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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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6
Libraries/LibJS/Runtime/AbstractOperations.cpp
View file

@ -1280,7 +1280,7 @@ ThrowCompletionOr<String> get_substitution(VM& vm, Utf16View const& matched, Utf
VERIFY(position <= string_length);
// 3. Let result be the empty String.
Utf16Data result;
StringBuilder result(StringBuilder::Mode::UTF16);
// 4. Let templateRemainder be replacementTemplate.
auto replace_template_string = TRY(replacement_template.to_utf16_string(vm));
@ -1451,7 +1451,7 @@ ThrowCompletionOr<String> get_substitution(VM& vm, Utf16View const& matched, Utf
auto ref_length = ref.length_in_code_units();
// k. Set result to the string-concatenation of result and refReplacement.
result.append(ref_replacement.utf16_span().data(), ref_replacement.length_in_code_units());
result.append(ref_replacement);
// j. Set templateRemainder to the substring of templateRemainder from refLength.
// NOTE: We do this step last because refReplacement may point to templateRemainder.
@ -1459,7 +1459,7 @@ ThrowCompletionOr<String> get_substitution(VM& vm, Utf16View const& matched, Utf
}
// 6. Return result.
return MUST(Utf16View { result }.to_utf8());
return MUST(result.utf16_string_view().to_utf8());
}
void DisposeCapability::visit_edges(GC::Cell::Visitor& visitor) const