co.c

#include <stdio.h>
#include <stdbool.h>

struct coro {
    void *state;
};

struct range_coro {
    struct coro base;
    int cur, max, step;
};

#define co_init(s)                                                             \
  if (!s->base.state) {                                                        \
    s->base.state = &&coroutine_init_label;                                    \
  } else {                                                                     \
    goto *s->base.state;                                                       \
  }                                                                            \
  coroutine_init_label:                                                        \
  do {                                                                         \
  } while (0)

#define co_yield__impl(s, val, counter)                                        \
  do {                                                                         \
    typeof(val) __ret__ = (val);                                               \
    s->base.state = &&yield_point##counter;                                    \
    return __ret__;                                                            \
    yield_point##counter:;                                                     \
  } while (0)
#define co_yield_impl(s, val, counter) co_yield__impl(s, val, counter)
#define co_yield(s, val) co_yield_impl(s, val, __COUNTER__)

int range(struct range_coro *rc)
{
    co_init(rc);

    while (rc->cur <= rc->max) {
        co_yield(rc, rc->cur);
        rc->cur += rc->step;
    }

    while (true) {
        co_yield(rc, -1);
    }
}

struct inc_mul_coro {
    struct coro base;
    int cur;
};

int inc_mul(struct inc_mul_coro *imc, int v)
{
    co_init(imc);

    while (true) {
        co_yield(imc, imc->cur * v);
        imc->cur++;
    }
}

struct inc_add_coro {
    struct coro base;
    int cur;
};

int inc_add(struct inc_add_coro *iac, int v)
{
    co_init(iac);

    while (true) {
        co_yield(iac, iac->cur * v);
        iac->cur++;
    }
}

int main(int argc, char *argv[])
{
    struct range_coro rc = { .cur = 0, .max = 100, .step = 7 };
    struct inc_mul_coro imc = {};
    struct inc_add_coro iac = {};
    int v;

    while ((v = range(&rc)) >= 0) {
        printf("%d\n", inc_add(&iac, inc_mul(&imc, v)));
    }

    printf("calling again: %d\n", range(&rc));
}