Implementing the queue with a fixed size array

We can implement a queue with an array and 2 indexes head and tail:

where:

head = the location of the front (first position) of the queue
tail = the location of the open spot at the tail (last position) of the queue

Implementing the queue with a fixed size array

The enqueue(x) method will append the element x at the tail of the queue:
Before enqueue(x):

After enqueue(x):

Implementing the queue with a fixed size array

The dequeue(x) method will remove the element x at the head of the queue:
Before dequeue(x):

After dequeue(x):

Problem with the implementation

Elements dequeued from the front of the array are unused:
We must move the queue content forward to reclaim the usage:

This may require many copy operations...

There is clever way to re-used the unused array elements -- discussed next