expr = expr + ident    // r1
     | ident           // r2

  | ident | +  | $ | expr |
0 |  s1   |    |   |  2   |
1 |  r2   | r2 | r2|      |
2 |       | s3 |acc|      |
3 |  s4   |    |   |  2   |
4 |  r1   | r1 | r1|      |

struct result {
    symbol nonterm;
    int tokens_to_reduce;
}

result state0(token* stream) {
    result res;
    switch(stream[0]) {
    case ident:
        res = state1(stream+1);
        break;
    default: throw error();
    }
    while(true) {
        if(res.tokens_to_reduce == 0) {
            switch(res.nonterm) {
            case expr:
                res = state2(stream);
                break;
            default: throw error();
            }
        }
        else return { res.non_term, res.tokens_to_reduce-1 };
    }
}

result state1(token* stream) {
    reduce2(); // needs arguments
    return { expr, 0 };
}

result state2(token* stream) {
    result res;
    switch(stream[0]) {
    case plus:
        res = state3(stream+1);
        break;
    case end:
        return accept(); // needs some work
    default: throw error();
    }
}

result state3(token* stream) {
    result res;
    switch(stream[0]) {
    case ident:
        res = state4(stream+1);
        break;
    default: throw error();
    }
    while(true) {
        if(res.tokens_to_reduce == 0) {
            switch(res.nonterm) {
            case expr:
                res = state2(stream);
                break;
            default: throw error();
            }
        }
        else return { res.non_term, res.tokens_to_reduce-1 };
    }
}

result state4(token* stream) {
    reduce1(); // needs arguments
    return { expr, 1 };
}