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[libc++][format][6/7] Optimizes formatted_size. #101835

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[libc++][format][6/7] Optimizes formatted_size. #101835

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51 changes: 29 additions & 22 deletions libcxx/include/__format/buffer.h
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Original file line number Diff line number Diff line change
Expand Up @@ -603,28 +603,6 @@ class _LIBCPP_TEMPLATE_VIS __format_buffer {
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(0);
return __size_;
}

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

// ***** ***** ***** LLVM-20 classes ***** ***** *****

// A dynamically growing buffer.
Expand Down Expand Up @@ -817,6 +795,35 @@ class _LIBCPP_TEMPLATE_VIS __format_to_n_buffer : private __buffer_selector<_Out
__max_output_size __max_output_size_;
};

// A buffer that counts the number of insertions.
//
// Since 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 : private __output_buffer<_CharT> {
public:
using _Base = __output_buffer<_CharT>;

[[nodiscard]] _LIBCPP_HIDE_FROM_ABI __formatted_size_buffer()
: _Base{nullptr, 0, __prepare_write, std::addressof(__max_output_size_)} {}

[[nodiscard]] _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return _Base::__make_output_iterator(); }

// This function does not need to be r-value qualified, however this is
// consistent with similar objects.
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI size_t __result() && { return __max_output_size_.__code_units_written(); }

private:
__max_output_size __max_output_size_{0};

_LIBCPP_HIDE_FROM_ABI static void
__prepare_write([[maybe_unused]] __output_buffer<_CharT>& __buffer, [[maybe_unused]] size_t __size_hint) {
// Note this function does not satisfy the requirement of giving a 1 code unit buffer.
_LIBCPP_ASSERT_INTERNAL(
false, "Since __max_output_size_.__max_size_ == 0 there should never be call to this function.");
}
};

// ***** ***** ***** LLVM-19 and LLVM-20 class ***** ***** *****

// A dynamically growing buffer intended to be used for retargeting a context.
Expand Down