class UTF16_File
{
    std::string FILENAME;
    UFILE* INPUT;

    UFILE* open() const {
        UFILE* file = u_fopen(FILENAME.c_str(),"r",nullptr,"UTF16");
        if( ! file ) throw std::invalid_argument("File couldn't open.");
        return file;
    }

    auto get_filesize() {
        if( fseek(u_fgetfile(INPUT),0,SEEK_END) != 0 )
            throw std::runtime_error(std::strerror(errno));
        auto position = ftell(u_fgetfile(INPUT));
        if( position == -1L )
            throw std::runtime_error(std::strerror(errno));
        u_frewind(INPUT);
        return position;
    }

    UTF16_File( const std::string& filename ) : FILENAME(filename),
        INPUT(open()) { }

    ~UTF16_File() {
        u_fclose(INPUT);
    }

    std::string str() {
        // The filesize is UTF-16 so it is double the size of the string needed
        auto length = get_filesize()/2;

        // Create a storage space for the file data
        UnicodeString buffer;
        UChar* bufmem = buffer.getBuffer(length);
        if( ! bufmem )
            throw std::runtime_error(std::strerror(errno));

        // Move the data into the storage space
        int32_t bytesread = u_file_read(bufmem,length,INPUT);
        if(bytesread < length-1) throw std::runtime_error(
            "File could not be read in its entirety!");
        buffer.releaseBuffer(bytesread);

        // Create a string to receive the converted data
        std::string retval;
        return buffer.toUTF8String(retval);
    }

    friend std::string UTF16_to_UTF8(const std::string&);
};

std::string UTF16_to_UTF8(const std::string& filename) {
    UTF16_File u16file(filename);
    return u16file.str();
}