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

typedef struct doubleStack {
  int capacity;
  int top1;
  int top2;
  int *arr;
} doubleStack;

doubleStack *createStack(int capacity) {
  doubleStack *new = malloc(sizeof(doubleStack));
  new->capacity = capacity;
  new->top1 = -1;
  new->top2 = capacity;

  if (capacity <= 0) {
    return NULL;
  }
  new->arr = malloc(sizeof(int) * capacity);

  return new;
}

void removeStack(doubleStack *st) {
  free(st->arr);
  free(st);
}

void push(doubleStack *st, int in, int side) {
  // side == 0: top1
  // side == 1: top2
  if (st->top1 == st->top2 - 1) {
    printf("ERR: array is full\n");
    return;
  }

  if (side == 0) {
    st->top1++;
    st->arr[st->top1] = in;
  } else {
    st->top2--;
    st->arr[st->top2] = in;
  }
}

int top(doubleStack *st, int side) {
  if (side == 0) {
    return st->arr[st->top1];
  } else {
    return st->arr[st->top2];
  }
}

int isEmpty(doubleStack *st, int side) {
  if (side == 0) {
    return (st->top1 == -1);
  } else {
    return (st->top2 == st->capacity);
  }
}

void pop(doubleStack *st, int side) {
  if (!isEmpty(st, side)) {
    if (side == 0) {
      st->top1--;
    } else {
      st->top2--;
    }
  } else {
    printf("ERR: array is empty\n");
  }
}

int main(void) {
  doubleStack* test = createStack(5);

  push(test, 1, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  push(test, 2, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  push(test, 3, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  push(test, 4, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  push(test, 5, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  push(test, 6, 0);
  printf("Top of left stack is %d\n", top(test, 0));
  pop(test, 0);
  push(test, 6, 0);
  printf("Top of left stack is %d\n", top(test, 0));

  pop(test, 0);
  pop(test, 0);

  push(test, 9, 1);
  printf("Top of right stack is %d\n", top(test, 1));
  push(test, 8, 1);
  printf("Top of right stack is %d\n", top(test, 1));

  removeStack(test);

  return 0;
}