MODULE Stacks;
IMPORT
  Object,
  Object:Boxed,
  Out := NPCT:Console;

TYPE
  Pool(E: Object.Object) = POINTER TO ARRAY OF E;
  Stack*(E: Object.Object) = POINTER TO StackDesc(E);
  StackDesc*(E: Object.Object) = RECORD
    pool: Pool(E);
    cap-,top: LONGINT;
  END;

  PROCEDURE (s: Stack(E)) INIT*(cap: LONGINT);
  BEGIN
    NEW(s.pool,cap);s.cap := cap;s.top := -1
  END INIT;

  PROCEDURE (s: Stack(E)) Top*(): E;
  BEGIN
    RETURN s.pool[s.top]
  END Top;

  PROCEDURE (s: Stack(E)) Push*(e: E);
  BEGIN
    INC(s.top);
    ASSERT(s.top < s.cap);
    s.pool[s.top] := e;
  END Push;

  PROCEDURE (s: Stack(E)) Pop*(): E;
  VAR
    resp: E;
  BEGIN
    ASSERT(s.top >= 0);
    resp := s.pool[s.top];DEC(s.top);
    RETURN resp
  END Pop;

  PROCEDURE (s: Stack(E)) IsEmpty(): BOOLEAN;
  BEGIN
    RETURN s.top < 0
  END IsEmpty;

  PROCEDURE (s: Stack(E)) Size*(): LONGINT;
  BEGIN
    RETURN s.top + 1
  END Size;

  PROCEDURE Test;
  VAR
    s: Stack(Boxed.LongInt);
  BEGIN
    s := NEW(Stack(Boxed.LongInt),100);
    s.Push(NEW(Boxed.LongInt,10));
    s.Push(NEW(Boxed.LongInt,100));
    Out.String("size: ");Out.Int(s.Size(),0);Out.Ln;
    Out.String("pop: ");Out.Object(s.Pop());Out.Ln;
    Out.String("top: ");Out.Object(s.Top());Out.Ln;
    Out.String("size: ");Out.Int(s.Size(),0);Out.Ln
  END Test;

BEGIN
  Test
END Stacks.