Zustandsbehaftetes Re-entrant Coroutine-Trampolin über Switch-Fallthrough-Zustandsautomat

#include <stdio.h>
 
typedef struct {
    int state;
    int index;
} GeneratorState;
 
#define COROUTINE_BEGIN(ctx) switch((ctx)->state) { case 0:
#define COROUTINE_YIELD(ctx, val) do { (ctx)->state = __LINE__; return (val); case __LINE__:; } while(0)
#define COROUTINE_END(ctx) default: (ctx)->state = -1; return -1; }
 
int generate_fibonacci(GeneratorState *ctx) {
    static int a = 0, b = 1;
    COROUTINE_BEGIN(ctx);
    
    while (ctx->index < 5) {
        int next = a + b;
        a = b;
        b = next;
        ctx->index++;
        COROUTINE_YIELD(ctx, a);
    }
    
    COROUTINE_END(ctx);
}