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

#define MAX_STACK 1024
#define STACK_SENTINEL -1

struct Stack {
  int l[MAX_STACK];
};

struct Stack *Stack_create()
{
  struct Stack *s = malloc(sizeof(struct Stack));
  if(!s) { return NULL; }

  int i = 0;

  for(i = 0; i < MAX_STACK; i++) {
    s->l[i] = STACK_SENTINEL;
  }

  return s;
}

int Stack_push(struct Stack *s, int v)
{
  if(v <= STACK_SENTINEL) {
    return STACK_SENTINEL;
  }

  int i = 0;
  for(i = 0; i < MAX_STACK; i++) {
    if(s->l[i] == STACK_SENTINEL) {
      s->l[i] = v;
      return i;
    }
  }

  return STACK_SENTINEL;
}

int Stack_pop(struct Stack *s)
{
  int i = 0;
  int v = 0;
  for(i = 0; i < MAX_STACK; i++) {
    if(s->l[i] == STACK_SENTINEL) {
      v = s->l[i-1];
      s->l[i-1] = STACK_SENTINEL;
      return v;
    }
  }

  return STACK_SENTINEL;
}

int Stack_size(struct Stack *s)
{
  int i = 0;
  for(i = 0; i < MAX_STACK; i++) {
    if(s->l[i] == STACK_SENTINEL) {
      return i;
    }
  }

  return STACK_SENTINEL;
}

int main(int argc, char *argv[])
{
  struct Stack *s = Stack_create();

  printf("size: %d\n", Stack_size(s));
  Stack_push(s, 10);
  Stack_push(s, 20);
  Stack_push(s, 40);

  printf("size: %d\n", Stack_size(s));
  printf("pop: %d\n", Stack_pop(s));
  printf("pop: %d\n", Stack_pop(s));
  printf("size: %d\n", Stack_size(s));
  printf("pop: %d\n", Stack_pop(s));
  printf("pop: %d\n", Stack_pop(s));

  return 0;
}