read

std::shared_ptr<Object> Read(Tokenizer* tokenizer);

std::shared_ptr<Cell> ReadList(Tokenizer* tokenizer) {
    if (tokenizer->IsEnd()) {
        throw SyntaxError("nothing more to parse");
    }
    auto t = tokenizer->GetToken();
    // empty list
    if (t.index() == 1 && (std::get<BracketToken>(t) == BracketToken::CLOSE)) {
        tokenizer->Next();
        return nullptr;
    }
    //not empty list or pair
    auto first_elem = Read(tokenizer);
    if (tokenizer->IsEnd()) {
        throw SyntaxError("list unexpectedly finished");
    }
    std::vector<std::shared_ptr<Cell>> nodes;
    std::shared_ptr<Cell> lst(new Cell(first_elem, nullptr));
    nodes.push_back(lst);
    while(true) {
        auto nxt = tokenizer->GetToken();
        //end of list
        if (nxt.index() == 1 && (std::get<BracketToken>(nxt) == BracketToken::CLOSE)) {
            tokenizer->Next();
            break;
        //pair
        } else if (nxt.index() == 4) {
            auto last = nodes.back();
            nodes.pop_back();
            tokenizer->Next(); //next after a dot
            auto second_elem = Read(tokenizer);
            std::shared_ptr<Cell> p(new Cell(last->head_, second_elem));
            auto br = tokenizer->GetToken();
            if (br.index() != 1 || (std::get<BracketToken>(br) != BracketToken::CLOSE)) {
                throw SyntaxError("invalid pair syntax");
            }
            nodes.push_back(p);
        } else {
            auto second_elem = Read(tokenizer);
            nodes.emplace_back(new Cell(second_elem, nullptr));
        }
    }
    auto rec = nodes.back();
    while (nodes.size() > 1) {
        nodes.pop_back();
        nodes.back()->tail_ = rec;
        rec = nodes.back();
    }
    return nodes.back();
}

std::shared_ptr<Object> Read(Tokenizer* tokenizer) {
    if (tokenizer->IsEnd()) {
        throw SyntaxError("nothing to parse");
    }
    auto t = tokenizer->GetToken();
    tokenizer->Next();
    if (t.index() == 0) {
        return std::shared_ptr<Number>(new Number(std::get<ConstantToken>(t).value_));
    } else if (t.index() == 1) {
        switch(std::get<BracketToken>(t)) {
            case BracketToken::CLOSE : throw SyntaxError("unexpected )");
            case BracketToken::OPEN : {
                std::cout << "reading a list: \n";
                return ReadList(tokenizer);
            }
        }
    } else if (t.index() == 2) {
        return std::shared_ptr<Symbol>(new Symbol(std::get<SymbolToken>(t).name_));
    } else if (t.index() == 3) {
        throw SyntaxError("unexpected '");
    } else if (t.index() == 4) {
        throw SyntaxError("unexpected .");
    }
}