-- This is the generic specification for a queue abstract data type.
Size : Positive;
type ItemType is private;
package QueuePkg is
type Queue is limited private;
Queue_Empty, Queue_Full : exception;
function IsEmpty (Q : Queue) return Boolean;
function IsFull (Q : Queue) return Boolean;
procedure Enqueue (Item : ItemType; Q : in out Queue);
procedure Dequeue ( Q : in out Queue);
function Front (Q : Queue) return ItemType;
type QueueElements is array(0..Size-1) of ItemType;
-- Implemented by accessing an array in a CIRCULAR order.
-- The array is accessed in circular order by using "mod Size"
-- Remember that (x mod Size) is in the range (0..Size-1)
-- Back points to the spot where the NEXT enqueued item is to go!
-- CurSize contains the number of elements currently on the queue
-- A queue is empty when the next element to be added would go into
-- the Front position (ie when the Front and Back have the same value)
-- A queue is full when the next element to add would fill the last
-- open position in the queue [ie when (Back + 1) mod Size = Front]
type Queue is record
Elements : QueueElements;
Front, Back, CurSize : Natural := 0;