-- File Queueg.adb
--  This is a generic queue package, built on top of a generic list
--  package (listg) that handles most of the pointer manipulation.
--  It will later be used in a graph traversal program. 

package body Queueg is

   procedure Copy (From: in Queue;
                   To: in out Queue) is
      index, trail: List;
      begin
         Copy (From.Front, To.Front);   -- copy list pointed to by Front
         if Empty (To.Front) then
            Clear (To.Rear);
         else
            index := To.Front;
            while not Empty (Rest (index)) loop
               index := Rest (index);
            end loop;
            To.Rear := index;
         end if;
      end Copy;

   procedure Clear (AQueue: in out Queue) is
      begin
         Clear (AQueue.Front);
         Clear (AQueue.Rear);
      end Clear;

   procedure Add (Item: in Element;
                  ToQueue: in out Queue) is
      temp: List;
      begin
         Cons (Item, temp);
         Append (temp, ToQueue.Front);
         if Empty (ToQueue.Rear) then
            ToQueue.Rear := ToQueue.Front;
         else
            ToQueue.Rear := Rest (ToQueue.Rear);
         end if;
      exception
         when Overflow => raise Overflow;
      end Add;

   procedure Pop (AQueue: in out Queue) is 
      begin
         if Empty (AQueue) then
            raise Underflow;
         else
            AQueue.Front := Rest (AQueue.Front);
            if Empty (AQueue.Front) then
               Clear (AQueue.Rear);
            end if;
         end if;
      end Pop;

   function Equal (Left: in Queue;
                   Right: in Queue) return Boolean is
      begin
         return Equal (Left.Front, Right.Front);
      end Equal;

   function Empty (AQueue: in Queue) return Boolean is
      begin
         return Empty (AQueue.Front);
      end Empty;

   function First (OfQueue: in Queue) return Element is
     -- datum of front of queue
      begin
         if Empty (OfQueue.Front) then
            raise Underflow;
         else
            return First (OfQueue.Front);
         end if;
      end First;

end Queueg;