(****
 *
 * This file illustrates how to specify formally the requirement that a
 * collection object contains a certain number of elements.  It is defined
 * using the TestTool spec in particular, but is applicable in general to
 * counting the number of elements in any collection.
 *
 *)

obj Q has t:Type and a:Answer;		(* Simplified question object *)
obj Type has tf:TF or mc:MC;		(* Simplified question type *)
obj Answer;				(* Dont care about the answer here *)
obj TF;
obj MC;

obj Test has ql:QL and OtherStuff;	(* Simplified test object *)
obj OtherStuff;				(* Dont care about other stuff *)
obj QL has Q*;				(* Question list is the collection in
					 * which we want to count the number of
					 * questions of a particular type *)

obj Criteria has hmtf:HowManyTF;	(* Simplified test const'n criteria *)
obj HowManyTF is integer;

op BuildTest(QL, c:Criteria)->t:Test
    post:
	(*
	 * The question list in the output test should have as many true/false
	 * questions as requested in the input criteria.
	 *)
	CountTF(t.ql, c.hmtf);
end;

(*
 * Aux function CountTF recursively traverses the given question list, counting
 * the number of TF questions it finds, returning true if exactly the given
 * number is found, false otherwise.
 *)
function CountTF(ql:QL, hmtf:HowManyTF)->boolean =

    if (ql = nil) and (hmtf = 0)	(* We succeed when list is empty and *)
    then true				(*  we expect to find 0 TF questions.*)
    else				(* We fail when list is empty and    *)
	if (ql = nil) and (hmtf != 0)   (*  we expect to find more TF ques-  *)
	then false			(*  tions or we have found too many. *)
	else				(* Otherwise,			     *)
	    if ql[1].t isa tf		(*  if ql[1] is a TF question	     *)
	    then CountTF(ql[2:#ql],	(*  then chk for 1 fewer TF's in the *)
		 hmtf-1)		(*    rest of the list		     *)
	    else CountTF(ql[2:#ql],	(*  else chk for the same number of  *)
		 hmtf);			(*    TFs in the rest of the list    *)