(* example showing the use of difference lists, and SML timers *)

(* the vanilla-flavor reverse1, which uses the append operator *)
fun reverse1([]) = []
  | reverse1(x::xs) = reverse1(xs)@[x];

(* the not so obvious, but maybe faster version *)
fun auxReverse(nil, M) = M
  | auxReverse(x::xs, ys) = auxReverse(xs, x::ys);
fun reverse2([]) = []
  | reverse2(x::xs) = auxReverse(x::xs, nil);

(* test them a little *)
val l1 = [3,1,4,1,5,9,2,6,5,3];
reverse1(l1) = reverse2(l1);

(* a funtion to run them many times *)
fun runIt(F: int list -> int list, n:int, maxn:int) = 
    if n=maxn then F l1
    else (runIt(F, n + 1, maxn); F l1);

(* and a function to time them *)
fun clockIt(F: int list -> int list, n:int, maxn:int) = 
    let
       val t1 = Timer.startCPUTimer();
    in (runIt(F, n, maxn);
        let
           val recTimer = Timer.checkCPUTimer(t1);
           val t1gc = Time.toString(#gc(recTimer));             
           val t1sys = Time.toString(#sys(recTimer));
           val t1usr = Time.toString(#usr(recTimer));
        in print("gc is "^t1gc^" sys is "^t1sys^" usr is "^t1usr^"\n")
        end)
    end;

SMLofNJ.Internals.GC.messages(false);
print("Timing of reverse1\n");
clockIt(reverse1,1,3000000);
print("Timing of reverse2\n");
clockIt(reverse2,1,3000000);