[libc++][format][4/6] Improve formatted_size.

[Mordante]

Summary

Use a specialized "buffer" to count the number of insertions instead of
using a string as storage type.

Depends on D110497.

[Mordante]

Needs to be updated due to the removal of __put in D110495.

[Mordante]

Rebased and adjusted to changes in __put.

[Mordante]

Rebased to test with wchar_t changes.

[Mordante]

Rebased and make changes based on D112361.

[vitaut]

In general looks good, just a few minor comments inline. However, the discussion in earlier diffs may affect __formatted_size_buffer API.

libcxx/include/__format/buffer.h

271

Why not const _CharT*?

275

Why &&-qualified?

libcxx/include/format

511–531

I suggest adding __vformatted_size and force-inlining formatted_size to reduce template bloat.

Also why move this block?

[Mordante]

libcxx/include/__format/buffer.h

275

Not really required here, but to keep consistent with the other "retrieve result" functions in these buffers.

libcxx/include/format

511–531

It's moved so format_to_n can use formatted_size.

[Mordante]

Rebased and addresses last review comment.

[Mordante]

libcxx/include/format

511–531

Since formatted_size no longer depends on format_to_n I moved it to its original location.

[Mordante]

Rebased.

[ldionne]

libcxx/include/__format/buffer.h

262

libcxx/include/format

692

This applies to other free function calls in this diff as well.

[Mordante]

Rebase and address review comments.

libcxx/include/__format/buffer.h

 // -*- 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
 //
 //===----------------------------------------------------------------------===//
 
_LIBCPP_HIDE_FROM_ABI _OutIt out() && {
     return _VSTD::move(__writer_).out();
   }
 
 private:
   [[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 <__formatter::__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 __size) {
+    __size_ += __size;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI size_t result() && {
+    __output_.flush();
+    return __size_;
+  }
+
+private:
+  __internal_storage<_CharT> __storage_;
+  __output_buffer<_CharT> __output_{__storage_.begin(), __storage_.capacity(),
+                                    this};
+  size_t __size_{0};
+};
+
 } // namespace __format
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_BUFFER_H

libcxx/include/format

 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
 _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
 format(wstring_view __fmt, const _Args&... __args) {
   return _VSTD::vformat(__fmt, _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
 format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, string_view __fmt,
             const _Args&... __args) {
   // TODO FMT Improve PoC: using std::string is inefficient.
   string __str = _VSTD::vformat(__fmt, _VSTD::make_format_args(__args...));
   iter_difference_t<_OutIt> __s = __str.size();
   iter_difference_t<_OutIt> __m =
       _VSTD::clamp(__n, iter_difference_t<_OutIt>(0), __s);
   __out_it = _VSTD::copy_n(__str.begin(), __m, _VSTD::move(__out_it));
   return {_VSTD::move(__out_it), __s};
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <output_iterator<const wchar_t&> _OutIt, class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
 format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, wstring_view __fmt,
             const _Args&... __args) {
   // TODO FMT Improve PoC: using std::string is inefficient.
   wstring __str = _VSTD::vformat(__fmt, _VSTD::make_wformat_args(__args...));
   iter_difference_t<_OutIt> __s = __str.size();
   iter_difference_t<_OutIt> __m =
       _VSTD::clamp(__n, iter_difference_t<_OutIt>(0), __s);
   __out_it = _VSTD::copy_n(__str.begin(), __m, _VSTD::move(__out_it));
   return {_VSTD::move(__out_it), __s};
 }
 #endif
 
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI size_t __vformatted_size(basic_string_view<_CharT> __fmt, auto __args) {
+  __format::__formatted_size_buffer<_CharT> __buffer;
+  _VSTD::__format::__vformat_to(basic_format_parse_context{__fmt, __args.__size()},
+                                _VSTD::__format_context_create(__buffer.make_output_iterator(), __args));
+  return _VSTD::move(__buffer).result();
+}
+
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(string_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(__fmt, _VSTD::make_format_args(__args...)).size();
+_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t formatted_size(string_view __fmt,
+                                                                                              const _Args&... __args) {
+  return __vformatted_size(__fmt, basic_format_args{_VSTD::make_format_args(__args...)});
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(wstring_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(__fmt, _VSTD::make_wformat_args(__args...)).size();
+_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t formatted_size(wstring_view __fmt,
+                                                                                              const _Args&... __args) {
+  return __vformatted_size(__fmt, basic_format_args{_VSTD::make_wformat_args(__args...)});
 }
 #endif
 
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
 requires(output_iterator<_OutIt, const _CharT&>) _LIBCPP_HIDE_FROM_ABI _OutIt
     __vformat_to(
format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc,
   iter_difference_t<_OutIt> __s = __str.size();
   iter_difference_t<_OutIt> __m =
       _VSTD::clamp(__n, iter_difference_t<_OutIt>(0), __s);
   __out_it = _VSTD::copy_n(__str.begin(), __m, _VSTD::move(__out_it));
   return {_VSTD::move(__out_it), __s};
 }
 #endif
 
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI size_t __vformatted_size(locale __loc, basic_string_view<_CharT> __fmt, auto __args) {
+  __format::__formatted_size_buffer<_CharT> __buffer;
+  _VSTD::__format::__vformat_to(
+      basic_format_parse_context{__fmt, __args.__size()},
+      _VSTD::__format_context_create(__buffer.make_output_iterator(), __args, _VSTD::move(__loc)));
+  return _VSTD::move(__buffer).result();
+}
+
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, string_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt,
-                        _VSTD::make_format_args(__args...))
-      .size();
+_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t formatted_size(locale __loc,
+                                                                                              string_view __fmt,
+                                                                                              const _Args&... __args) {
+  return __vformatted_size(_VSTD::move(__loc), __fmt, basic_format_args{_VSTD::make_format_args(__args...)});
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, wstring_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt,
-                        _VSTD::make_wformat_args(__args...))
-      .size();
+_LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t formatted_size(locale __loc,
+                                                                                              wstring_view __fmt,
+                                                                                              const _Args&... __args) {
+  return __vformatted_size(_VSTD::move(__loc), __fmt, basic_format_args{_VSTD::make_wformat_args(__args...)});
 }
 #endif
 
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 #endif // _LIBCPP_FORMAT