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LibJS: Implement UTF-16 surrogate pair concatenation without iteration

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Performance of string concatenation regressed in a57e2f9. That commit
iterates over the LHS string to find the last code unit, to check if it
is a high surrogate. Instead, first look at the 3rd-to-last byte in the
UTF-8 encoded string to check if it is a 3-byte code point; then decode
just those bytes to check if we have a high surrogate. Similarly, check
the first 3 bytes of the RHS string to check if we have a low surrogate.
This commit is contained in:
Timothy Flynn 2022-01-17 22:02:50 -05:00 • committed by Linus Groh
parent 567b3a4810
commit 444b2d9ec2
1 changed files with 27 additions and 20 deletions
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47
Userland/Libraries/LibJS/Runtime/Value.cpp
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@ -995,31 +995,38 @@ static PrimitiveString* concatenate_strings(GlobalObject& global_object, Primiti
return js_string(vm, Utf16String(move(combined)));
}
Utf8View lhs_string { lhs.string() };
Utf8View rhs_string { rhs.string() };
auto const& lhs_string = lhs.string();
auto const& rhs_string = rhs.string();
StringBuilder builder(lhs_string.length() + rhs_string.length());
Optional<u16> high_surrogate;
for (auto it = lhs_string.begin(); it != lhs_string.end(); ++it) {
if (!it.peek(1).has_value() && Utf16View::is_high_surrogate(*it) && !rhs_string.is_empty())
high_surrogate = *it;
else
builder.append_code_point(*it);
}
auto return_combined_strings = [&]() {
builder.append(lhs_string);
builder.append(rhs_string);
return js_string(vm, builder.to_string());
};
if (high_surrogate.has_value()) {
auto low_surrogate = *rhs_string.begin();
// Surrogates encoded as UTF-8 are 3 bytes.
if ((lhs_string.length() < 3) || (rhs_string.length() < 3))
return return_combined_strings();
if (Utf16View::is_low_surrogate(low_surrogate)) {
builder.append_code_point(Utf16View::decode_surrogate_pair(*high_surrogate, low_surrogate));
rhs_string = rhs_string.substring_view(3); // A low surrogate encoded as UTF-8 is 3 bytes.
} else {
builder.append_code_point(*high_surrogate);
}
}
auto lhs_leading_byte = static_cast<u8>(lhs_string[lhs_string.length() - 3]);
auto rhs_leading_byte = static_cast<u8>(rhs_string[0]);
if ((lhs_leading_byte & 0xf0) != 0xe0)
return return_combined_strings();
if ((rhs_leading_byte & 0xf0) != 0xe0)
return return_combined_strings();
auto high_surrogate = *Utf8View(lhs_string.substring_view(lhs_string.length() - 3)).begin();
auto low_surrogate = *Utf8View(rhs_string).begin();
if (!Utf16View::is_high_surrogate(high_surrogate) || !Utf16View::is_low_surrogate(low_surrogate))
return return_combined_strings();
builder.append(lhs_string.substring_view(0, lhs_string.length() - 3));
builder.append_code_point(Utf16View::decode_surrogate_pair(high_surrogate, low_surrogate));
builder.append(rhs_string.substring_view(3));
builder.append(rhs_string.as_string());
return js_string(vm, builder.to_string());
}