in src/codec/string.cpp [87:95]
void encode_utf8(std::string &out, unsigned p) {
if (json_likely(p <= 0x7F)) {
encode_utf8_1(out, p);
} else if (json_likely(p <= 0x07FF)) {
encode_utf8_2(out, p);
} else {
encode_utf8_3(out, p);
}
}