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LibJS: Decode UTF-16 surrogate pairs during string literal construction

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Rather than deferring this decoding to PrimitiveString, we can decode
surrogate pairs when parsing the string. This prevents a string copy
when constructing the PrimitiveString.
This commit is contained in:
Timothy Flynn 2021-08-02 10:06:42 -04:00 • committed by Andreas Kling
parent 27d555bab0
commit 4c2cc419f9
2 changed files with 28 additions and 14 deletions
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11
Userland/Libraries/LibJS/Runtime/PrimitiveString.cpp
View file

@ -63,17 +63,6 @@ PrimitiveString* js_string(Heap& heap, String string)
return &heap.vm().single_ascii_character_string(ch);
}
// UTF-8 strings must first be transcoded to UTF-16, even though they are stored as String objects
// internally, to parse encoded surrogate pairs. As an optimization to reduce string copying, only
// perform that transcoding if there are non-ASCII codepoints in the string.
for (auto it : string) {
auto ch = static_cast<u8>(it);
if (!is_ascii(ch)) {
auto utf16_string = AK::utf8_to_utf16(string);
return js_string(heap, Utf16View { utf16_string });
}
}
return heap.allocate_without_global_object<PrimitiveString>(move(string));
}

31
Userland/Libraries/LibJS/Token.cpp
View file

@ -10,6 +10,7 @@
#include <AK/CharacterTypes.h>
#include <AK/GenericLexer.h>
#include <AK/StringBuilder.h>
#include <AK/Utf16View.h>
namespace JS {
@ -102,6 +103,16 @@ String Token::string_value(StringValueStatus& status) const
return {};
};
auto decode_surrogate = [&lexer]() -> Optional<u16> {
u16 surrogate = 0;
for (int j = 0; j < 4; ++j) {
if (!lexer.next_is(is_ascii_hex_digit))
return {};
surrogate = (surrogate << 4u) | hex2int(lexer.consume());
}
return surrogate;
};
StringBuilder builder;
while (!lexer.is_eof()) {
// No escape, consume one char and continue
@ -157,10 +168,24 @@ String Token::string_value(StringValueStatus& status) const
}
lexer.ignore();
} else {
for (int j = 0; j < 4; ++j) {
if (!lexer.next_is(is_ascii_hex_digit))
auto high_surrogate = decode_surrogate();
if (!high_surrogate.has_value())
return encoding_failure(StringValueStatus::MalformedUnicodeEscape);
code_point = (code_point << 4u) | hex2int(lexer.consume());
if (Utf16View::is_high_surrogate(*high_surrogate) && lexer.consume_specific("\\u"sv)) {
auto low_surrogate = decode_surrogate();
if (!low_surrogate.has_value())
return encoding_failure(StringValueStatus::MalformedUnicodeEscape);
if (Utf16View::is_low_surrogate(*low_surrogate)) {
code_point = Utf16View::decode_surrogate_pair(*high_surrogate, *low_surrogate);
} else {
builder.append_code_point(*high_surrogate);
code_point = *low_surrogate;
}
} else {
code_point = *high_surrogate;
}
}
builder.append_code_point(code_point);