-- File listg.adb
--  This package implements a generic list, using access types
--  The list is intended to be used as part of lab5, being used
--  to implement a queue


package body Listg is 

   procedure Copy (Source: in List;
                   Destination: in out List) is
     from, to: List;
     
     begin
        from := Source;
        if Source = null then
           Destination := null;
        else
           Destination := new Node' (Datum => Source.Datum,
                                     Tail => null);
           from := Source.Tail;
           to := Destination;
           while from /= null loop
              to.Tail := new Node' (Datum => from.Datum,
                                    Tail => null);
              to := to.Tail;
              from := from.Tail;
           end loop;
        end if;
     exception
        when Storage_Error => raise Overflow;
     end Copy;
 
   procedure Clear (AList: in out List) is
      begin
         AList := null;
      end Clear;

   procedure Cons (Item: in Element;
                   AndList: in out List) is
        -- insert a new node containing Item at the front of the list
      begin
         AndList := new Node' (Datum => Item,   
                               Tail => AndList);
      exception
         when Storage_Error => raise Overflow;
      end Cons;

   procedure Append (AList: in List;
                     ToList: in out List) is
        -- make AList the tail of ToList
      index: List;
      begin
         if ToList = null then
            ToList := AList;
         else
            index := ToList;
            while index.Tail /= null loop
               index := index.Tail;
            end loop;
            index.Tail := AList;
         end if;
      end Append;

   function Equal (Left: in List;
                   Right: in List) return Boolean is
     lindex, rindex: List;
     begin
        lindex := Left;
        rindex := Right;
        while lindex /= null loop
              if lindex.Datum /= rindex.Datum then
                 return False;
              else
                 lindex := lindex.Tail;
                 rindex := rindex.Tail;
              end if;
         end loop;
         return (rindex = null);
      exception
         when Constraint_Error =>
            return False;
      end Equal;

   function Empty (AList: in List) return Boolean is
      begin
         return (AList = null);
      end Empty;

   function First (OfList: in List) return Element is
        -- return element stored in first node of list
      begin
         if OfList = null then
            raise Underflow;
         else
            return OfList.Datum;
         end if;
      end First;

   function Rest (OfList: in List) return List is
        -- return list following first node
      begin
         if OfList = null then
            raise Underflow;
         else
            return OfList.Tail;
         end if;
      end Rest;

end Listg;