Improve optimisation for a list source in "CountDown"

Author: atifaziz

MoreLinq/CountDown.cs

@@ -65,15 +65,11 @@ public static IEnumerable<TResult> CountDown<T, TResult>(this IEnumerable<T> sou
 
             IEnumerable<TResult> IterateList(IListLike<T> list)
             {
-                var countdown = Math.Min(count, list.Count);
+                var listCount = list.Count;
+                var countdown = Math.Min(count, listCount);
 
-                for (var i = 0; i < list.Count; i++)
-                {
-                    var cd = list.Count - i <= count
-                           ? --countdown
-                           : (int?) null;
-                    yield return resultSelector(list[i], cd);
-                }
+                for (var i = 0; i < listCount; i++)
+                    yield return resultSelector(list[i], listCount - i <= count ? --countdown : null);
             }
 
             IEnumerable<TResult> IterateCollection(int i)