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

typedef struct Stacks stacks;
struct Stacks {
  int *stack;
  uint s1, s2;
  uint l1, l2;
};

stacks *init(uint s1, uint s2) {
  stacks *s = malloc(sizeof(stacks));
  s->stack = malloc(sizeof(int) * (s1 + s2));
  s->s1 = s1, s->s2 = s2;
  s->l1 = s->l2 = 0;

  return s;
}

void push(uint stack, stacks *s, int elem) {
  if (stack != 1 && stack != 2)
    fprintf(stderr, "Invalid stack number\n");

  if (stack == 1) {
    if (s->l1 == s->s1) {
      fprintf(stderr, "Stack 1 Overflow");
      return;
    }
    s->stack[s->l1++] = elem;
    return;
  }

  if (s->l2 == s->s2) {
    fprintf(stderr, "Stack 2 Overflow");
    return;
  }
  s->stack[s->s1 + s->l2++] = elem;
}

int pop(uint stack, stacks *s) {
  if (stack != 1 && stack != 2)
    fprintf(stderr, "Invalid stack number\n");

  if (stack == 1) {
    if (!s->l1) {
      fprintf(stderr, "Stack 1 Underflow");
      exit(1);
    }
    return s->stack[--s->l1];
  }

  if (!s->l2) {
    fprintf(stderr, "Stack 2 Underflow");
    exit(1);
  }
  return s->stack[s->s1 + --s->l2];
}

int main() {
  int s1 = 8, s2 = 4; // sizes
  stacks *s = init(s1, s2);
  push(1, s, 3);
  push(1, s, 99);
  push(2, s, 6);
  push(1, s, 4);

  printf("%d ", pop(1, s));
  printf("%d", pop(2, s));

  free(s->stack);
  free(s);
}