/*
 * modified from textbook example
 * HBF
*/
#include <stdio.h>
#define MAX 10

void enqueue_left(int [], int *, int *, int);
void enqueue_right(int [], int *, int *, int);
void pop_left(int [], int *, int *);
void pop_right(int [], int *, int *);
int peek_left(int [], int);
int peek_right(int [], int);
void display(int [], int, int);

int main() {
  int deque[MAX];
  int left = -1, right = -1;

  int i;
  for (i=0; i<5; i++) {
    enqueue_left(deque, &left, &right, i);
    enqueue_right(deque, &left, &right,i+5);
  }

  printf("\nDisplay all: ");
  display(deque, left, right);

  printf("\nPeek left: %d\n", peek_left(deque, left));
  printf("\nPeek right: %d\n", peek_right(deque, right));
    
  printf("\nDelete left and then right: ");
  pop_left(deque, &left, &right);
  pop_right(deque, &left, &right);

  printf("\nDisplay all: ");
  display(deque, left, right);

  return 0;
}

void enqueue_right(int deque[], int *left, int *right, int val) {
  if ((*left == 0 && *right == MAX - 1) || (*left == *right + 1)) {
    printf("\n OVERFLOW");
    return;
  }
  if (*left == -1) /* if queue is initially empty */
  {
    *left = 0;
    *right = 0;
  } else {
    if (*right == MAX - 1) /*right is at last position of queue */
      *right = 0;
    else
      *right = *right + 1;
  }
  deque[*right] = val;
}

void enqueue_left(int deque[], int *left, int *right, int val)  {
  if ((*left == 0 && *right == MAX - 1) || (*left == *right + 1)) {
    printf("\n Overflow");
    return;
  }
  if (*left == -1)/*If queue is initially empty*/
  {
    *left = 0;
    *right = 0;
  } else {
    if (*left == 0)
      *left = MAX - 1;
    else
      *left = *left - 1;
  }
  deque[*left] = val;
}

void pop_left(int deque[], int *left, int *right) {
  if (*left == -1) {
    printf("\n UNDERFLOW");
    return;
  }
  if (*left == *right) /*Queue has only one element */
  {
    *left = -1;
    *right = -1;
  } else {
    if (*left == MAX - 1)
      *left = 0;
    else
      *left = *left + 1;
  }
}

void pop_right(int deque[], int *left, int *right) {
  if (*left == -1) {
    printf("\n UNDERFLOW");
    return;
  }
  if (*left == *right) /*queue has only one element*/
  {
    *left = -1;
    *right = -1;
  } else {
    if (*right == 0)
      *right = MAX - 1;
    else
      *right = *right - 1;
  }
}

int peek_left(int deque[], int left) {
  if (left == -1) {
    printf("\n UNDERFLOW");
    return -1;
  }
  return deque[left];
}

int peek_right(int deque[], int right) {
  if (right == -1) {
    printf("\n UNDERFLOW");
    return -1;
  }
  return deque[right];
}


void display(int deque[], int left, int right) {
  int front = left, rear = right;
  if (front == -1) {
    printf("\nQUEUE IS EMPTY");
    return;
  }
  if (front <= rear) {
    while (front <= rear) {
      printf("%d ", deque[front]);
      front++;
    }
  } else {
    while (front <= MAX - 1) {
      printf("%d ", deque[front]);
      front++;
    }
    front = 0;
    while (front <= rear) {
      printf("%d ", deque[front]);
      front++;
    }
  }
}

/*
Display all: 4 3 2 1 0 5 6 7 8 9
Peek left: 4

Peek right: 9

Delete left and then right:
Display all: 3 2 1 0 5 6 7 8
*/