharT>, __writer_direct<_OutIt, _CharT>,
                                           __writer_iterator<_OutIt, _CharT>>>;
};

/// The generic formatting buffer.
template <class _OutIt, __fmt_char_type _CharT>
requires(output_iterator<_OutIt, const _CharT&>) class _LIBCPP_TEMPLATE_VIS
    __format_buffer {
  using _Storage =
      conditional_t<__enable_direct_output<_OutIt, _CharT>,
                    __direct_storage<_CharT>, __internal_storage<_CharT>>;

public:
  _LIBCPP_HIDE_FROM_ABI explicit __format_buffer(_OutIt __out_it)
    requires(same_as<_Storage, __internal_storage<_CharT>>)
      : __output_(__storage_.__begin(), __storage_.__buffer_size, this), __writer_(_VSTD::move(__out_it)) {}

  _LIBCPP_HIDE_FROM_ABI explicit __format_buffer(_OutIt __out_it) requires(
      same_as<_Storage, __direct_storage<_CharT>>)
      : __output_(_VSTD::__unwrap_iter(__out_it), size_t(-1), this),
        __writer_(_VSTD::move(__out_it)) {}

  _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return __output_.__make_output_iterator(); }

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT* __ptr, size_t __n) { __writer_.__flush(__ptr, __n); }

  _LIBCPP_HIDE_FROM_ABI _OutIt __out_it() && {
    __output_.__flush();
    return _VSTD::move(__writer_).__out_it();
  }

private:
  _LIBCPP_NO_UNIQUE_ADDRESS _Storage __storage_;
  __output_buffer<_CharT> __output_;
  typename __writer_selector<_OutIt, _CharT>::type __writer_;
};

/// A buffer that counts the number of insertions.
///
/// Since \ref formatted_size only needs to know the size, the output itself is
/// discarded.
template <__fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __formatted_size_buffer {
public:
  _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return __output_.__make_output_iterator(); }

  _LIBCPP_HIDE_FROM_ABI void __flush(const _CharT*, size_t __n) { __size_ += __n; }

  _LIBCPP_HIDE_FROM_ABI size_t __result() && {
    __output_.__flush();
    return __size_;
  }

private:
  __internal_storage<_CharT> __storage_;
  __output_buffer<_CharT> __output_{__storage_.__begin(), __storage_.__buffer_size, this};
  size_t __size_{0};
};

/// The base of a buffer that counts and limits the number of insertions.
template <class _OutIt, __fmt_char_type _CharT, bool>
  requires(output_iterator<_OutIt, const _CharT&>)
struct _LIBCPP_TEMPLATE_VIS __format_to_n_buffer_base {
  using _Size = iter_difference_t<_OutIt>;

public:
  _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer_base(_OutIt __out_it, _Size __max_size)
      : __writer_(_VSTD::move(__out_it)), __max_size_(_VSTD::max(_Size(0), __max_size)) {}

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT* __ptr, size_t __n) {
    if (_Size(__size_) <= __max_size_)
      __writer_.__flush(__ptr, _VSTD::min(_Size(__n), __max_size_ - __size_));
    __size_ += __n;
  }

protected:
  __internal_storage<_CharT> __storage_;
  __output_buffer<_CharT> __output_{__storage_.__begin(), __storage_.__buffer_size, this};
  typename __writer_selector<_OutIt, _CharT>::type __writer_;

  _Size __max_size_;
  _Size __size_{0};
};

/// The base of a buffer that counts and limits the number of insertions.
///
/// This version is used when \c __enable_direct_output<_OutIt, _CharT> == true.
///
/// This class limits the size available to the direct writer so it will not
/// exceed the maximum number of code units.
template <class _OutIt, __fmt_char_type _CharT>
  requires(output_iterator<_OutIt, const _CharT&>)
class _LIBCPP_TEMPLATE_VIS __format_to_n_buffer_base<_OutIt, _CharT, true> {
  using _Size = iter_difference_t<_OutIt>;

public:
  _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer_base(_OutIt __out_it, _Size __max_size)
      : __output_(_VSTD::__unwrap_iter(__out_it), __max_size, this),
        __writer_(_VSTD::move(__out_it)),
        __max_size_(__max_size) {
    if (__max_size <= 0) [[unlikely]]
      __output_.__reset(__storage_.__begin(), __storage_.__buffer_size);
  }

  _LIBCPP_HIDE_FROM_ABI void __flush(_CharT* __ptr, size_t __n) {
    // A __flush to the direct writer happens in the following occasions:
    // - The format function has written the maximum number of allowed code
    //   units. At this point it's no longer valid to write to this writer. So
    //   switch to the internal storage. This internal storage doesn't need to
    //   be written anywhere so the __flush for that storage writes no output.
    // - Like above, but the next "mass write" operation would overflow the
    //   buffer. In that case the buffer is pre-emptively switched. The still
    //   valid code units will be written separately.
    // - The format_to_n function is finished. In this case there's no need to
    //   switch the buffer, but for simplicity the buffers are still switched.
    // When the __max_size <= 0 the constructor already switched the buffers.
    if (__size_ == 0 && __ptr != __storage_.__begin()) {
      __writer_.__flush(__ptr, __n);
      __output_.__reset(__storage_.__begin(), __storage_.__buffer_size);
    } else if (__size_ < __max_size_) {
      // Copies a part of the internal buffer to the output up to n characters.
      // See __output_buffer<_CharT>::__flush_on_overflow for more information.
      _Size __s = _VSTD::min(_Size(__n), __max_size_ - __size_);
      std::copy_n(__ptr, __s, __writer_.__out_it());
      __writer_.__flush(__ptr, __s);
    }

    __size_ += __n;
  }

protected:
  __internal_storage<_CharT> __storage_;
  __output_buffer<_CharT> __output_;
  __writer_direct<_OutIt, _CharT> __writer_;

  _Size __max_size_;
  _Size __size_{0};
};

/// The buffer that counts and limits the number of insertions.
template <class _OutIt, __fmt_char_type _CharT>
  requires(output_iterator<_OutIt, const _CharT&>)
struct _LIBCPP_TEMPLATE_VIS __format_to_n_buffer final
    : public __format_to_n_buffer_base< _OutIt, _CharT, __enable_direct_output<_OutIt, _CharT>> {
  using _Base = __format_to_n_buffer_base<_OutIt, _CharT, __enable_direct_output<_OutIt, _CharT>>;
  using _Size = iter_difference_t<_OutIt>;

public:
  _LIBCPP_HIDE_FROM_ABI explicit __format_to_n_buffer(_OutIt __out_it, _Size __max_size)
      : _Base(_VSTD::move(__out_it), __max_size) {}
  _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return this->__output_.__make_output_iterator(); }

  _LIBCPP_HIDE_FROM_ABI format_to_n_result<_OutIt> __result() && {
    this->__output_.__flush();
    return {_VSTD::move(this->__writer_).__out_it(), this->__size_};
  }
};

// A dynamically growing buffer intended to be used for retargeting a context.
//
// P2286 Formatting ranges adds range formatting support. It allows the user to
// specify the minimum width for the entire formatted range.  The width of the
// range is not known until the range is formatted. Formatting is done to an
// output_iterator so there's no guarantee it would be possible to add the fill
// to the front of the output. Instead the range is formatted to a temporary
// buffer and that buffer is formatted as a string.
//
// There is an issue with that approach, the format context used in
// std::formatter<T>::format contains the output iterator used as part of its
// type. So using this output iterator means there needs to be a new format
// context and the format arguments need to be retargeted to the new context.
// This retargeting is done by a basic_format_context specialized for the
// __iterator of this container.
//
// This class uses its own buffer management, since using vector
// would lead to a circular include with formatter for vector<bool>.
template <__fmt_char_type _CharT>
class _LIBCPP_TEMPLATE_VIS __retarget_buffer {
  using _Alloc = allocator<_CharT>;

public:
  using value_type = _CharT;

  struct __iterator {
    using difference_type = ptrdiff_t;
    using value_type      = _CharT;

    _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(__retarget_buffer& __buffer)
        : __buffer_(std::addressof(__buffer)) {}
    _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator=(const _CharT& __c) {
      __buffer_->push_back(__c);
      return *this;
    }
    _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator=(_CharT&& __c) {
      __buffer_->push_back(__c);
      return *this;
    }

    _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator*() { return *this; }
    _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator++() { return *this; }
    _LIBCPP_HIDE_FROM_ABI constexpr __iterator operator++(int) { return *this; }
    __retarget_buffer* __buffer_;
  };

  __retarget_buffer(const __retarget_buffer&)            = delete;
  __retarget_buffer& operator=(const __retarget_buffer&) = delete;

  _LIBCPP_HIDE_FROM_ABI explicit __retarget_buffer(size_t __size_hint) {
    // When the initial size is very small a lot of resizes happen
    // when elements added. So use a hard-coded minimum size.
    //
    // Note a size < 2 will not work
    // - 0 there is no buffer, while push_back requires 1 empty element.
    // - 1 multiplied by the grow factor is 1 and thus the buffer never
    //   grows.
    auto __result = std::__allocate_at_least(__alloc_, std::max(__size_hint, 256 / sizeof(_CharT)));
    __ptr_        = __result.ptr;
    __capacity_   = __result.count;
  }

  _LIBCPP_HIDE_FROM_ABI ~__retarget_buffer() {
    ranges::destroy_n(__ptr_, __size_);
    allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __capacity_);
  }

  _LIBCPP_HIDE_FROM_ABI __iterator __make_output_iterator() { return __iterator{*this}; }

  _LIBCPP_HIDE_FROM_ABI void push_back(_CharT __c) {
    std::construct_at(__ptr_ + __size_, __c);
    ++__size_;

    if (__size_ == __capacity_)
      __grow_buffer();
  }

  template <__fmt_char_type _InCharT>
  _LIBCPP_HIDE_FROM_ABI void __copy(basic_string_view<_InCharT> __str) {
    size_t __n = __str.size();
    if (__size_ + __n >= __capacity_)
      // Push_back requires the buffer to have room for at least one character.
      __grow_buffer(__size_ + __n + 1);

    std::uninitialized_copy_n(__str.data(), __n, __ptr_ + __size_);
    __size_ += __n;
  }

  template <__fmt_char_type _InCharT, class _UnaryOperation>
  _LIBCPP_HIDE_FROM_ABI void __transform(const _InCharT* __first, const _InCharT* __last, _UnaryOperation __operation) {
    _LIBCPP_ASSERT_UNCATEGORIZED(__first <= __last, "not a valid range");

    size_t __n = static_cast<size_t>(__last - __first);
    if (__size_ + __n >= __capacity_)
      // Push_back requires the buffer to have room for at least one character.
      __grow_buffer(__size_ + __n + 1);

    std::uninitialized_default_construct_n(__ptr_ + __size_, __n);
    std::transform(__first, __last, __ptr_ + __size_, std::move(__operation));
    __size_ += __n;
  }

  _LIBCPP_HIDE_FROM_ABI void __fill(size_t __n, _CharT __value) {
    if (__size_ + __n >= __capacity_)
      // Push_back requires the buffer to have room for at least one character.
      __grow_buffer(__size_ + __n + 1);

    std::uninitialized_fill_n(__ptr_ + __size_, __n, __value);
    __size_ += __n;
  }

  _LIBCPP_HIDE_FROM_ABI basic_string_view<_CharT> __view() { return {__ptr_, __size_}; }

private:
  _LIBCPP_HIDE_FROM_ABI void __grow_buffer() { __grow_buffer(__capacity_ * 1.6); }

  _LIBCPP_HIDE_FROM_ABI void __grow_buffer(size_t __capacity) {
    _LIBCPP_ASSERT_UNCATEGORIZED(__capacity > __capacity_, "the buffer must grow");
    auto __result = std::__allocate_at_least(__alloc_, __capacity);
    auto __guard  = std::__make_exception_guard([&] {
      allocator_traits<_Alloc>::deallocate(__alloc_, __result.ptr, __result.count);
    });
    // This shouldn't throw, but just to be safe. Note that at -O1 this
    // guard is optimized away so there is no runtime overhead.
    std::uninitialized_move_n(__ptr_, __size_, __result.ptr);
    __guard.__complete();
    ranges::destroy_n(__ptr_, __size_);
    allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __capacity_);

    __ptr_      = __result.ptr;
    __capacity_ = __result.count;
  }
  _LIBCPP_NO_UNIQUE_ADDRESS _Alloc __alloc_;
  _CharT* __ptr_;
  size_t __capacity_;
  size_t __size_{0};
};

} // namespace __format

#endif //_LIBCPP_STD_VER >= 20

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___FORMAT_BUFFER_H