QueueUsingCircularArray.c

struct Queue { 
    int a[5]; // queue with 5 elements 
    int h; // head (first element of queue) 
    int t; // tail (right after last element) 
};

/* 
  This implementation can add infinite numbers of elements to the queue (even though it might be full) 
  because it is a circular queue implementation. If the array is full, elements are dropped. If this isn't reasonable
  add a isFull check before adding
*/ 

void enqueue(struct Queue* q, int val) {
    q->a[q->t] = x;
    q->t = (q->t + 1) % n;
}

int dequeue(struct Queue* q) {
    int i = q->a[q->h];
    q->h = (q->h + 1) % n;
    return i;
}

bool isFull(struct Queue* q) {
    // q->t is guaranteed to be in bounds after last dequeue
    // but q->t + 1 might be out of bounds, so we need to 
    // modulo again 
    return (q->t + 1) % n == q->h;
}