-- File stacks.adb
--  Body for a simple stack package, implemented as a linked list
--  The stack is intended to be used in lab4, to help implement an
--  infix to postfix algorithm
--  Stack elements are characters

package body Stacks is

-- constructors

  procedure Copy (Source : in Stack;
                  Destination : in out Stack) is
     from, to: Stack;
     
     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;
     end Copy;

  procedure Clear (AStack : in out Stack) is
     begin
        AStack := null;
     end Clear;

  procedure Push (Item : in Character;
                  OnStack : in out Stack) is
     begin
        OnStack := new Node' (Datum => Item,
                              Tail => OnStack);   
     end Push;

  procedure Pop (AStack : in out Stack) is
     begin
        if AStack = null then
           raise underflow;
        else 
           AStack := AStack.Tail;
        end if;
     end Pop;
           
                  
-- selectors

  function Equal (Left, Right : in Stack) return Boolean is
     lindex, rindex: Stack;
     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 Depth (OfStack : in Stack) return Natural is
     count: integer := 0;
     index: Stack;
     begin
        index := OfStack;
        while index /= null loop
           index := index.Tail;
           count := count + 1;
        end loop;
        return count;
     end Depth;

  function Empty (AStack : in Stack) return Boolean is
     begin
        return (AStack = null);
     end Empty;

  function Top (OfStack : in Stack) return Character is
     begin
        if OfStack = null then
           raise underflow;
        else
           return (OfStack.Datum);
        end if;
     end Top;

end Stacks;