Resumable Function Coroutines via Interleaved Switch-Case Statements

#include <stdio.h>
 
typedef struct {
    int state;
    int i;
} GeneratorState;
 
int step_generator(GeneratorState *ctx) {
    switch (ctx->state) {
        case 0:
            for (ctx->i = 0; ctx->i < 3; ctx->i++) {
                ctx->state = 1;
                return ctx->i * 10;
                case 1:;
            }
    }
    ctx->state = -1;
    return -1;
}
 
int main(void) {
    GeneratorState gen = { .state = 0, .i = 0 };
    int val;
    while ((val = step_generator(&gen)) != -1) {
        printf("Yielded: %d\n", val);
    }
    return 0;
}