@Override
    public void transferByteRangeTo(Output output, boolean utf8String, int fieldNumber,
            boolean repeated) throws IOException
    {
        final int length = readRawVarint32();
        if (length < 0)
            throw ProtobufException.negativeSize();

        output.writeByteRange(utf8String, fieldNumber, buffer, offset, length, repeated);

        offset += length;
    }
Read a 64-bit little-endian integer from the internal byte buffer.
protostuff-core/src/main/java/io/protostuff/ByteArrayInput.java