Method utf8_remainder

include/pybind11/iostream.h:58–91

Computes how many bytes at the end of the buffer are part of an incomplete sequence of UTF-8 bytes. Precondition: pbase() < pptr()

    // incomplete sequence of UTF-8 bytes.
    // Precondition: pbase() < pptr()
    size_t utf8_remainder() const {
        const auto rbase = std::reverse_iterator<char *>(pbase());
        const auto rpptr = std::reverse_iterator<char *>(pptr());
        auto is_ascii = [](char c) { return (static_cast<unsigned char>(c) & 0x80) == 0x00; };
        auto is_leading = [](char c) { return (static_cast<unsigned char>(c) & 0xC0) == 0xC0; };
        auto is_leading_2b = [](char c) { return static_cast<unsigned char>(c) <= 0xDF; };
        auto is_leading_3b = [](char c) { return static_cast<unsigned char>(c) <= 0xEF; };
        // If the last character is ASCII, there are no incomplete code points
        if (is_ascii(*rpptr)) {
            return 0;
        }
        // Otherwise, work back from the end of the buffer and find the first
        // UTF-8 leading byte
        const auto rpend = rbase - rpptr >= 3 ? rpptr + 3 : rbase;
        const auto leading = std::find_if(rpptr, rpend, is_leading);
        if (leading == rbase) {
            return 0;
        }
        const auto dist = static_cast<size_t>(leading - rpptr);
        size_t remainder = 0;

        if (dist == 0) {
            remainder = 1; // 1-byte code point is impossible
        } else if (dist == 1) {
            remainder = is_leading_2b(*leading) ? 0 : dist + 1;
        } else if (dist == 2) {
            remainder = is_leading_3b(*leading) ? 0 : dist + 1;
        }
        // else if (dist >= 3), at least 4 bytes before encountering an UTF-8
        // leading byte, either no remainder or invalid UTF-8.
        // Invalid UTF-8 will cause an exception later when converting
        // to a Python string, so that's not handled here.
        return remainder;
    }

    // This function must be non-virtual to be called in a destructor.
    int _sync() {