#include <stdio.h>
#include <stdlib.h>
#include <math.h>

typedef int (*seq_func)(void *);
#define SEQ_BASE seq_func f; int output

/* sort of polymorphing data structure */
typedef struct { SEQ_BASE; } gen_t;

int seq_next(void *state)
{
	return ((gen_t*)state)->output = (*(seq_func*)state)(state);
}

typedef struct {
	SEQ_BASE;
	int pos, n;
} power_gen_t;

int power_next(void *s)
{
	return (int)pow(++((power_gen_t*)s)->pos, ((power_gen_t*)s)->n);
}

void *power_seq(int n)
{
	power_gen_t *s = malloc(sizeof(power_gen_t));
	s->output = -1;
	s->f = power_next;
	s->n = n;
	s->pos = -1;
	return s;
}

typedef struct {
	SEQ_BASE;
	void *in, *without;
} filter_gen_t;

int filter_next(void *s)
{
	gen_t *in = ((filter_gen_t*)s)->in, *wo = ((filter_gen_t*)s)->without;

	do{
		seq_next(in);
		while (wo->output < in->output)
			seq_next(wo);
	} while(wo->output == in->output);

	return in->output;
}

void* filter_seq(gen_t *in, gen_t *without)
{
	filter_gen_t *filt = malloc(sizeof(filter_gen_t));
	filt->in = in;
	filt->without = without;
	filt->f = filter_next;
	filt->output = -1;
	return filt;
}

int main()
{
	int i;
	void *s = filter_seq(power_seq(2), power_seq(3));

	for (i = 0; i < 20; i++) seq_next(s);
	for (i = 0; i < 10; i++)
		printf("%d\n", seq_next(s));

	return 0;
}