typedef struct Visitor {
    Tokens tokens;
    size_t current;
} Visitor;

Token
peek_token(const Visitor *visitor) {
    return visitor->tokens.buf[visitor->current];
}

Token
next_token(Visitor *visitor) {
    return visitor->tokens.buf[visitor->current++];
}

bool
has_next_token(const Visitor *visitor) {
    return visitor->current < visitor->tokens.size;
}

Object *
parse_fixnum(Token tok) {
    ssize_t num = 0;
    int sign = 1;
    for (size_t i = 0; i < tok.value.n; i++) {
        char c = tok.value.start[i];
        if (c == '-') {
            sign = -1;
            continue;
        }
        num = num * 10 + (c - '0');
    }
    return make_fixnum(num * sign);
}

Object * parse_tree(Visitor *vs);

Object *
parse_list(Visitor *vs) {
    Token tok = peek_token(vs);
    if (tok.type == TOKEN_EOF) {
        return obj_err;
    }
    Object *next_obj = parse_tree(vs);
    if (next_obj == obj_err) {
        return obj_err;
    }
    Object *root = make_pair(next_obj, obj_nil);
    Object *list = root;
    while (has_next_token(vs)) {
        Token tok = peek_token(vs);
        if (tok.type == TOKEN_RPAREN) {
            next_token(vs);
            break;
        }
        if (tok.type == TOKEN_EOF) {
            free_objects(root);
            return obj_err;
        }
        next_obj = parse_tree(vs);
        if (next_obj == obj_err) {
            free_objects(root);
            return obj_err;
        }
        list->cdr = make_pair(next_obj, obj_nil);
        list = list->cdr;
    }
    return root;
}

Object *
parse_tree(Visitor *vs) {
    Token tok = next_token(vs);
    switch (tok.type) {
        case TOKEN_FIXNUM: {
            return parse_fixnum(tok);
        } break;
        case TOKEN_TRUE: {
            return obj_true;
        } break;
        case TOKEN_FALSE: {
            return obj_false;
        } break;
        case TOKEN_RPAREN: {
            error_push((Error){
                .type = ERR_TYPE_PARSER,
                .value = ERR_UNBALANCED_PAREN,
                .line = tok.line,
                .col = tok.column,
            });
            return obj_err;
        } break;
        case TOKEN_QUOTE: {
            Object *quote_sym = make_symbol((StringView){"quote", 5});
            Object *next_obj = parse_tree(vs);
            if (next_obj == obj_err) {
                free_objects(quote_sym);
                return obj_err;
            }
            return make_pair(quote_sym, make_pair(next_obj, obj_nil));
        } break;
        case TOKEN_LPAREN: {
            Object *obj = parse_list(vs);
            if (obj == obj_err) {
                error_push((Error){
                    .type = ERR_TYPE_PARSER,
                    .value = ERR_UNBALANCED_PAREN,
                    .line = tok.line,
                    .col = tok.column,
                });
            }
            return obj;
        } break;
        case TOKEN_STRING: {
            Object *obj = make_string();
            append_string(obj, tok.value);
            return obj;
        } break;
        case TOKEN_SYMBOL: {
            return make_symbol(tok.value);
        } break;
        case TOKEN_NIL: {
            return obj_nil;
        } break;
        default: {
            break;
        } break;
    }
    error_push((Error){
        .type = ERR_TYPE_PARSER,
        .value = ERR_EOF_REACHED,
        .line = tok.line,
        .col = tok.column,
    });
    return obj_err;
}