ENQUEUEand `DEQUEUE to detect underflow and overflow of a queue.
I shall go with the pseudo-code version, since I'm too lazy to bother figuring out how to test it in C.
We need to do a slight modification, since the current version provides no way
to tell whether a queue is empty or full. We should have
Q.head == NIL when
the queue is empty and
Q.head == Q.tail when the queue is full. An empty
queue is initializes with
NIL in its head and
Q.tail =. We need to update
Q.head when a
DEQUEUE operation causes the queue to become empty.
ENQUEUE(Q, x) if Q.head == Q.tail error "Queue overflow" Q[Q.tail] = x if Q.head == NIL Q.head = Q.tail if Q.tail == Q.length Q.tail = 1 else Q.tail = Q.tail + 1 DEQUEUE(Q) if Q.head == NIL error "Queue underflow" x = Q[Q.head] if Q.head == Q.length Q.head = 1 else Q.head = Q.head + 1 if Q.head == Q.tail Q.head = NIL return x