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AK: Support UTF-16 string formatting

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The underlying storage used during string formatting is StringBuilder.
To support UTF-16 strings, this patch allows callers to specify a mode
during StringBuilder construction. The default mode is UTF-8, for which
StringBuilder remains unchanged.

In UTF-16 mode, we treat the StringBuilder's internal ByteBuffer as a
series of u16 code units. Appending a single character will append 2
bytes for that character (cast to a char16_t). Appending a StringView
will transcode the string to UTF-16.

Utf16String also gains the same memory optimization that we added for
String, where we hand-off the underlying buffer to Utf16String to avoid
having to re-allocate.

In the future, we may want to further optimize for ASCII strings. For
example, we could defer committing to the u16-esque storage until we
see a non-ASCII code point.
This commit is contained in:
Timothy Flynn 2025-06-17 16:08:30 -04:00 • committed by Tim Flynn
parent fe676585f5
commit 2803d66d87
Notes: github-actions[bot] 2025-07-18 16:47:24 +00:00
11 changed files with 362 additions and 55 deletions
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11
Libraries/LibWeb/DOM/CharacterData.cpp
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@ -87,12 +87,11 @@ WebIDL::ExceptionOr<void> CharacterData::replace_data(size_t offset, size_t coun
auto inserted_data_result = MUST(AK::utf8_to_utf16(data));
auto after_data = utf16_view.substring_view(offset + count);
Utf16Data full_data;
full_data.ensure_capacity(before_data.length_in_code_units() + inserted_data_result.data.size() + after_data.length_in_code_units());
full_data.append(before_data.utf16_span().data(), before_data.length_in_code_units());
full_data.extend(inserted_data_result.data);
full_data.append(after_data.utf16_span().data(), after_data.length_in_code_units());
Utf16View full_view { full_data };
StringBuilder full_data(StringBuilder::Mode::UTF16, before_data.length_in_code_units() + inserted_data_result.data.size() + after_data.length_in_code_units());
full_data.append(before_data);
full_data.append(inserted_data_result.data);
full_data.append(after_data);
auto full_view = full_data.utf16_string_view();
bool characters_are_the_same = utf16_view == full_view;
auto old_data = m_data;