这几天在研究编译器，为了熟悉一下 FSA 在 C 语言中应该怎么实现，做了一些练习。我发现被很多代码规范所不齿的 goto 语句貌似在写状态机的时候非常有用，能够写出可读性很高的代码。如果不使用 goto，必然会引入一个状态标志，还有一大堆 switch/case，可读性反而下降了。

不妨想想看在 Ruby 中应该怎么实现？应该用些什么技巧才能避免一大堆判断语句呢？我对元编程理解不是很深入，没有丝毫头绪啊

/*
 * init  = initial state
 * op_in = operator loaded in
 * eql   = equal sign loaded in
 *
 * |    States   |   input    |   transition to
 * |             |            |
 * |    init     |   digit    |   init
 * |    init     |   * + -    |   op_in ( with target transfer )
 * |    init     |   key_ac   |   init ( with resetting )
 * |    op_in    |   digit    |   op_in
 * |    op_in    |   * + -    |   op_in ( with calculation being done )
 * |    op_in    |   key_ac   |   init ( with resetting )
 * |    op_in    |   key_eql  |   init ( with calculation being done )
 */

/*
 * where calculation actually being done.
 * @param: [int op] loaded operator
 * @param: [int lhs] left hand side operand
 * @param: [int rhs] right hand side operand
 * @return: result of the calculation
 */
static int do_calc(int op, int lhs, int rhs);
extern int read_key();

void calculator(void)
{
    some_init_proc();
    int i;
    int lhs, rhs, *target;
    int state, op;

    lhs = rhs = 0;
    target = &lhs;
    display_num(0);

    // State INIT
    while (1) {
init:
        i = read_key();
        if (i < 10) {
            *target *= 10;
            *target += i;
            display_num(*target);
        } else if (i == KEY_AC) {
            lhs = rhs = 0;
            target = &lhs;
            display_num(0);
            goto init;
        } else if (i == '+' || i == '-' || i == '*') {
            target = &rhs;
            op = i;
            goto op_in;
        }
    }

    // State OP_IN
    while (1) {
op_in:
        i = read_key();
        if (i < 10) {
            *target *= 10;
            *target += i;
            display_num(*target);
        } else if (i == KEY_AC) {
            lhs = rhs = 0;
            target = &lhs;
            display_num(0);
            goto init;
        } else if (i == '+' || i == '-' || i == '*') {
            target = &rhs;
            lhs = do_calc(op, lhs, rhs);
            display_num(lhs);
            op = i;
            goto op_in;
        } else if (i == KEY_EQL) {
            lhs = do_calc(op, lhs, rhs);
            display_num(lhs);
            goto equ;
        }
    }

    // State EQL
    while (1) {
eql:
        i = read_key();
        if (i == KEY_AC) {
            target = &lhs;
            lhs = rhs = 0;
            op = '+';
            goto init;
        }
    }
}

int do_calc(int op, int lhs, int rhs)
{
    switch (op) {
    case '+':
        return lhs + rhs;
    case '-':
        return lhs - rhs;
    case '*':
        return lhs * rhs;
    default:
        return lhs + rhs;
    }
}