-- file name listndbl.adb 2:51PM  9/1/95

with gnat.IO; 
use gnat.IO;			-- needed for printing message on exception
package body listn.dbl is

procedure insert(value:in element; 
                 afterX, inList: in out pointer) is
  t: pointer:= new dlistnode'(data => value, next => null, previous => null);
	-- note that, even though t is initialized as a dlistnode ptr, you still
	-- must use conversion to get access to non-listnode'class fields 
	-- in this case, "previous"
begin
  if inList = null
    then inList := t;              -- inserting into the empty list
    elsif afterX = null		   -- inserting at front of list
	 then dlistnode(inList.all).previous := t;
	      inList := t;
    else t.next := afterX.next;    -- insert after afterX   
	 dlistnode(t.all).previous := afterX;
	 dlistnode(t.next.all).previous := t;
         afterX.next := t;
  end if;
end insert;

function cons(x:in element; toList : in pointer) return pointer is
  L: pointer;
begin
  L := new dlistnode'(data => x, next => toList, previous => null);
  if toList /= null then dlistnode(toList.all).previous := L;  end if;
  return L;
end cons;

function prev(DL : in pointer) return pointer is
  noprevmt : exception;
begin
  if IsEmpty(DL)
  then raise noprevmt;
  else return dlistnode(DL.all).previous;
  end if;
exception
  when noprevmt => put_line("tried to find prev of empty list");
		   return null;
end prev;

procedure delete(x, precededByY, inList: in out pointer) is
begin
  if precededByY = null
    then inList := inList.next;    -- deleting first element
	 dlistnode(inList.all).previous := null;  -- fix previous pointer of new first element
    else precededByY.next := x.next;
	 dlistnode(x.next.all).previous := dlistnode(x.all).previous; 
  end if;
-- dispose(x); or free(x); if you choose to define it and do so
end delete;

end listn.dbl;